amplifier, load and source resistor

Discussion in 'General Electronics Chat' started by Xufyan, Jul 27, 2011.

  1. Xufyan

    Thread Starter Member

    Aug 3, 2010
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    Hey,
    when there is no Rs and Rl in an amplifier the voltage gain is very high however when we connect both these resistor the gain slightly reduces, i know why the gain reduces but how would i adjust these to resistors so that it doesn't effect the overall gain,
    i.e the gain without Rs and Rl must be equal to the gain with Rs and Rl

    is it possible ?
     
  2. Robert.Adams

    Active Member

    Feb 16, 2010
    112
    5
    Can you clarify what type of amplifier you're talking about? I think you're referring to the source resistance and the load resistance for some basic configuration.

    If so, the load resistance will always take the gain down when it is in place. The open source gain will always be higher than the loaded gain.

    For the source resistance, you could bypass it with a cap to increase gain.
     
  3. Xufyan

    Thread Starter Member

    Aug 3, 2010
    114
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    yes i am referring to basic configuration,
    one of my friend says yes it is possible to use such values of Rs and Rl that doesn't effect the Gain at all but you have to find out how ,
    you said i should use a cap with the source resistors , but it will not keep the gain as it was without those resistors,

    Edit: i have read Boylestad book and it is not mentioned any where that it is possible to do so ,
    The Voltage Gain (no load) is always greatest,
     
    Last edited: Jul 27, 2011
  4. Robert.Adams

    Active Member

    Feb 16, 2010
    112
    5
    Yes, the source resistor will divide the gain by (1+gm*Rs). This comes from taking the small signal model of the transistor and putting it into the circuit. For more on this, try googling: Common Source Source Degeneration.

    From the similar small signal model (I think you use the hybrid-pi model but I don't remember what its called anymore), you can see how the load resistance will pull the output down.
     
  5. Xufyan

    Thread Starter Member

    Aug 3, 2010
    114
    0
    wat about BJTs ?

     
  6. Robert.Adams

    Active Member

    Feb 16, 2010
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  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Source resistance is not generally a discrete resistor that you can bypass with a cap. It is usually an internal property of the source, and is not accessible. For example, a common emitter amplifier has fairly high output resistance, and it is not possible to bypass that resistance. If a CE amp's output is the source for the amplifier in question, then you are stuck with the CE amp's output impedance, unless you add an intermediate stage such as an emitter follower.
     
  8. Robert.Adams

    Active Member

    Feb 16, 2010
    112
    5
    Yes, but the source resistance I was speaking of is that used for achieving more linear gain in CS amplifier configurations (back when I assumed the question was on FETs). I believe you are talking about a non-ideal voltage/current source which has an internal resistance.
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I see that after re-reading your posts. As you imply, bypassing the source degeneration resistor in a CS amp will raise the gain but will introduce a lot of even harmonic distortion.
     
  10. Xufyan

    Thread Starter Member

    Aug 3, 2010
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    i might got the answer,
    in order to increase gain the input impedance must be very very high and the output impedance should be very very low....
    is it so ?
     
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