ESP1104 Introduction to Electronic Systems Khursheed
Tutorial 5
1) A more complete AC equivalent model for a BJT transistor includes a large shunt resistor, r0, at the transistor output terminals, indicating that it is not a perfect current source, as shown below
How would you expect inclusion of r0 to modify the voltage gain, input impedance and output impedance of the Committer Emitter Amplifier? Would they increase or decrease if r0 is taken into account?
2) Consider the common emitter BJT circuit shown below
1
r0
50 Ω
(a) Find the Q-point base current/voltage at the input side and then go on to calculate the collector current/voltage and voltage across the collector- emitter terminals, V
CE, at the Q-point conditions on the output side. Calculate the AC voltage gain of the amplifier, v0/vS. Assume all the coupling
capacitors are shorts at the signal frequency of 20 kHz. Assume the thermal voltage is 26 mV and the thermal coefficient (n) = 1 for the base-emitter junction.
C2
CE β=400
v0
2 (b) Select suitable values for the coupling capacitors (signal frequency of 20 kHz) and justify your choice
2 (c) Now consider the situation where the capacitor CE is omitted, modify the AC small signal equivalent circuit to incorporate this change and derive its effect on the voltage gain and input resistance of the circuit. Comment on how you would expect the value of CE to change the voltage gain and input resistance in general terms. How do you think this influence of CE on the amplifier’s voltage gain/input resistance might be useful?
2
