Integrated injection logic
Encyclopedia
Integrated injection logic (IIL, I2L, or I2L) is a class of digital circuit
s built with multiple collector bipolar junction transistor
s (BJT). 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 circuit
s. 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. Sometimes also known as Merged Transistor Logic.
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
. 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).
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.
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). This logical relationship is known as De Morgan's Theorem.
, and was commonly used before the advent of CMOS
logic by companies such as Motorola
(now Freescale) and Texas Instruments
. In 1975, Sinclair Radionics introduced one of the first consumer-grade digital watches, the Black Watch
, which used I2L technology.
Generally, I2L gates were constructed with transistors with 1, 2 or 3 separate collectors. This fan-out of up to 3 allowed 3-input NAND
or NOR
gates to be constructed very simply.
Digital circuit
Digital electronics represent signals by discrete bands of analog levels, rather than by a continuous range. All levels within a band represent the same signal state...
s built with multiple collector bipolar junction transistor
Bipolar junction transistor
|- align = "center"| || PNP|- align = "center"| || NPNA bipolar transistor is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons...
s (BJT). When introduced it had speed comparable to TTL yet was almost as low power as CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
, making it ideal for use in VLSI
Very-large-scale integration
Very-large-scale integration is the process of creating integrated circuits by combining thousands of transistors into a single chip. VLSI began in the 1970s when complex semiconductor and communication technologies were being developed. The microprocessor is a VLSI device.The first semiconductor...
(and larger) integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
s. 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. Sometimes also known as Merged Transistor Logic.
Operation
Current source
A current source is an electrical or electronic device that delivers or absorbs electric current. A current source is the dual of a voltage source. The term constant-current sink is sometimes used for sources fed from a negative voltage supply...
. 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).
Like direct-coupled transistor logic
Direct-coupled transistor logic
A special type of resistor–transistor logic , a direct-coupled transistor logic or DCTL gate is one wherein the bases of the transistors are connected directly to inputs without any base resistors. Without the base resistors, DCTL gates are more economical and simpler to fabricate onto integrated...
, there is no resistor between the output (collector) of one NPN transistor and the input (base) of the following transistor.
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). This logical relationship is known as De Morgan's Theorem.
Usage
I2L is relatively simple to construct on an integrated circuitIntegrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
, and was commonly used before the advent of CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
logic by companies such as Motorola
Motorola
Motorola, Inc. was an American multinational telecommunications company based in Schaumburg, Illinois, which was eventually divided into two independent public companies, Motorola Mobility and Motorola Solutions on January 4, 2011, after losing $4.3 billion from 2007 to 2009...
(now Freescale) and Texas Instruments
Texas Instruments
Texas Instruments Inc. , widely known as TI, is an American company based in Dallas, Texas, United States, which develops and commercializes semiconductor and computer technology...
. In 1975, Sinclair Radionics introduced one of the first consumer-grade digital watches, the Black Watch
Black Watch (wristwatch)
The Black Watch is an electronic wristwatch launched in September 1975 by Sinclair Radionics. It cost £24.95 ready-built, but was also available for £17.95, as a kit....
, which used I2L technology.
Generally, I2L gates were constructed with transistors with 1, 2 or 3 separate collectors. This fan-out of up to 3 allowed 3-input NAND
NAND gate
The Negated AND, NO AND or NAND gate is the opposite of the digital AND gate, and behaves in a manner that corresponds to the opposite of AND gate, as shown in the truth table on the right. A LOW output results only if both the inputs to the gate are HIGH...
or NOR
NOR gate
The NOR gate is a digital logic gate that implements logical NOR - it behaves according to the truth table to the right. A HIGH output results if both the inputs to the gate are LOW . If one or both input is HIGH , a LOW output results. NOR is the result of the negation of the OR operator...
gates to be constructed very simply.