Active load

Last updated April 22, 2023

An active load or dynamic load is a component or a circuit that functions as a current-stable nonlinear resistor.

Circuit design

In circuit design, an active load is a circuit component made up of active devices, such as transistors, intended to present a high small-signal impedance yet not requiring a large DC voltage drop, as would occur if a large resistor were used instead. Such large AC load impedances may be desirable, for example, to increase the AC gain of some types of amplifier. Most commonly the active load is the output part of a current mirror ^[1] and is represented in an idealized manner as a current source. Usually, it is only a constant-current resistor that is a part of the whole current source including a constant voltage source as well (the power supply V_CC on the figures below).

Common base example

In Figure 1 the load is a resistor, and the current through the resistor is determined by Ohm's law as:

I_{C}={\frac {V_{CC}-V_{\text{out}}}{R_{C}}}

.

As a consequence of this relation, the voltage drop across the resistor is tied to the current at the Q-point. If the bias current is fixed for some performance reason, any increase in load resistance automatically leads to a lower voltage for V_out. which in turn lowers the voltage drop V_CB between collector and base, limiting the signal swing at the amplifier output (if the output swing is larger than V_CB, the transistor is driven out of active mode during part of the signal cycle).

In contrast, using the active load of Figure 2, the AC impedance of the ideal current source is infinite regardless of the voltage drop V_CC − V_out, which allows even a large value of V_CB. and consequently a large output signal swing.

Differential amplifiers

Active loads are frequently used in op-amp differential input stages, in order to enormously increase the gain.

Practical limitations

In practice the ideal current source is replaced by a current mirror, which is less ideal in two ways. First, its AC resistance is large, but not infinite. Second, the mirror requires a small voltage drop to maintain operation (to keep the output transistors of the mirror in active mode). As a result, the current mirror does limit the allowable output voltage swing, but this limitation is much less than for a resistor, and also does not depend upon the choice of bias current, leaving more flexibility than a resistor in designing the circuit.

Test equipment

In the area of electronic test equipment, an active load is used for automatic testing of power supplies and other sources of electrical power to ensure that their output voltage and current are within their specifications over a range of load conditions, from no load to maximum load.

One approach to test loads uses a set of resistors of different values, and manual intervention. In contrast, an active load presents to the source a resistance value varied by electronic control, either by an analogue adjusting device such as a multi-turn potentiometer or, in automated test setups, by a digital computer. The load resistance can often be varied rapidly in order to test the power supply's transient response.

Just like a resistor, an active load converts the power supply's electrical energy to heat. The heat-dissipating devices (usually transistors) in an active load therefore have to be designed to withstand the resulting temperature rise, and are usually cooled by means of heatsinks.

For added convenience, active loads often include circuitry to measure the current and voltage delivered to the inputs, and may display these measurements on numeric readouts.

Related Research Articles

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In electronics, a common-base amplifier is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a current buffer or voltage amplifier.

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In electronics, a common collector amplifier is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a voltage buffer.

A current mirror is a circuit designed to copy a current through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading. The current being "copied" can be, and sometimes is, a varying signal current. Conceptually, an ideal current mirror is simply an ideal inverting current amplifier that reverses the current direction as well. Or it can consist of a current-controlled current source (CCCS). The current mirror is used to provide bias currents and active loads to circuits. It can also be used to model a more realistic current source.

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 current source is an electronic circuit that delivers or absorbs an electric current which is independent of the voltage across it.

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In the field of electronics, a technique where part of the output of a system is used at startup can be described as bootstrapping.

In electronics, biasing is the setting of DC operating conditions of an active device in an amplifier. Many electronic devices, such as diodes, transistors and vacuum tubes, whose function is processing time-varying (AC) signals, also require a steady (DC) current or voltage at their terminals to operate correctly. This current or voltage is called bias. The AC signal applied to them is superposed on this DC bias current or voltage.

Bipolar transistors must be properly biased to operate correctly. In circuits made with individual devices, biasing networks consisting of resistors are commonly employed. Much more elaborate biasing arrangements are used in integrated circuits, for example, bandgap voltage references and current mirrors. The voltage divider configuration achieves the correct voltages by the use of resistors in certain patterns. By selecting the proper resistor values, stable current levels can be achieved that vary only little over temperature and with transistor properties such as β.

A nullor is a theoretical two-port network consisting of a nullator at its input and a norator at its output. Nullors represent an ideal amplifier, having infinite current, voltage, transconductance and transimpedance gain. Its transmission parameters are all zero, that is, its input–output behavior is summarized with the matrix equation

References

↑ Richard C. Jaeger, Travis N. Blalock (2004). Microelectronic Circuit Design (Second ed.). New York: McGraw-Hill Professional. p. 1228. ISBN 0-07-250503-6.

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[Jaeger-1] Richard C. Jaeger, Travis N. Blalock (2004). Microelectronic Circuit Design (Second ed.). New York: McGraw-Hill Professional. p. 1228. ISBN 0-07-250503-6.

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