FET Amplifier HW Question

Discussion in 'Homework Help' started by yar, Nov 8, 2010.

  1. yar

    Thread Starter New Member

    Nov 8, 2010
    Hi All,

    Hopefully somebody is still up and reading this, HW is due tomorrow. :(

    The question is "for the circuit below in which g_fs=0.04 S and r_os=infinity calculate Amplification v_1/v_in and v_2/v_in assuming the capacitors are short circuits for AC.

    The circuit is shown in the image. The capacitor values and the input AC source were NOT given in the problem. I included them just so the circuit would simulate.


    I don't know what the heck to do. The book I'm using gives zero worked examples, and we covered this material very very quickly in class.

    Please, little help?
  2. yar

    Thread Starter New Member

    Nov 8, 2010
    I forgot to mention that the top Vout is V_1 and the bottom is V_2. The answers in the back of the book are A_1=.995 and A_2=-.995. How you arrive at those, however, I do not know...
  3. Terp

    Active Member

    Jun 6, 2008

    This is a small signal (AC) model type problem. Basic hints:

    1. Before you do any AC analysis you need to figure out the DC operating point, i.e. determining, without the "small signal" voltage signals, what are the DC node voltages and currents in the circuit. By the way, for that I think your DC gate voltage is missing. Without the gate voltage, this circuit will not work whatsoever.

    2. Only after you figure out these DC values will the AC stuff start to make sense. AC small signals in problems like these are basically "small signal" fluctuations around your DC offset values that you will have calculated in step 1 above.

    I will let you learn about the details from the website above, it's pretty nice little introductory stuff to get your feet wet. You need the the "Pi-Equivalent circuit" in that page for your AC analysis.
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009

    For this exercise you probably don't need to do all that work - as beneficial as it might be. By all means familiarize yourself with the linked material.

    All one really needs to do is plug the values into the small signal model.

    It's worth noting that the voltage at the source can be derived from the standard source follower small signal analysis. Any assumptions are explicitly stated in the original post. Since the source and drain (AC & DC) currents can be assumed to be the same for this case, then the drain AC voltage will be identical to the source AC voltage. Which is confirmed by the textbook answer.

    So Yar, I'd suggest you just Google "source follower analysis" to get the gist of the small signal analysis.

    With respect to the gate DC biasing thought, it might be apparent that the circuit is capable of operating as a 'self-bias' circuit. So the FET will probably bias correctly without the need for a separate gate bias input.