9.1 TRANSISTOR SWITCHES 423
9.1 TRANSISTOR SWITCHES
The basic element of logic circuits is the transistor switch, a simplified model of which is shown
in Figure 9.1.1. The control signal is the input voltagevin, which must lie in either the “low” range
or the “high” range for a digital circuit. Whenvinis low, the switch may take either the open or
the closed position; whenvinis high, the switch takes the other position. Looking at Figure 9.1.1,
closing the switch makesvoutzero, while opening the switch yields an output nearVCC, assuming
that not much current flows through the output terminal. IfVCCis chosen to lie in the high range
andV=0 is inside the low range, then allowed values ofvincontrol the switch and give rise to
allowed values ofvout. A common special circuit in which a high input yields a low output, and
vice versa, is known as aninverter, which performs the complement operation.
A typical inverter circuit using a bipolar transistor is shown in Figure 9.1.2(a), and the
operation of the BJT switch is illustrated in Figure 9.1.2(b). The KVL around the collector–
emitter loop is given by
vCE=VCC−RCiC (9.1.1)
from which the load line may be drawn on the collector–emitter characteristic. The KVL around
the base–emitter loop is given by
vBE=Vi−RBiB (9.1.2)
and the corresponding load line may be drawn on the base–emitter characteristic.
If the input is at zero volts, that is logic 0, then the base current is zero, and the operating
point is at➀, as shown in Figure 9.1.2(b). The transistor is then said to becut off, or simplyoff,
when only a very small value of collector current (ICcutoff∼=ICEO) flows. Thus, in the cutoff state
the output voltage is given by
Vo=VCEcutoff∼=VCC−RCICEO∼=VCC=5V (i.e., logic 1 level) (9.1.3)
If the inputVichanges to+5 V (logic 1 level), the base current is given by
IBsat=
Vi−vBE
RB
(9.1.4)
wherevBE∼= 0 .7 V, which is thethreshold voltage VT. Supposing thatRBis chosen so as to drive
the transistor into saturation such thatIBsat= 50 μA, the operating point switches to point➁on
the collector–emitter characteristic, when the transistor is said to besaturated, or simplyon.In
this saturated stateVCEsat=Vsat, which is typically 0.2 to 0.3 V, depending oniB. The collector
current in saturation is given by
ICsat=
VCC−Vsat
RC
∼=VCC
RC
(9.1.5)
vout
vin
R
VCC Figure 9.1.1Model of transistor switch.
