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Class A BJT Amplifiers
62 questions By Tony R. Kuphaldt
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Question 31 of 62
Predict how all transistor currents (IB, IC, and IE) and the output voltage signal will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults):
- Capacitor Cout fails open:
- Solder bridge (short) past resistor R1:
- Resistor R1 fails open:
- Resistor RC fails open:
- Resistor RE fails open:
- Capacitor Cbypass fails shorted:
For each of these conditions, explain why the resulting effects will occur.
Reveal answer- Capacitor Cout fails open: Transistor currents unaffected, no output signal.
- Solder bridge (short) past resistor R1: Transistor saturates (large increase in all currents), no output signal.
- Resistor R1 fails open: All transistor currents fall to zero (transistor in complete cutoff mode), no output signal.
- Resistor RC fails open: Transistor base current will decrease, zero collector current, greatly decreased emitter current, no output signal.
- Resistor RE fails open: All transistor currents fall to zero (transistor in complete cutoff mode), no output signal.
- Capacitor Cbypass fails shorted: All transistor currents fall to zero (transistor in complete cutoff mode), no output signal.
Notes:The purpose of this question is to approach the domain of circuit troubleshooting from a perspective of knowing what the fault is, rather than only knowing what the symptoms are. Although this is not necessarily a realistic perspective, it helps students build the foundational knowledge necessary to diagnose a faulted circuit from empirical data. Questions such as this should be followed (eventually) by other questions asking students to identify likely faults based on measurements.
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Question 32 of 62
The voltage gain of this amplifier circuit, unlike other amplifier configurations, is completely independent of the load resistor value:
No matter what the resistance of the load, the amplifier’s voltage gain remains the same. Explain why this is so.
Reveal answerI’ll let you determine the answer to this question on your own!
Notes:I have found it helpful to approach this amplifier circuit from the perspective of a constant DC input voltage, qualitatively analyzing the voltage across the load resistor as it increases from a power-up condition. Students will see that the transistor goes into cutoff mode if the load voltage ever exceeds the total DC input voltage at the transistor’s base terminal, and will saturate if the load voltage ever falls below Vin - 0.7 volts.
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Question 33 of 62
Temperature changes are well known to affect transistor operation. For instance, if we were to apply a constant voltage between the base and emitter of a transistor and increase its temperature over time, the collector current would increase:
First, describe why the collector current changes, if the input voltage is held constant. Then, determine the relative degree of output voltage change (∆Vout) resulting from this thermal effect in the following two amplifier circuits:
What is different in the responses of these two circuits to temperature changes? Why does one circuit respond so much differently than the other?
If both these amplifier circuits had AC signal inputs, and were biased for Class A operation, what effect would an increase in temperature have on each of them? State your answer in terms of AC voltage gain and Q-point.
Reveal answerThe collector current of a warming transistor increases even with constant input voltage because its intrinsic emitter resistance decreases with increased charge carrier activity.
In the common-emitter amplifier circuit, the output voltage will change substantially with changes in transistor temperature. In the common-collector circuit, the output voltage will hardly change at all as transistor temperature changes.
When amplifying AC input signals, the common-emitter amplifier’s voltage gain will increase, while the common-collector amplifier’s voltage gain will remain at unity. Likewise, the common-emitter amplifier’s Q point will shift substantially, while the common-collector amplifier’s Q point will not.
Notes:Discuss the impact of emitter resistance change on base current, and then transfer this concept to the two amplifier circuits and note the effects. Students should immediately realize the effects of this change in the common-emitter circuit, but the effects in the common-collector circuit will be a bit more difficult to follow. Work with your students in the analysis of the common-collector circuit, noting the effect changes in load voltage (voltage across the resistor) have on base current.
This question also previews the concept of negative feedback, which will be essential to your students’ understanding of electronic circuits later in their studies.
