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determine the gain of an amplifier

Discussion in 'Electronic Design' started by tiger66, Mar 18, 2007.

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  1. tiger66

    tiger66 Guest

    Hi All
    I am just wondering if I only have the .dc plot of an amplifier to get
    the Vin bias point (0.75V) and given the 1mV input signal that's added
    to the bias point. Is it possible to determine the gain of the
    amplifier?

    Thanks in advance for your help
     
  2. Tom Bruhns

    Tom Bruhns Guest


    Not with only that information. If you also have the circuit, it may
    be possible, depending on the accuracy you want and the details of the
    circuit.

    Cheers,
    Tom
     
  3. tiger66

    tiger66 Guest

    So, I have the bias point (0.75V) and the 1 mV input signal voltage
    added to the bias point (0.75+0.001)= 0.751V
    Then doesn't Vout/Vin = gain?

    Or am I complete wrong on this ?

    Thanks
     
  4. D from BC

    D from BC Guest

    No....
    In basic design practice, the bias point and signal gain are often
    taken as being independent especially for an amplifier that doesn't
    distort the signal about the bias point.
    However, there are circuits which vary the bias point to vary signal
    gain but I'll guess this is not the circuit in question.

    D from BC
     
  5. If you are amplifying a DC signal, yes. Otherwise, the component models
    used for determining the DC operating point of an amplifier (treating
    capacitors as open circuit, for example) don't give you the correct
    answer for an AC signal.
     
  6. Eeyore

    Eeyore Guest

    You mean Vout/Vin ?
    Yes. Measure the slope of the graph.

    Graham
     
  7. Eeyore

    Eeyore Guest

    In fact deltaVout /deltaVin = gain.

    Measure the slope of the graph at this point.

    Graham
     
  8. Tom Bruhns

    Tom Bruhns Guest

    ....
    Ah, OK, I didn't understand your original posting...I didn't realize
    that you had run (or proposed to run) the DC operating point for two
    values of input. In that case, the DC gain between two points (the
    input and output) should be clear, as you suggest. That doesn't apply
    at other frequencies, however, and will in general change if you put a
    load on the amplifier. It also assumes linearity; if the gain is
    high, that may not be valid.
    analysis. Why not run a .AC or .TRAN analysis? The .AC will easily
    give you gain as a function of frequency, assuming linearity about the
    determined DC operating point, and the .TRAN can give you gain at
    discrete frequencies, and show nonlinearities as well.

    Cheers,
    Tom
     
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