436 DIGITAL CIRCUITS
Figure 9.3.5CMOS gates with
two inputs. (a) NAND gate.
(b)NOR gate.allowed values ofvin. The reader is encouraged to reason out these statements. Since the gate
conducts well in either direction when the switch is on, the circuit is known to bereciprocal,in
which case the labels “input” and “output” can be interchanged.
In addition to TTL, NMOS, and CMOS, several other logic families have been developed
to suit various purposes. Emitter-coupled logic (ECL) is a high-speed bipolar technology with
high power consumption. Transistors operate in the active mode and do not saturate in ECL;
propagation delays per gate are as short as 1 ns. Besides high power consumption, the logic swing
(which is the difference between high and low voltage levels) is small, being on the order of 1 V,
which makes the ECL vulnerable to random noise voltages, thereby leading to errors.
The integrated-injection logic (IIL or I^2 L) is a compact, low-power bipolar technology
competitive with MOS technology for LSI. This technology offers packing densities ten times
larger than TTL: 100 to 200 gates per square millimeter as compared with probably 10 to 20 for
TTL. Power consumption and delay time have a kind of trade-off relationship in IIL technology.