Integrated injection logic

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Simplified schematic of an I2L inverter. I2L npn inversor 1.png
Simplified schematic of an I2L inverter.

Integrated injection logic (IIL, I2L, or I2L) is a class of digital circuits built with multiple collector bipolar junction transistors (BJT). [1] When introduced it had speed comparable to TTL yet was almost as low power as CMOS, making it ideal for use in VLSI (and larger) integrated circuits. Although the logic voltage levels are very close (High: 0.7V, Low: 0.2V), I2L has high noise immunity because it operates by current instead of voltage. I2L was developed in 1971 by Siegfried K. Wiedmann and Horst H. Berger who originally called it merged-transistor logic (MTL). [2]

Bipolar junction transistor transistor that uses both electron and hole charge carriers.In contrast,unipolar transistors such as field-effect transistors,only use one kind of charge carrier.For their operation,BJTs use 2 junctions between 2 semiconductor types,n-type and p-type

A bipolar junction transistor is a type of transistor that uses both electrons and holes as charge carriers.

Transistor–transistor logic (TTL) is a logic family built from bipolar junction transistors. Its name signifies that transistors perform both the logic function and the amplifying function ; it is the same naming convention used in resistor–transistor logic (RTL) and diode–transistor logic (DTL).

CMOS Technology for constructing integrated circuits

Complementary metal–oxide–semiconductor (CMOS), also known as complementary-symmetry metal–oxide–semiconductor (COS-MOS), is a type of MOSFET fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSFETs for logic functions. CMOS technology is used for constructing integrated circuits (ICs), including microprocessors, microcontrollers, memory chips, and other digital logic circuits. CMOS technology is also used for analog circuits such as image sensors, data converters, RF circuits, and highly integrated transceivers for many types of communication.



I2L NOR gate with two inputs, two outputs, and one voltage input for the current injector transistor I2L NOR Gate.svg
I2L NOR gate with two inputs, two outputs, and one voltage input for the current injector transistor

The I2L inverter gate is constructed with a PNP common base current source transistor and an NPN common emitter open collector inverter transistor. On a wafer, these two transistors are merged together. A small voltage (around 0.6 volts) is supplied to the emitter of the current source transistor to control the current supplied to the inverter transistor. Transistors are used for current sources on integrated circuits because they are much smaller than resistors.

Because the inverter is open collector, a wired AND operation may be performed by connecting an output from each of two or more gates together. Thus the fan-out of an output used in such a way is one. However, additional outputs may be produced by adding more collectors to the inverter transistor. The gates can be constructed very simply with just a single layer of interconnect metal.

In digital electronics, the fan-out of a logic gate output is the number of gate inputs it can drive.

In a discrete implementation of an I2L circuit, bipolar NPN transistors with multiple collectors can be replaced with multiple discrete 3-terminal NPN transistors connected in parallel having their bases connected together and their emitters connected likewise. Similarly, the merged PNP current injector transistor and the NPN inverter transistor can be implemented as separate discrete components. The current source transistor may be replaced with a resistor from the positive supply to the base of the inverter transistor, since discrete resistors are smaller and less expensive than discrete transistors.


IIL circuit IIL circuit.png
IIL circuit

The heart of an I2L circuit is the common emitter open collector inverter. Typically, an inverter consists of an NPN transistor with the emitter connected to ground and the base biased with a forward current from the current source. The input is supplied to the base as either a current sink (low logic level) or as a high-z floating condition (high logic level). The output of an inverter is at the collector. Likewise, it is either a current sink (low logic level) or a high-z floating condition (high logic level).

Current source electronic circuit that delivers or absorbs an electric current which is independent of the voltage across it; dual of a voltage source

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

Like direct-coupled transistor logic, there is no resistor between the output (collector) of one NPN transistor and the input (base) of the following transistor.

Direct-coupled transistor logic (DCTL) is similar to resistor–transistor logic (RTL) but the input transistor bases are connected directly to the collector outputs without any base resistors. Consequently, DCTL gates have fewer components, are more economical, and are simpler to fabricate onto integrated circuits than RTL gates. Unfortunately, DCTL has much smaller signal levels, has more susceptibility to ground noise, and requires matched transistor characteristics. The transistors are also heavily overdriven; that is a good feature in that it reduces the saturation voltage of the output transistors, but it also slows the circuit down due to a high stored charge in the base. Gate fan-out is limited due to "current hogging": if the transistor base-emitter voltages are not well matched, then the base-emitter junction of one transistor may conduct most of the input drive current at such a low base-emitter voltage that other input transistors fail to turn on.

To understand how the inverter operates, it is necessary to understand the current flow. If the bias current is shunted to ground (low logic level), the transistor turns off and the collector floats (high logic level). If the bias current is not shunted to ground because the input is high-z (high logic level), the bias current flows through the transistor to the emitter, switching on the transistor, and allowing the collector to sink current (low logic level). Because the output of the inverter can sink current but cannot source current, it is safe to connect the outputs of multiple inverters together to form a wired AND gate. When the outputs of two inverters are wired together, the result is a two-input NOR gate because the configuration (NOT A) AND (NOT B) is equivalent to NOT (A OR B) (per De Morgan's Theorem).

De Morgans laws pair of transformation rules that are both valid rules of inference

In propositional logic and boolean algebra, De Morgan's laws are a pair of transformation rules that are both valid rules of inference. They are named after Augustus De Morgan, a 19th-century British mathematician. The rules allow the expression of conjunctions and disjunctions purely in terms of each other via negation.

Due to internal parasitic capacitance in transistors, higher currents sourced into the base of the inverter transistor result in faster switching speeds, and since the voltage difference between high and low logic levels is smaller for I2L than other bipolar logic families (around 0.5 volts instead of around 3.3 or 5 volts), losses due to charging and discharging parasitic capacitances are minimized.


I2L is relatively simple to construct on an integrated circuit, and was commonly used before the advent of CMOS logic by companies such as Motorola (now Freescale) [3] and Texas Instruments (e.g. SBP0400). In 1975, Sinclair Radionics introduced one of the first consumer-grade digital watches, the Black Watch, which used I2L technology. [4] In the late 1970s, RCA used I²L in its CA3162 ADC 3 digit meter integrated circuit. In 1979, HP introduced a frequency measurement instrument based on a HP-made custom LSI chip that uses integrated injection logic (I2L) for low power consumption and high density, enabling portable battery operation, and also some emitter function logic (EFL) circuits where high speed is needed. [5]

Related Research Articles

Inverter (logic gate) logic gate implementing negation

In digital logic, an inverter or NOT gate is a logic gate which implements logical negation. The truth table is shown on the right.

Darlington transistor compound structure consisting of two bipolar transistors

In electronics, a multi-transistor configuration called darlington pair, or the Darlington configuration is a compound structure of a particular design made by two bipolar transistors connected in such a way that the current amplified by the first transistor is amplified further by the second one. This configuration gives a much higher current gain than each transistor taken separately.

Emitter-coupled logic high-speed integrated circuit bipolar transistor logic family

In electronics, emitter-coupled logic (ECL) is a high-speed integrated circuit bipolar transistor logic family. ECL uses an overdriven BJT differential amplifier with single-ended input and limited emitter current to avoid the saturated region of operation and its slow turn-off behavior. As the current is steered between two legs of an emitter-coupled pair, ECL is sometimes called current-steering logic (CSL), current-mode logic (CML) or current-switch emitter-follower (CSEF) logic.

Common base

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.

Differential amplifier electronic amplifier, a circuit component

A differential amplifier is a type of electronic amplifier that amplifies the difference between two input voltages but suppresses any voltage common to the two inputs. It is an analog circuit with two inputs and and one output in which the output is ideally proportional to the difference between the two voltages

Schmitt trigger

In electronics, a Schmitt trigger is a comparator circuit with hysteresis implemented by applying positive feedback to the noninverting input of a comparator or differential amplifier. It is an active circuit which converts an analog input signal to a digital output signal. The circuit is named a "trigger" because the output retains its value until the input changes sufficiently to trigger a change. In the non-inverting configuration, when the input is higher than a chosen threshold, the output is high. When the input is below a different (lower) chosen threshold the output is low, and when the input is between the two levels the output retains its value. This dual threshold action is called hysteresis and implies that the Schmitt trigger possesses memory and can act as a bistable multivibrator. There is a close relation between the two kinds of circuits: a Schmitt trigger can be converted into a latch and a latch can be converted into a Schmitt trigger.

Resistor–transistor logic (RTL) is a class of digital circuits built using resistors as the input network and bipolar junction transistors (BJTs) as switching devices. RTL is the earliest class of transistorized digital logic circuit used; other classes include diode–transistor logic (DTL) and transistor–transistor logic (TTL). RTL circuits were first constructed with discrete components, but in 1961 it became the first digital logic family to be produced as a monolithic integrated circuit. RTL integrated circuits were used in the Apollo Guidance Computer, whose design was begun in 1961 and which first flew in 1966.

Diode–transistor logic (DTL) is a class of digital circuits that is the direct ancestor of transistor–transistor logic. It is called so because the logic gating function is performed by a diode network and the amplifying function is performed by a transistor.

Buffer amplifier electronic amplifier, a circuit component

A buffer amplifier is one that provides electrical impedance transformation from one circuit to another, with the aim of preventing the signal source from being affected by whatever currents that the load may be produced with. The signal is 'buffered from' load currents. Two main types of buffer exist: the voltage buffer and the current buffer.

Common emitter

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

Common collector

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.

Transconductance, also infrequently called mutual conductance, is the electrical characteristic relating the current through the output of a device to the voltage across the input of a device. Conductance is the reciprocal of resistance.

In computer engineering, a logic family may refer to one of two related concepts. A logic family of monolithic digital integrated circuit devices is a group of electronic logic gates constructed using one of several different designs, usually with compatible logic levels and power supply characteristics within a family. Many logic families were produced as individual components, each containing one or a few related basic logical functions, which could be used as "building-blocks" to create systems or as so-called "glue" to interconnect more complex integrated circuits. A "logic family" may also refer to a set of techniques used to implement logic within VLSI integrated circuits such as central processors, memories, or other complex functions. Some such logic families use static techniques to minimize design complexity. Other such logic families, such as domino logic, use clocked dynamic techniques to minimize size, power consumption and delay.

IC power-supply pin

Almost all integrated circuits (ICs) have at least two pins that connect to the power rails of the circuit in which they are installed. These are known as the power-supply pins. However, the labeling of the pins varies by IC family and manufacturer.

Diode logic constructs Boolean logic gates from diodes acting

Diode logic (DL), or diode-resistor logic (DRL), is the construction of Boolean logic gates from diodes. Diode logic was used extensively in the construction of early computers, where semiconductor diodes could replace bulky and costly active vacuum tube elements. The most common use for diode logic is in diode–transistor logic (DTL) integrated circuits that, in addition to diodes, include inverter logic for power gain and signal restoration.

Open collector

An open collector is a common type of output found on many integrated circuits (IC), which behaves like a switch that is either connected to ground or disconnected.

A Wilson current mirror is a three-terminal circuit that accepts an input current at the input terminal and provides a "mirrored" current source or sink output at the output terminal. The mirrored current is a precise copy of the input current. It may be used as a Wilson current source by applying a constant bias current to the input branch as in Fig. 2. The circuit is named after George R. Wilson, an integrated circuit design engineer who worked for Tektronix. Wilson devised this configuration in 1967 when he and Barrie Gilbert challenged each other to find an improved current mirror overnight that would use only three transistors. Wilson won the challenge.


  1. Hart, K.; Slob, A. (Oct 1972). "Integrated Injection Logic: A New Approach to LSI". IEEE Journal of Solid-State Circuits. 7 (5): 346–351. doi:10.1109/jssc.1972.1052891.
  2. Siegfried K. Wiedmann, Horst H. Berger (1972). "Merged-transistor logic (MTL)-a low-cost bipolar logic concept". IEEE Journal of Solid-State Circuits. 7 (5): 340–346. doi:10.1109/JSSC.1972.1052890.
  3. Jarrett, Robert. "A monolithic speed-control micro-system for automotive applications". IEEE.ORG. IEEE. Retrieved 21 June 2014.
  4. "Clive Sinclair's 1982 Practical Computing Interview" . Retrieved 21 June 2014.
  5. "HP memory project: Time, Frequency Standard & Counter"

Further reading