One way in which the different types of resistance can be distinguished is in the directions of current and electric power between a circuit and an electronic component. The illustrations below, with a rectangle representing the component attached to a circuit, summarize how the different types work:
The voltage v and current i variables in an electrical component must be defined according to the passive sign convention; positive conventional current is defined to enter the positive voltage terminal; this means power P flowing from the circuit into the component is defined to be positive, while power flowing from the component into the circuit is negative.^{ [25] }^{ [31] } This applies to both DC and AC current. The diagram shows the directions for positive values of the variables.  
In a positive static resistance, , so v and i have the same sign.^{ [24] } Therefore, from the passive sign convention above, conventional current (flow of positive charge) is through the device from the positive to the negative terminal, in the direction of the electric field E (decreasing potential).^{ [25] } so the charges lose potential energy doing work on the device, and electric power flows from the circuit into the device,^{ [24] }^{ [29] } where it is converted to heat or some other form of energy (yellow). If AC voltage is applied, and periodically reverse direction, but the instantaneous always flows from the higher potential to the lower potential.  
In a power source, ,^{ [23] } so and have opposite signs.^{ [24] } This means current is forced to flow from the negative to the positive terminal.^{ [23] } The charges gain potential energy, so power flows out of the device into the circuit:^{ [23] }^{ [24] }. Work (yellow) must be done on the charges by some power source in the device to make them move in this direction against the force of the electric field.  
In a passive negative differential resistance, , only the AC component of the current flows in the reverse direction. The static resistance is positive^{ [6] }^{ [9] }^{ [21] } so the current flows from positive to negative: . But the current (rate of charge flow) decreases as the voltage increases. So when a timevarying (AC) voltage is applied in addition to a DC voltage (right), the timevarying current and voltage components have opposite signs, so .^{ [37] } This means the instantaneous AC current flows through the device in the direction of increasing AC voltage , so AC power flows out of the device into the circuit. The device consumes DC power, some of which is converted to AC signal power which can be delivered to a load in the external circuit,^{ [7] }^{ [37] } enabling the device to amplify the AC signal applied to it.^{ [11] } 
r_{diff} > 0 Positive differential resistance  r_{diff} < 0 Negative differential resistance  

R_{static} > 0 Passive: Consumes net power  Positive resistances:
 Passive negative differential resistances:

R_{static} < 0 Active: Produces net power  Power sources:
 "Active resistors" Positive feedback amplifiers used in:

In an electronic device, the differential resistance , the static resistance , or both, can be negative,^{ [24] } so there are three categories of devices (fig. 2–4 above, and table) which could be called "negative resistances".
The term "negative resistance" almost always means negative differential resistance .^{ [3] }^{ [17] }^{ [20] } Negative differential resistance devices have unique capabilities: they can act as oneport amplifiers,^{ [3] }^{ [11] }^{ [13] }^{ [38] } increasing the power of a timevarying signal applied to their port (terminals), or excite oscillations in a tuned circuit to make an oscillator.^{ [37] }^{ [38] }^{ [39] } They can also have hysteresis.^{ [15] }^{ [16] } It is not possible for a device to have negative differential resistance without a power source,^{ [40] } and these devices can be divided into two categories depending on whether they get their power from an internal source or from their port:^{ [16] }^{ [37] }^{ [39] }^{ [41] }^{ [42] }
Occasionally ordinary power sources are referred to as "negative resistances"^{ [20] }^{ [27] }^{ [32] }^{ [51] } (fig. 3 above). Although the "static" or "absolute" resistance of active devices (power sources) can be considered negative (see Negative static resistance section below) most ordinary power sources (AC or DC), such as batteries, generators, and (non positive feedback) amplifiers, have positive differential resistance (their source resistance).^{ [52] }^{ [53] } Therefore, these devices cannot function as oneport amplifiers or have the other capabilities of negative differential resistances.
Electronic components with negative differential resistance include these devices:
Electric discharges through gases also exhibit negative differential resistance,^{ [63] }^{ [64] } including these devices
In addition, active circuits with negative differential resistance can also be built with amplifying devices like transistors and op amps, using feedback.^{ [43] }^{ [37] }^{ [47] } A number of new experimental negative differential resistance materials and devices have been discovered in recent years.^{ [67] } The physical processes which cause negative resistance are diverse,^{ [12] }^{ [56] }^{ [67] } and each type of device has its own negative resistance characteristics, specified by its current–voltage curve.^{ [10] }^{ [43] }
A point of some confusion is whether ordinary resistance ("static" or "absolute" resistance, ) can be negative.^{ [68] }^{ [72] } In electronics, the term "resistance" is customarily applied only to passive materials and components^{ [30] } – such as wires, resistors and diodes. These cannot have as shown by Joule's law .^{ [29] } A passive device consumes electric power, so from the passive sign convention . Therefore, from Joule's law .^{ [23] }^{ [27] }^{ [29] } In other words, no material can conduct electric current better than a "perfect" conductor with zero resistance.^{ [6] }^{ [73] } For a passive device to have would violate either conservation of energy ^{ [3] } or the second law of thermodynamics,^{ [39] }^{ [44] }^{ [68] }^{ [71] }(diagram). Therefore, some authors^{ [6] }^{ [29] }^{ [69] } state that static resistance can never be negative.
However it is easily shown that the ratio of voltage to current v/i at the terminals of any power source (AC or DC) is negative.^{ [27] } For electric power (potential energy) to flow out of a device into the circuit, charge must flow through the device in the direction of increasing potential energy, conventional current (positive charge) must move from the negative to the positive terminal.^{ [23] }^{ [36] }^{ [44] } So the direction of the instantaneous current is out of the positive terminal. This is opposite to the direction of current in a passive device defined by the passive sign convention so the current and voltage have opposite signs, and their ratio is negative
This can also be proved from Joule's law ^{ [23] }^{ [27] }^{ [68] }
This shows that power can flow out of a device into the circuit () if and only if .^{ [23] }^{ [24] }^{ [32] }^{ [68] } Whether or not this quantity is referred to as "resistance" when negative is a matter of convention. The absolute resistance of power sources is negative,^{ [3] }^{ [24] } but this is not to be regarded as "resistance" in the same sense as positive resistances. The negative static resistance of a power source is a rather abstract and not very useful quantity, because it varies with the load. Due to conservation of energy it is always simply equal to the negative of the static resistance of the attached circuit (right).^{ [27] }^{ [42] }
Work must be done on the charges by some source of energy in the device, to make them move toward the positive terminal against the electric field, so conservation of energy requires that negative static resistances have a source of power.^{ [3] }^{ [23] }^{ [39] }^{ [44] } The power may come from an internal source which converts some other form of energy to electric power as in a battery or generator, or from a separate connection to an external power supply circuit^{ [44] } as in an amplifying device like a transistor, vacuum tube, or op amp.
A circuit cannot have negative static resistance (be active) over an infinite voltage or current range, because it would have to be able to produce infinite power.^{ [10] } Any active circuit or device with a finite power source is "eventually passive".^{ [49] }^{ [74] }^{ [75] } This property means if a large enough external voltage or current of either polarity is applied to it, its static resistance becomes positive and it consumes power^{ [74] }
Therefore, the ends of the I–V curve will eventually turn and enter the 1st and 3rd quadrants.^{ [75] } Thus the range of the curve having negative static resistance is limited,^{ [10] } confined to a region around the origin. For example, applying a voltage to a generator or battery (graph, above) greater than its opencircuit voltage^{ [76] } will reverse the direction of current flow, making its static resistance positive so it consumes power. Similarly, applying a voltage to the negative impedance converter below greater than its power supply voltage V_{s} will cause the amplifier to saturate, also making its resistance positive.
In a device or circuit with negative differential resistance (NDR), in some part of the I–V curve the current decreases as the voltage increases:^{ [21] }
The I–V curve is nonmonotonic (having peaks and troughs) with regions of negative slope representing negative differential resistance.
Passive negative differential resistances have positive static resistance;^{ [3] }^{ [6] }^{ [21] } they consume net power. Therefore, the I–V curve is confined to the 1st and 3rd quadrants of the graph,^{ [15] } and passes through the origin. This requirement means (excluding some asymptotic cases) that the region(s) of negative resistance must be limited,^{ [17] }^{ [77] } and surrounded by regions of positive resistance, and cannot include the origin.^{ [3] }^{ [10] }
Negative differential resistances can be classified into two types:^{ [16] }^{ [77] }
Most devices have a single negative resistance region. However devices with multiple separate negative resistance regions can also be fabricated.^{ [67] }^{ [81] } These can have more than two stable states, and are of interest for use in digital circuits to implement multivalued logic.^{ [67] }^{ [81] }
An intrinsic parameter used to compare different devices is the peaktovalley current ratio (PVR),^{ [67] } the ratio of the current at the top of the negative resistance region to the current at the bottom (see graphs, above):
The larger this is, the larger the potential AC output for a given DC bias current, and therefore the greater the efficiency
A negative differential resistance device can amplify an AC signal applied to it^{ [11] }^{ [13] } if the signal is biased with a DC voltage or current to lie within the negative resistance region of its I–V curve.^{ [7] }^{ [12] }
The tunnel diode circuit (see diagram) is an example.^{ [82] } The tunnel diode TD has voltage controlled negative differential resistance.^{ [54] } The battery adds a constant voltage (bias) across the diode so it operates in its negative resistance range, and provides power to amplify the signal. Suppose the negative resistance at the bias point is . For stability must be less than .^{ [36] } Using the formula for a voltage divider, the AC output voltage is^{ [82] }
In a normal voltage divider, the resistance of each branch is less than the resistance of the whole, so the output voltage is less than the input. Here, due to the negative resistance, the total AC resistance is less than the resistance of the diode alone so the AC output voltage is greater than the input . The voltage gain is greater than one, and increases without limit as approaches .
The diagrams illustrate how a biased negative differential resistance device can increase the power of a signal applied to it, amplifying it, although it only has two terminals. Due to the superposition principle the voltage and current at the device's terminals can be divided into a DC bias component () and an AC component ().
Since a positive change in voltage causes a negative change in current , the AC current and voltage in the device are 180° out of phase.^{ [7] }^{ [57] }^{ [36] }^{ [84] } This means in the AC equivalent circuit (right), the instantaneous AC current Δi flows through the device in the direction of increasing AC potential Δv, as it would in a generator.^{ [36] } Therefore, the AC power dissipation is negative; AC power is produced by the device and flows into the external circuit.^{ [85] }
With the proper external circuit, the device can increase the AC signal power delivered to a load, serving as an amplifier,^{ [36] } or excite oscillations in a resonant circuit to make an oscillator. Unlike in a two port amplifying device such as a transistor or op amp, the amplified signal leaves the device through the same two terminals (port) as the input signal enters.^{ [86] }
In a passive device, the AC power produced comes from the input DC bias current,^{ [21] } the device absorbs DC power, some of which is converted to AC power by the nonlinearity of the device, amplifying the applied signal. Therefore, the output power is limited by the bias power^{ [21] }
The negative differential resistance region cannot include the origin, because it would then be able to amplify a signal with no applied DC bias current, producing AC power with no power input.^{ [3] }^{ [10] }^{ [21] } The device also dissipates some power as heat, equal to the difference between the DC power in and the AC power out.
The device may also have reactance and therefore the phase difference between current and voltage may differ from 180° and may vary with frequency.^{ [8] }^{ [42] }^{ [87] } As long as the real component of the impedance is negative (phase angle between 90° and 270°),^{ [84] } the device will have negative resistance and can amplify.^{ [87] }^{ [88] }
The maximum AC output power is limited by size of the negative resistance region ( in graphs above)^{ [21] }^{ [89] }
The reason that the output signal can leave a negative resistance through the same port that the input signal enters is that from transmission line theory, the AC voltage or current at the terminals of a component can be divided into two oppositely moving waves, the incident wave, which travels toward the device, and the reflected wave, which travels away from the device.^{ [90] } A negative differential resistance in a circuit can amplify if the magnitude of its reflection coefficient , the ratio of the reflected wave to the incident wave, is greater than one.^{ [17] }^{ [85] }
The "reflected" (output) signal has larger amplitude than the incident; the device has "reflection gain".^{ [17] } The reflection coefficient is determined by the AC impedance of the negative resistance device, , and the impedance of the circuit attached to it, .^{ [85] } If and then and the device will amplify. On the Smith chart, a graphical aide widely used in the design of high frequency circuits, negative differential resistance corresponds to points outside the unit circle , the boundary of the conventional chart, so special "expanded" charts must be used.^{ [17] }^{ [91] }
Because it is nonlinear, a circuit with negative differential resistance can have multiple equilibrium points (possible DC operating points), which lie on the I–V curve.^{ [92] } An equilibrium point will be stable, so the circuit converges to it within some neighborhood of the point, if its poles are in the left half of the s plane (LHP), while a point is unstable, causing the circuit to oscillate or "latch up" (converge to another point), if its poles are on the jω axis or right half plane (RHP), respectively.^{ [93] }^{ [94] } In contrast, a linear circuit has a single equilibrium point that may be stable or unstable.^{ [95] }^{ [96] } The equilibrium points are determined by the DC bias circuit, and their stability is determined by the AC impedance of the external circuit. However, because of the different shapes of the curves, the condition for stability is different for VCNR and CCNR types of negative resistance:^{ [86] }^{ [97] }
.
.
For general negative resistance circuits with reactance, the stability must be determined by standard tests like the Nyquist stability criterion.^{ [102] } Alternatively, in high frequency circuit design, the values of for which the circuit is stable are determined by a graphical technique using "stability circles" on a Smith chart.^{ [17] }
For simple nonreactive negative resistance devices with and the different operating regions of the device can be illustrated by load lines on the I–V curve^{ [77] }(see graphs).
The DC load line (DCL) is a straight line determined by the DC bias circuit, with equation
where is the DC bias supply voltage and R is the resistance of the supply. The possible DC operating point(s) (Q points) occur where the DC load line intersects the I–V curve. For stability^{ [103] }
The AC load line (L_{1} − L_{3}) is a straight line through the Q point whose slope is the differential (AC) resistance facing the device. Increasing rotates the load line counterclockwise. The circuit operates in one of three possible regions (see diagrams), depending on .^{ [77] }
In addition to the passive devices with intrinsic negative differential resistance above, circuits with amplifying devices like transistors or op amps can have negative resistance at their ports.^{ [3] }^{ [37] } The input or output impedance of an amplifier with enough positive feedback applied to it can be negative.^{ [47] }^{ [38] }^{ [107] }^{ [108] } If is the input resistance of the amplifier without feedback, is the amplifier gain, and is the transfer function of the feedback path, the input resistance with positive shunt feedback is^{ [3] }^{ [109] }
So if the loop gain is greater than one, will be negative. The circuit acts like a "negative linear resistor"^{ [3] }^{ [45] }^{ [50] }^{ [110] } over a limited range,^{ [42] } with I–V curve having a straight line segment through the origin with negative slope (see graphs).^{ [67] }^{ [24] }^{ [26] }^{ [35] }^{ [106] } It has both negative differential resistance and is active
and thus obeys Ohm's law as if it had a negative value of resistance −R,^{ [67] }^{ [46] } over its linear range (such amplifiers can also have more complicated negative resistance I–V curves that do not pass through the origin).
In circuit theory these are called "active resistors".^{ [24] }^{ [28] }^{ [48] }^{ [49] } Applying a voltage across the terminals causes a proportional current out of the positive terminal, the opposite of an ordinary resistor.^{ [26] }^{ [45] }^{ [46] } For example, connecting a battery to the terminals would cause the battery to charge rather than discharge.^{ [44] }
Considered as oneport devices, these circuits function similarly to the passive negative differential resistance components above, and like them can be used to make oneport amplifiers and oscillators^{ [3] }^{ [11] } with the advantages that:
The I–V curve can have voltagecontrolled ("N" type) or currentcontrolled ("S" type) negative resistance, depending on whether the feedback loop is connected in "shunt" or "series".^{ [26] }
Negative reactances (below) can also be created, so feedback circuits can be used to create "active" linear circuit elements, resistors, capacitors, and inductors, with negative values.^{ [37] }^{ [46] } They are widely used in active filters ^{ [42] }^{ [50] } because they can create transfer functions that cannot be realized with positive circuit elements.^{ [111] } Examples of circuits with this type of negative resistance are the negative impedance converter (NIC), gyrator, Deboo integrator,^{ [50] }^{ [112] } frequency dependent negative resistance (FDNR),^{ [46] } and generalized immittance converter (GIC).^{ [42] }^{ [98] }^{ [113] }
If an LC circuit is connected across the input of a positive feedback amplifier like that above, the negative differential input resistance can cancel the positive loss resistance inherent in the tuned circuit.^{ [114] } If this will create in effect a tuned circuit with zero AC resistance (poles on the jω axis).^{ [39] }^{ [107] } Spontaneous oscillation will be excited in the tuned circuit at its resonant frequency, sustained by the power from the amplifier. This is how feedback oscillators such as Hartley or Colpitts oscillators work.^{ [41] }^{ [115] } This negative resistance model is an alternate way of analyzing feedback oscillator operation.^{ [14] }^{ [36] }^{ [104] }^{ [108] }^{ [116] }^{ [117] }^{ [118] }All linear oscillator circuits have negative resistance^{ [36] }^{ [84] }^{ [104] }^{ [117] } although in most feedback oscillators the tuned circuit is an integral part of the feedback network, so the circuit does not have negative resistance at all frequencies but only near the oscillation frequency.^{ [119] }
A tuned circuit connected to a negative resistance which cancels some but not all of its parasitic loss resistance (so ) will not oscillate, but the negative resistance will decrease the damping in the circuit (moving its poles toward the jω axis), increasing its Q factor so it has a narrower bandwidth and more selectivity.^{ [114] }^{ [120] }^{ [121] }^{ [122] } Q enhancement, also called regeneration, was first used in the regenerative radio receiver invented by Edwin Armstrong in 1912^{ [107] }^{ [121] } and later in "Q multipliers".^{ [123] } It is widely used in active filters.^{ [122] } For example, RF integrated circuits use integrated inductors to save space, consisting of a spiral conductor fabricated on chip. These have high losses and low Q, so to create high Q tuned circuits their Q is increased by applying negative resistance.^{ [120] }^{ [122] }
Circuits which exhibit chaotic behavior can be considered quasiperiodic or nonperiodic oscillators, and like all oscillators require a negative resistance in the circuit to provide power.^{ [124] } Chua's circuit, a simple nonlinear circuit widely used as the standard example of a chaotic system, requires a nonlinear active resistor component, sometimes called Chua's diode.^{ [124] } This is usually synthesized using a negative impedance converter circuit.^{ [124] }
A common example of an "active resistance" circuit is the negative impedance converter (NIC)^{ [45] }^{ [46] }^{ [115] }^{ [125] } shown in the diagram. The two resistors and the op amp constitute a negative feedback noninverting amplifier with gain of 2.^{ [115] } The output voltage of the opamp is
So if a voltage is applied to the input, the same voltage is applied "backwards" across , causing current to flow through it out of the input.^{ [46] } The current is
So the input impedance to the circuit is^{ [76] }
The circuit converts the impedance to its negative. If is a resistor of value , within the linear range of the op amp the input impedance acts like a linear "negative resistor" of value .^{ [46] } The input port of the circuit is connected into another circuit as if it was a component. An NIC can cancel undesired positive resistance in another circuit,^{ [126] } for example they were originally developed to cancel resistance in telephone cables, serving as repeaters.^{ [115] }
By replacing in the above circuit with a capacitor () or inductor (), negative capacitances and inductances can also be synthesized.^{ [37] }^{ [46] } A negative capacitance will have an I–V relation and an impedance of
where . Applying a positive current to a negative capacitance will cause it to discharge; its voltage will decrease. Similarly, a negative inductance will have an I–V characteristic and impedance of
A circuit having negative capacitance or inductance can be used to cancel unwanted positive capacitance or inductance in another circuit.^{ [46] } NIC circuits were used to cancel reactance on telephone cables.
There is also another way of looking at them. In a negative capacitance the current will be 180° opposite in phase to the current in a positive capacitance. Instead of leading the voltage by 90° it will lag the voltage by 90°, as in an inductor.^{ [46] } Therefore, a negative capacitance acts like an inductance in which the impedance has a reverse dependence on frequency ω; decreasing instead of increasing like a real inductance^{ [46] } Similarly a negative inductance acts like a capacitance that has an impedance which increases with frequency. Negative capacitances and inductances are "nonFoster" circuits which violate Foster's reactance theorem.^{ [127] } One application being researched is to create an active matching network which could match an antenna to a transmission line over a broad range of frequencies, rather than just a single frequency as with current networks.^{ [128] } This would allow the creation of small compact antennas that would have broad bandwidth,^{ [128] } exceeding the Chu–Harrington limit.
Negative differential resistance devices are widely used to make electronic oscillators.^{ [7] }^{ [43] }^{ [129] } In a negative resistance oscillator, a negative differential resistance device such as an IMPATT diode, Gunn diode, or microwave vacuum tube is connected across an electrical resonator such as an LC circuit, a quartz crystal, dielectric resonator or cavity resonator ^{ [117] } with a DC source to bias the device into its negative resistance region and provide power.^{ [130] }^{ [131] } A resonator such as an LC circuit is "almost" an oscillator; it can store oscillating electrical energy, but because all resonators have internal resistance or other losses, the oscillations are damped and decay to zero.^{ [21] }^{ [39] }^{ [115] } The negative resistance cancels the positive resistance of the resonator, creating in effect a lossless resonator, in which spontaneous continuous oscillations occur at the resonator's resonant frequency.^{ [21] }^{ [39] }
Negative resistance oscillators are mainly used at high frequencies in the microwave range or above, since feedback oscillators function poorly at these frequencies.^{ [14] }^{ [116] } Microwave diodes are used in low to mediumpower oscillators for applications such as radar speed guns, and local oscillators for satellite receivers. They are a widely used source of microwave energy, and virtually the only solidstate source of millimeter wave ^{ [132] } and terahertz energy^{ [129] } Negative resistance microwave vacuum tubes such as magnetrons produce higher power outputs,^{ [117] } in such applications as radar transmitters and microwave ovens. Lower frequency relaxation oscillators can be made with UJTs and gasdischarge lamps such as neon lamps.
The negative resistance oscillator model is not limited to oneport devices like diodes but can also be applied to feedback oscillator circuits with two port devices such as transistors and tubes.^{ [116] }^{ [117] }^{ [118] }^{ [133] } In addition, in modern high frequency oscillators, transistors are increasingly used as oneport negative resistance devices like diodes. At microwave frequencies, transistors with certain loads applied to one port can become unstable due to internal feedback and show negative resistance at the other port.^{ [37] }^{ [88] }^{ [116] } So high frequency transistor oscillators are designed by applying a reactive load to one port to give the transistor negative resistance, and connecting the other port across a resonator to make a negative resistance oscillator as described below.^{ [116] }^{ [118] }
The common Gunn diode oscillator (circuit diagrams)^{ [21] } illustrates how negative resistance oscillators work. The diode D has voltage controlled ("N" type) negative resistance and the voltage source biases it into its negative resistance region where its differential resistance is . The choke RFC prevents AC current from flowing through the bias source.^{ [21] } is the equivalent resistance due to damping and losses in the series tuned circuit , plus any load resistance. Analyzing the AC circuit with Kirchhoff's Voltage Law gives a differential equation for , the AC current^{ [21] }
Solving this equation gives a solution of the form^{ [21] }
This shows that the current through the circuit, , varies with time about the DC Q point, . When started from a nonzero initial current the current oscillates sinusoidally at the resonant frequency ω of the tuned circuit, with amplitude either constant, increasing, or decreasing exponentially, depending on the value of α. Whether the circuit can sustain steady oscillations depends on the balance between and , the positive and negative resistance in the circuit:^{ [21] }
Practical oscillators are designed in region (3) above, with net negative resistance, to get oscillations started.^{ [118] } A widely used rule of thumb is to make .^{ [17] }^{ [134] } When the power is turned on, electrical noise in the circuit provides a signal to start spontaneous oscillations, which grow exponentially. However, the oscillations cannot grow forever; the nonlinearity of the diode eventually limits the amplitude.
At large amplitudes the circuit is nonlinear, so the linear analysis above does not strictly apply and differential resistance is undefined; but the circuit can be understood by considering to be the "average" resistance over the cycle. As the amplitude of the sine wave exceeds the width of the negative resistance region and the voltage swing extends into regions of the curve with positive differential resistance, the average negative differential resistance becomes smaller, and thus the total resistance and the damping becomes less negative and eventually turns positive. Therefore, the oscillations will stabilize at the amplitude at which the damping becomes zero, which is when .^{ [21] }
Gunn diodes have negative resistance in the range −5 to −25 ohms.^{ [135] } In oscillators where is close to ; just small enough to allow the oscillator to start, the voltage swing will be mostly limited to the linear portion of the I–V curve, the output waveform will be nearly sinusoidal and the frequency will be most stable. In circuits in which is far below , the swing extends further into the nonlinear part of the curve, the clipping distortion of the output sine wave is more severe,^{ [134] } and the frequency will be increasingly dependent on the supply voltage.
Negative resistance oscillator circuits can be divided into two types, which are used with the two types of negative differential resistance – voltage controlled (VCNR), and current controlled (CCNR)^{ [91] }^{ [103] }
Most oscillators are more complicated than the Gunn diode example, since both the active device and the load may have reactance (X) as well as resistance (R). Modern negative resistance oscillators are designed by a frequency domain technique due to K. Kurokawa.^{ [88] }^{ [118] }^{ [136] } The circuit diagram is imagined to be divided by a "reference plane" (red) which separates the negative resistance part, the active device, from the positive resistance part, the resonant circuit and output load (right).^{ [137] } The complex impedance of the negative resistance part depends on frequency ω but is also nonlinear, in general declining with the amplitude of the AC oscillation current I; while the resonator part is linear, depending only on frequency.^{ [88] }^{ [117] }^{ [137] } The circuit equation is so it will only oscillate (have nonzero I) at the frequency ω and amplitude I for which the total impedance is zero.^{ [88] } This means the magnitude of the negative and positive resistances must be equal, and the reactances must be conjugate ^{ [85] }^{ [117] }^{ [118] }^{ [137] }
For steadystate oscillation the equal sign applies. During startup the inequality applies, because the circuit must have excess negative resistance for oscillations to start.^{ [85] }^{ [88] }^{ [118] }
Alternately, the condition for oscillation can be expressed using the reflection coefficient.^{ [85] } The voltage waveform at the reference plane can be divided into a component V_{1} travelling toward the negative resistance device and a component V_{2} travelling in the opposite direction, toward the resonator part. The reflection coefficient of the active device is greater than one, while that of the resonator part is less than one. During operation the waves are reflected back and forth in a round trip so the circuit will oscillate only if^{ [85] }^{ [117] }^{ [137] }
As above, the equality gives the condition for steady oscillation, while the inequality is required during startup to provide excess negative resistance. The above conditions are analogous to the Barkhausen criterion for feedback oscillators; they are necessary but not sufficient,^{ [118] } so there are some circuits that satisfy the equations but do not oscillate. Kurokawa also derived more complicated sufficient conditions,^{ [136] } which are often used instead.^{ [88] }^{ [118] }
Negative differential resistance devices such as Gunn and IMPATT diodes are also used to make amplifiers, particularly at microwave frequencies, but not as commonly as oscillators.^{ [86] } Because negative resistance devices have only one port (two terminals), unlike twoport devices such as transistors, the outgoing amplified signal has to leave the device by the same terminals as the incoming signal enters it.^{ [12] }^{ [86] } Without some way of separating the two signals, a negative resistance amplifier is bilateral; it amplifies in both directions, so it suffers from sensitivity to load impedance and feedback problems.^{ [86] } To separate the input and output signals, many negative resistance amplifiers use nonreciprocal devices such as isolators and directional couplers.^{ [86] }
One widely used circuit is the reflection amplifier in which the separation is accomplished by a circulator .^{ [86] }^{ [138] }^{ [139] }^{ [140] } A circulator is a nonreciprocal solidstate component with three ports (connectors) which transfers a signal applied to one port to the next in only one direction, port 1 to port 2, 2 to 3, and 3 to 1. In the reflection amplifier diagram the input signal is applied to port 1, a biased VCNR negative resistance diode N is attached through a filter F to port 2, and the output circuit is attached to port 3. The input signal is passed from port 1 to the diode at port 2, but the outgoing "reflected" amplified signal from the diode is routed to port 3, so there is little coupling from output to input. The characteristic impedance of the input and output transmission lines, usually 50Ω, is matched to the port impedance of the circulator. The purpose of the filter F is to present the correct impedance to the diode to set the gain. At radio frequencies NR diodes are not pure resistive loads and have reactance, so a second purpose of the filter is to cancel the diode reactance with a conjugate reactance to prevent standing waves.^{ [140] }^{ [141] }
The filter has only reactive components and so does not absorb any power itself, so power is passed between the diode and the ports without loss. The input signal power to the diode is
The output power from the diode is
So the power gain of the amplifier is the square of the reflection coefficient^{ [138] }^{ [140] }^{ [141] }
is the negative resistance of the diode −r. Assuming the filter is matched to the diode so ^{ [140] } then the gain is
The VCNR reflection amplifier above is stable for .^{ [140] } while a CCNR amplifier is stable for . It can be seen that the reflection amplifier can have unlimited gain, approaching infinity as approaches the point of oscillation at .^{ [140] } This is a characteristic of all NR amplifiers,^{ [139] } contrasting with the behavior of twoport amplifiers, which generally have limited gain but are often unconditionally stable. In practice the gain is limited by the backward "leakage" coupling between circulator ports.
Masers and parametric amplifiers are extremely low noise NR amplifiers that are also implemented as reflection amplifiers; they are used in applications like radio telescopes.^{ [141] }
Negative differential resistance devices are also used in switching circuits in which the device operates nonlinearly, changing abruptly from one state to another, with hysteresis.^{ [15] } The advantage of using a negative resistance device is that a relaxation oscillator, flipflop or memory cell can be built with a single active device,^{ [81] } whereas the standard logic circuit for these functions, the EcclesJordan multivibrator, requires two active devices (transistors). Three switching circuits built with negative resistances are
Some instances of neurons display regions of negative slope conductances (RNSC) in voltageclamp experiments.^{ [142] } The negative resistance here is implied were one to consider the neuron a typical Hodgkin–Huxley style circuit model.
Negative resistance was first recognized during investigations of electric arcs, which were used for lighting during the 19th century.^{ [143] } In 1881 Alfred Niaudet^{ [144] } had observed that the voltage across arc electrodes decreased temporarily as the arc current increased, but many researchers thought this was a secondary effect due to temperature.^{ [145] } The term "negative resistance" was applied by some to this effect, but the term was controversial because it was known that the resistance of a passive device could not be negative.^{ [68] }^{ [145] }^{ [146] } Beginning in 1895 Hertha Ayrton, extending her husband William's research with a series of meticulous experiments measuring the I–V curve of arcs, established that the curve had regions of negative slope, igniting controversy.^{ [65] }^{ [145] }^{ [147] } Frith and Rodgers in 1896^{ [145] }^{ [148] } with the support of the Ayrtons^{ [65] } introduced the concept of differential resistance, dv/di, and it was slowly accepted that arcs had negative differential resistance. In recognition of her research, Hertha Ayrton became the first woman voted for induction into the Institute of Electrical Engineers.^{ [147] }
George Francis FitzGerald first realized in 1892 that if the damping resistance in a resonant circuit could be made zero or negative, it would produce continuous oscillations.^{ [143] }^{ [149] } In the same year Elihu Thomson built a negative resistance oscillator by connecting an LC circuit to the electrodes of an arc,^{ [105] }^{ [150] } perhaps the first example of an electronic oscillator. William Duddell, a student of Ayrton at London Central Technical College, brought Thomson's arc oscillator to public attention.^{ [105] }^{ [143] }^{ [147] } Due to its negative resistance, the current through an arc was unstable, and arc lights would often produce hissing, humming, or even howling noises. In 1899, investigating this effect, Duddell connected an LC circuit across an arc and the negative resistance excited oscillations in the tuned circuit, producing a musical tone from the arc.^{ [105] }^{ [143] }^{ [147] } To demonstrate his invention Duddell wired several tuned circuits to an arc and played a tune on it.^{ [143] }^{ [147] } Duddell's "singing arc" oscillator was limited to audio frequencies.^{ [105] } However, in 1903 Danish engineers Valdemar Poulsen and P. O. Pederson increased the frequency into the radio range by operating the arc in a hydrogen atmosphere in a magnetic field,^{ [151] } inventing the Poulsen arc radio transmitter, which was widely used until the 1920s.^{ [105] }^{ [143] }
By the early 20th century, although the physical causes of negative resistance were not understood, engineers knew it could generate oscillations and had begun to apply it.^{ [143] } Heinrich Barkhausen in 1907 showed that oscillators must have negative resistance.^{ [84] } Ernst Ruhmer and Adolf Pieper discovered that mercury vapor lamps could produce oscillations, and by 1912 AT&T had used them to build amplifying repeaters for telephone lines.^{ [143] }
In 1918 Albert Hull at GE discovered that vacuum tubes could have negative resistance in parts of their operating ranges, due to a phenomenon called secondary emission.^{ [9] }^{ [36] }^{ [152] } In a vacuum tube when electrons strike the plate electrode they can knock additional electrons out of the surface into the tube. This represents a current away from the plate, reducing the plate current.^{ [9] } Under certain conditions increasing the plate voltage causes a decrease in plate current. By connecting an LC circuit to the tube Hull created an oscillator, the dynatron oscillator. Other negative resistance tube oscillators followed, such as the magnetron invented by Hull in 1920.^{ [60] }
The negative impedance converter originated from work by Marius Latour around 1920.^{ [153] }^{ [154] } He was also one of the first to report negative capacitance and inductance.^{ [153] } A decade later, vacuum tube NICs were developed as telephone line repeaters at Bell Labs by George Crisson and others,^{ [26] }^{ [127] } which made transcontinental telephone service possible.^{ [127] } Transistor NICs, pioneered by Linvill in 1953, initiated a great increase in interest in NICs and many new circuits and applications developed.^{ [125] }^{ [127] }
Negative differential resistance in semiconductors was observed around 1909 in the first pointcontact junction diodes, called cat's whisker detectors, by researchers such as William Henry Eccles ^{ [155] }^{ [156] } and G. W. Pickard.^{ [156] }^{ [157] } They noticed that when junctions were biased with a DC voltage to improve their sensitivity as radio detectors, they would sometimes break into spontaneous oscillations.^{ [157] } However the effect was not pursued.
The first person to exploit negative resistance diodes practically was Russian radio researcher Oleg Losev, who in 1922 discovered negative differential resistance in biased zincite (zinc oxide) point contact junctions.^{ [157] }^{ [158] }^{ [159] }^{ [160] }^{ [161] } He used these to build solidstate amplifiers, oscillators, and amplifying and regenerative radio receivers, 25 years before the invention of the transistor.^{ [155] }^{ [159] }^{ [161] }^{ [162] } Later he even built a superheterodyne receiver.^{ [161] } However his achievements were overlooked because of the success of vacuum tube technology. After ten years he abandoned research into this technology (dubbed "Crystodyne" by Hugo Gernsback),^{ [162] } and it was forgotten.^{ [161] }
The first widely used solidstate negative resistance device was the tunnel diode, invented in 1957 by Japanese physicist Leo Esaki.^{ [67] }^{ [163] } Because they have lower parasitic capacitance than vacuum tubes due to their small junction size, diodes can function at higher frequencies, and tunnel diode oscillators proved able to produce power at microwave frequencies, above the range of ordinary vacuum tube oscillators. Its invention set off a search for other negative resistance semiconductor devices for use as microwave oscillators,^{ [164] } resulting in the discovery of the IMPATT diode, Gunn diode, TRAPATT diode, and others. In 1969 Kurokawa derived conditions for stability in negative resistance circuits.^{ [136] } Currently negative differential resistance diode oscillators are the most widely used sources of microwave energy,^{ [80] } and many new negative resistance devices have been discovered in recent decades.^{ [67] }
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the power of a signal. It is a twoport electronic circuit that uses electric power from a power supply to increase the amplitude of a signal applied to its input terminals, producing a proportionally greater amplitude signal at its output. The amount of amplification provided by an amplifier is measured by its gain: the ratio of output voltage, current, or power to input. An amplifier is a circuit that has a power gain greater than one.
An operational amplifier is a DCcoupled highgain electronic voltage amplifier with a differential input and, usually, a singleended output. In this configuration, an opamp produces an output potential that is typically hundreds of thousands of times larger than the potential difference between its input terminals. Operational amplifiers had their origins in analog computers, where they were used to perform mathematical operations in many linear, nonlinear, and frequencydependent circuits.
The electrical resistance of an object is a measure of its opposition to the flow of electric current. The inverse quantity is electrical conductance, and is the ease with which an electric current passes. Electrical resistance shares some conceptual parallels with the notion of mechanical friction. The SI unit of electrical resistance is the ohm (Ω), while electrical conductance is measured in siemens (S).
A gyrator is a passive, linear, lossless, twoport electrical network element proposed in 1948 by Bernard D. H. Tellegen as a hypothetical fifth linear element after the resistor, capacitor, inductor and ideal transformer. Unlike the four conventional elements, the gyrator is nonreciprocal. Gyrators permit network realizations of two(ormore)port devices which cannot be realized with just the conventional four elements. In particular, gyrators make possible network realizations of isolators and circulators. Gyrators do not however change the range of oneport devices that can be realized. Although the gyrator was conceived as a fifth linear element, its adoption makes both the ideal transformer and either the capacitor or inductor redundant. Thus the number of necessary linear elements is in fact reduced to three. Circuits that function as gyrators can be built with transistors and opamps using feedback.
In electronics, a commonemitter amplifier is one of three basic singlestage bipolarjunctiontransistor (BJT) amplifier topologies, typically used as the voltage amplifier.
In electronics, a common collector amplifier is one of three basic singlestage bipolar junction transistor (BJT) amplifier topologies, typically used as a voltage buffer.
In electronics, the dynatron oscillator, invented in 1918 by Albert Hull at General Electric, is an obsolete vacuum tube electronic oscillator circuit which uses a negative resistance characteristic in early tetrode vacuum tubes, caused by a process called secondary emission. It was the first negative resistance vacuum tube oscillator. The dynatron oscillator circuit was used to a limited extent as beat frequency oscillators (BFOs), and local oscillators in vacuum tube radio receivers as well as in scientific and test equipment from the 1920s to the 1940s but became obsolete around World War 2 due to the variability of secondary emission in tubes.
A tunnel diode or Esaki diode is a type of semiconductor diode that has effectively "negative resistance" due to the quantum mechanical effect called tunneling. It was invented in August 1957 by Leo Esaki, Yuriko Kurose, and Takashi Suzuki when they were working at Tokyo Tsushin Kogyo, now known as Sony. In 1973, Esaki received the Nobel Prize in Physics, jointly with Brian Josephson, for discovering the electron tunneling effect used in these diodes. Robert Noyce independently devised the idea of a tunnel diode while working for William Shockley, but was discouraged from pursuing it. Tunnel diodes were first manufactured by Sony in 1957, followed by General Electric and other companies from about 1960, and are still made in low volume today.
The output impedance of an electrical network is the measure of the opposition to current flow (impedance), both static (resistance) and dynamic (reactance), into the load network being connected that is internal to the electrical source. The output impedance is a measure of the source's propensity to drop in voltage when the load draws current, the source network being the portion of the network that transmits and the load network being the portion of the network that consumes.
A network, in the context of electronics, is a collection of interconnected components. Network analysis is the process of finding the voltages across, and the currents through, all network components. There are many techniques for calculating these values. However, for the most part, the techniques assume linear components. Except where stated, the methods described in this article are applicable only to linear network analysis.
A current source is an electronic circuit that delivers or absorbs an electric current which is independent of the voltage across it.
A Colpitts oscillator, invented in 1918 by American engineer Edwin H. Colpitts, is one of a number of designs for LC oscillators, electronic oscillators that use a combination of inductors (L) and capacitors (C) to produce an oscillation at a certain frequency. The distinguishing feature of the Colpitts oscillator is that the feedback for the active device is taken from a voltage divider made of two capacitors in series across the inductor.
An IMPATT diode is a form of highpower semiconductor diode used in highfrequency microwave electronics devices. They have negative resistance and are used as oscillators and amplifiers at microwave frequencies. They operate at frequencies of about 3 and 100 GHz, or higher. The main advantage is their highpower capability; single IMPATT diodes can produce continuous microwave outputs of up to 3 kilowatts, and pulsed outputs of much higher power. These diodes are used in a variety of applications from lowpower radar systems to proximity alarms. A major drawback of IMPATT diodes is the high level of phase noise they generate. This results from the statistical nature of the avalanche process.
A Gunn diode, also known as a transferred electron device (TED), is a form of diode, a twoterminal semiconductor electronic component, with negative resistance, used in highfrequency electronics. It is based on the "Gunn effect" discovered in 1962 by physicist J. B. Gunn. Its largest use is in electronic oscillators to generate microwaves, in applications such as radar speed guns, microwave relay data link transmitters, and automatic door openers.
A current–voltage characteristic or I–V curve is a relationship, typically represented as a chart or graph, between the electric current through a circuit, device, or material, and the corresponding voltage, or potential difference across it.
This article illustrates some typical operational amplifier applications. A nonideal operational amplifier's equivalent circuit has a finite input impedance, a nonzero output impedance, and a finite gain. A real opamp has a number of nonideal features as shown in the diagram, but here a simplified schematic notation is used, many details such as device selection and power supply connections are not shown. Operational amplifiers are optimised for use with negative feedback, and this article discusses only negativefeedback applications. When positive feedback is required, a comparator is usually more appropriate. See Comparator applications for further information.
An active load or dynamic load is a component or a circuit that functions as a currentstable nonlinear resistor.
The operational transconductance amplifier (OTA) is an amplifier whose differential input voltage produces an output current. Thus, it is a voltage controlled current source (VCCS). There is usually an additional input for a current to control the amplifier's transconductance. The OTA is similar to a standard operational amplifier in that it has a high impedance differential input stage and that it may be used with negative feedback.
A clamper is an electronic circuit that fixes either the positive or the negative peak excursions of a signal to a defined value by shifting its DC value. The clamper does not restrict the peaktopeak excursion of the signal, it moves the whole signal up or down so as to place the peaks at the reference level. A diode clamp consists of a diode, which conducts electric current in only one direction and prevents the signal exceeding the reference value; and a capacitor, which provides a DC offset from the stored charge. The capacitor forms a time constant with the resistor load, which determines the range of frequencies over which the clamper will be effective.
The Miller theorem refers to the process of creating equivalent circuits. It asserts that a floating impedance element, supplied by two voltage sources connected in series, may be split into two grounded elements with corresponding impedances. There is also a dual Miller theorem with regards to impedance supplied by two current sources connected in parallel. The two versions are based on the two Kirchhoff's circuit laws.
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