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Class A BJT Amplifiers
62 questions By Tony R. Kuphaldt
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Question 40 of 62
A student builds this common-emitter amplifier so they he may amplify the audio signals from a microphone to power a speaker:
Unfortunately, the results are considerably less than expected: although some sound does come out of the speaker, it is not enough to be considered a success. Another student inspects the design and cryptically mumbles something about “poor impedance matching,” leaving the first student somewhat confused.
Explain what impedance matching means in this context, where the mis-match might be in this circuit, and what might be done to correct it.
Reveal answerI won’t reveal all the answers here, but I will provide a visual hint:
Ideally, the impedance-matching transformer will have a turns ratio of approximately 30:1 to match the output impedance of the amplifier circuit with the impedance of the speaker.
Notes:Note to your students that the addition of a transformer is not the only viable option for solving this impedance mis-match problem. One could add another stage of transistor amplification (of the right type, of course).
One point not touched upon in the answer is a possible mis-match of impedances between the microphone and the amplifier input. Since the microphone impedance was not specified, one cannot tell whether there is an impedance mis-match or not.
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Question 41 of 62
Each of the following faults will cause this audio amplifier circuit to stop working. Determine what diagnostic voltage measurement(s) would positively identify each one of the faults.
- Microphone coil fails open:
- Capacitor C1 fails shorted:
- Resistor R1 fails open:
- Resistor R2 fails open:
- Capacitor C3 fails open:
- Transformer T1 primary winding fails open:
Reveal answer- Microphone coil fails open: No AC voltage at all across microphone terminals when sound is present.
- Capacitor C1 fails shorted: DC voltage present across microphone terminals.
- Resistor R1 fails open: Full DC supply voltage dropped across R1, no DC voltage dropped across R2 (could indicate a shorted R2 as well - no way to tell unless a resistance measurement is taken).
- Resistor R2 fails open: Increased DC voltage drop across R2, decreased DC voltage drop across R1, reasonable transistor DC voltages (VE 0.7 volts less than VB, VC as expected based on value of VE and R3, R4 values) indicate that Q1 is probably not the source of the trouble.
- Capacitor C3 fails open: Larger-than-normal AC voltage at collector terminal, with no AC voltage present across transformer primary winding.
- Transformer T1 primary winding fails open: Larger-than-normal AC voltage across transformer primary winding, with no AC voltage across transformer secondary winding.
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 42 of 62
Suppose this microphone amplifier circuit used to function fine, but now has stopped outputting any sound at all:
Initial diagnostic measurements show all quiescent (DC) voltages to be normal. From this data, where would you suspect the problem is, and where would you suspect the problem is not?
Reveal answerThe problem is not in any of the four resistors, or the transistor. The most likely components to suspect at this point would be the microphone, capacitors, transformer, and/or speaker.
Notes:Ask your students why the normal DC voltage measurements indicate healthy resistors and transistor. How can we quickly eliminate those components as being faulty based on simple DC voltage measurements?
