Question 7.9: The characteristic curves shown in Fig. 7.37 relate to a tra...
The characteristic curves shown in Fig. 7.37 relate to a transistor operating in common-emitter mode. If the transistor is operated with I_B = 30 μA, a load resistor of 1.2 kΩ and an 18 V supply, determine the quiescent values of collector voltage and current ( V_{CQ} and I_{CQ} ). Also determine the peak–peak output voltage that would be produced by an input signal of 40 μA peak–peak.
First we need to construct the load line. The two ends of the load line will correspond to V_{CC} (18 V) on the collector-emitter voltage axis and (18V/1.2 kΩ or 15 mA) on the collector current axis. Next we locate the operating point (or quiescent point) from the point of intersection of the I_B = 30 μA characteristic and the load line. Having located the operating point we can read off the quiescent (no-signal) values of collector-emitter voltage ( V_{CQ} ) and collector current ( I_{CQ} ). Hence:
V_{CQ}=9.2 \ V and I_{CQ} = 7.3 mA
Next we can determine the maximum and minimum values of collector-emitter voltage by locating the appropriate intercept points on Fig. 7.37.
Note that the maximum and minimum values of base current will be (30 μA + 20 μA) on positive peaks of the signal and (30 μA − 20 μA) on negative peaks of the signal. The maximum and minimum values of V_{CE} are, respectively, 14.8 V and 3.3 V. Hence the output voltage swing will be (14.8 V − 3.3 V) or 11.5 V peak–peak.