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). 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. 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). If the bias current is shunted to ground (low logic level), the transistor turns off and the collector floats (high logic level). To understand how the inverter operates, it is necessary to understand the current flow. 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. Likewise, it is either a current sink (low logic level) or a high-z floating condition (high logic level). The output of an inverter is at the collector. 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). 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 heart of an I2L circuit is the common emitter open collector inverter. 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. 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. The gates can be constructed very simply with just a single layer of interconnect metal. However, additional outputs may be produced by adding more collectors to the inverter transistor. Thus the fan-out of an output used in such a way is one. Transistors are used for current sources on integrated circuits because they are much smaller than resistors.īecause the inverter is open collector, a wired AND operation may be performed by connecting an output from each of two or more gates together. A small voltage (around 1 volts) is supplied to the emitter of the current source transistor to control the current supplied to the inverter transistor. On a wafer, these two transistors are merged. The I2L inverter gate is constructed with a PNP common base current source transistor and an NPN common emitter open collector inverter transistor (i.e. Ī disadvantage of this logic family is that the gates draw power when not switching unlike with CMOS.Ĭonstruction I2L NOR gate with two inputs, two outputs, and one voltage input for the current injector transistor Berger who originally called it merged-transistor logic (MTL). I2L was developed in 1971 by Siegfried K. 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. The gates can be made smaller with this logic family than with CMOS because complementary transistors are not needed. 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. Integrated injection logic ( IIL, I 2L, or I2L) is a class of digital circuits built with multiple collector bipolar junction transistors (BJT). Two-BJT transistor digital logic Simplified schematic of an I2L inverter.
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