Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Output gain of 2-stage amp.

  1. Jun 9, 2013 #1


    User Avatar
    Gold Member

    Hello all. I am currently trying to find the gain of a 2 stage amp. According to some notes I have, it is:

    A = A1 x A2 x (rin2 / (rin2 + ro1))
    A1 = gain of first stage
    A2 = gain of second stage
    rin2 = input resistance of second stage
    ro1 = output resistance of first stage

    Let p = (rin2 / (rin2 + ro1)) so that A = A1 x A2 x p

    I understand conceptually why 'p' comes in. It is due to loading of the stages together (right?).
    But I don't understand how to get that factor ('p') mathematically. Please explain. Thank you.

    PS: If a schematic of the amplifier is needed please let me know.
  2. jcsd
  3. Jun 10, 2013 #2
    In electronic circuits, we always hooking the output of something (output of a amplifier, output of a voltage input source...) to the input of something else (load resistor, input stage of a next amplifier...).
    After we draw this in a simplified schematics we see our old friend the voltage divider.


    p = Voltage divider equation

    So the second stage input voltage is not equal to

    Vin2 = Vin x A1 because we have a voltage divider in the circuit, so the second stage input voltage is equal to.

    Vin2 = Vin x A1 x Rin/(Rout + Rin)
    Last edited: Jun 10, 2013
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook