ABSTRACT
A MOSFET is usually used such that a voltage at the gate terminal of the MOSFET controls a current between its source and drain. When the voltages at the gate terminals of MOSFETs that constitute a logic gate are regarded as input variables
x
,
y
, and so on, the voltage at the output terminal represents the output function
f
. Here,
x
= 1 means a high voltage and
x
= 0 a low voltage. For example, MOSFETs in the logic gate in CMOS shown in Fig. 15.1(a) are used in this manner and the output function
f
represents . But a MOSFET can be used such that an input variable
x
applied at the source or drain of the MOSFET is delivered to the other or not, depending on whether a voltage applied at the gate terminal of the MOSFET is high or low. For example, the n-channel MOSFET shown in Fig. 15.1(b) works in this manner and the MOSFET used in this manner is called a
transfer gate
or, more often, a
pass transistor
. When the control voltage
c
is high, the MOSFET becomes conductive and the input voltage
x
appears at the output terminal
f
(henceforth, let letters,
f, x,
and others represent terminals as well as voltages or signal values), no matter whether the voltage
x
is high or low. When the control voltage
c
is low, the MOSFET becomes non-conductive and the input voltage at
x
does not appear at the output terminal
f
. Pass transistors have been used for simplification of transistor circuits. Logic functions can be realized
with fewer MOSFETs than logic networks of logic gates where MOSFETs are used like those in Fig. 15.1(a). The circuit in Fig. 15.2, for example, realizes the even-parity function
. This circuit works in the following manner. When
x
and
y
are both low voltages, n-channel MOSFETs, 1 and 2, are nonconductive and consequently no current flows from the power supply
V
to the terminal
x
or
y
. Thus, the output voltage
f
is high because it is the same as the power supply voltage at
V
. When
x
is a low voltage and
y
is a high voltage, MOSFET 1 becomes conductive and 2 is non-conductive. Consequently, a current flows from the power supply to
x
because
x
is a low voltage. Thus, the output voltage at
f
is low. Continuing the analysis, we have the truth table shown in Fig. 15.3(a). Then we can derive the truth table for function in Fig. 15.3(b) by regarding low and high voltages as 0 and 1, respectively. A logic gate for this function in CMOS requires 8 MOSFETs, as shown in Fig. 15.4, whereas the circuit realized with pass transistors in Fig. 15.2 requires only three MOSFETs. Notice that inputs
x
and
y
are connected to the sources of MOSFETs 1 and 2, unlike MOSFET in ordinary logic gates. Signal
x
at the source of MOSFET 1 is either sent to the drain or not, according to whether or not its MOSFET gate has a high voltage.
