BJT Amplifier Troubleshooting

Discrete Semiconductor Devices and Circuits

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  • Question 1

    As an instructor of electronics, I am called upon frequently to help students troubleshoot their malfunctioning lab circuits. When I approach a student’s self-built circuit to troubleshoot it, though, I often begin the process with a very different mindset than if I were troubleshooting a malfunctioning circuit on a real job site.

    Aside from different safety considerations and a very different work environment, what else do you think I might consider differently when approaching a student-built circuit? Specifically, how might the range of probable faults differ between a professionally-installed electronic system that malfunctions and a student’s lab project that malfunctions? What generalizations might you make about this difference in troubleshooting perspective, regarding the construction and operational history of the circuit in question?

    Reveal answer
  • Question 2

    Examine the following “component” stereo system closely:

    The CD player generates the audio signal to be amplified, while the equalizer/preamp modifies the tone of the signal to suit the listener’s preferences and the power amplifier provides adequate power to drive the speakers.

    Suppose this system has a problem: no sound at all coming out of either speaker. All components in the system are turned on, as indicated by power lights on the front panels. All control knobs seem to be set to their proper positions, as well. The CD player indicates the disk is being played, and that it is presently playing a song. Despite all these good indicators, though, no sound is heard from the speakers.

    Being prepared at all times to troubleshoot electronic systems, you have a digital multimeter close by which you may use to check for the presence of audio signals (set the meter to measure AC millivolts). All audio signal cables (including the speaker cables) may be unplugged to provide access for your meter’s test probes.

    At what point in the system would you begin testing for the presence of an audio signal? Explain why you chose that point, and describe your subsequent actions based on the results of that test.

    Reveal answer
  • Question 3

    Here are a few good steps to take prior to applying any specific troubleshooting strategies to a malfunctioning amplifier circuit:

    Measure the output signal with an oscilloscope.
    Determine if the amplifier is receiving a good input signal.
    Check to see that the amplifier is receiving good-quality power.

    Explain why taking these simple steps may save a lot of time in the troubleshooting process. For example, why bother checking the amplifier’s output signal if you already know it isn’t outputting what it’s supposed to? What, exactly, constitutes “good-quality” power for an amplifier circuit?

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  • Question 4

    The three-stage amplifier shown here has a problem. Despite being supplied with good, “clean” DC power and an adequate input signal to amplify, there is no output signal whatsoever:

    Explain how you would use the “divide and conquer” or “divide by two” strategy of troubleshooting to locate the amplification stage where the fault is. (This is where you divide the signal path into different sections, then test for good signal at points along that path so as to narrow the problem down to one-half of the circuit, then to one-quarter of the circuit, etc.)

    Show the lines of demarcation where you would divide the circuit into distinct sections, and identify input and output test points for each of those sections.

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  • Question 5

    In order to successfully troubleshoot any electronic circuit to the component level, one must have a good understanding of each component’s function within the context of that circuit. Transistor amplifiers are no exception to this rule. The following schematic shows a simple, two-stage audio amplifier circuit:

    Identify the role of the following components in this audio amplifier circuit:

    The 0.47 μF capacitor connected to the microphone
    The 220 k Ω and 27 k Ω resistor pair
    The 4.7 μF electrolytic capacitor connected across the 1.5 k Ω resistor
    The 33 μF electrolytic capacitor connected to the speaker
    The 47 μF electrolytic capacitor connected to the power supply rail

    Additionally, answer the following questions concerning the circuit’s design:

    What configuration is each stage (common-base, common-collector, common-emitter)?
    Why not just use one transistor stage to drive the speaker? Why is an additional stage necessary?
    What might happen if the 47 μF “decoupling” capacitor were not in the circuit?
    Why does the second stage of the amplifier not need its own voltage divider to set bias voltage as the first stage does?
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  • Question 6

    Often times, component failures in transistor circuits will cause significant shifting of DC (quiescent) parameters. This is a benefit for the troubleshooter, as it means many faults may be located simply by measuring DC voltages (with no signal input) and comparing those voltages against what is expected. The most difficult part, though, is determining what DC voltage levels to expect at various points in an amplifier circuit.

    Examine this two-stage audio amplifier circuit, and estimate the DC voltages at all the points marked by bold letters and arrows (A through G), with reference to ground. Assume that conducting PN junctions will drop 0.7 volts, that loading effects on the voltage divider are negligible, and that the transistor’s collector and emitter currents are virtually the same magnitude:








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  • Question 7

    Study this audio amplifier circuit closely:

    Then, determine whether the DC voltage at each test point (VTP1 through VTP6) with respect to ground will increase, decrease, or remain the same for each of the given fault conditions:

    Fault VTP1 VTP2 VTP3 VTP4 VTP5 VTP6

    R1 failed open Same Same

    R2 failed open Same Same

    R3 failed open Same Same

    R4 failed open Same Same

    R5 failed open Same Same

    Short between TP2 and ground Same Same

    C2 failed shorted Same Same

    Q1 collector failed open Same Same

    When analyzing component faults, consider only one fault at a time. That is, for each row in the table, you should analyze the circuit as though the only fault in it is the one listed in the far left column of that row.

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  • Question 8

    The likelihood that a given component will fail in the “open” mode is quite often not the same as the likelihood that it will fail “shorted.” Based on the research you do and your own personal experience with troubleshooting electronic circuits, determine whether the following components are more likely to fail open or fail shorted (this includes partial, or high-resistance, shorts):

    Bipolar transistors:

    I encourage you to research information on these devices’ failure modes, as well as glean from your own experiences building and troubleshooting electronic circuits.

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  • Question 9

    Suppose you were troubleshooting the following amplifier circuit, and found the output signal to be “clipped” on the negative peaks:

    If you knew that this amplifier was a new design, and might not have all its components properly sized, what type of problem would you suspect in the circuit? Please be as specific as possible.

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  • Question 10

    Suppose you were troubleshooting the following amplifier circuit, and found the output signal to be symmetrically “clipped” on both the positive and negative peaks:

    If you knew that this amplifier was a new design, and might not have all its components properly sized, what type of problem would you suspect in the circuit? Please be as specific as possible.

    Reveal answer
  • Question 11

    This class-B audio power amplifier circuit has a problem: its output is very distorted, resembling half of a sine wave when tested with an input signal from a function generator:

    List some of the possible faults in this system, based on the output signal shown by the oscilloscope. Also, determine which components, if any, are known to be good based on the same data:

    Possible faults in the system:
    Fault #1:
    Fault #2:
    Fault #3:
    Components known to be okay in the system:
    Component #1:
    Component #2:
    Component #3:
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