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Amplifier with feedback

  1. Mar 7, 2007 #1

    Suppose I have an amplifier with feedback, how would I measure the input and output resistances (using instruments)?

    If the input of the amplifier is a voltage source, the input resistance would simply be V(signal) / I(signal).
    Now if the output of the amplifier is a current, would I just need to hook a load resistance and measure the voltage across and then divide by the output current?

    Lastly, is it possible to measure beta (the feedback gain), or does that have to be done mathematically?

    Many Thanks
  2. jcsd
  3. Mar 7, 2007 #2
    Yes, that's how you do it. R = V/I for input and output. For amplifiers, it is more useful to measure input and output impedance Z = V/I, as there is that imaginary component.

    It is impractical to measure current at higher frequencies, therefore this method is seldom used. The preferred method is using a network analyzer, which measures S parameters very accurately. The advantage of that is S parameters can be converted to any type of parameters including the impedance.

    If you are working on simple low frequency BJT amp, then input and output resistances can be easily calculated.

    I'm not sure which beta are you referring to? Transistor current gain? or negative feedback as some books use beta? In either case both can be measured and calculated.
  4. Mar 8, 2007 #3
    Hi, thanks for your reply!

    By beta I was referring to the negative feedback, not the transistor current gain (beta or hfe).

    (The amplifiers in question will have to be tested with a maximum frequency of 10kHz)
  5. Mar 8, 2007 #4
    At 10 kHz, you probably don't have to use a network analyzer. Since the frequency is so low, you can get away with a DC measurement.
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