Op Amp Question.

Discussion in 'Homework Help' started by coffwf04, May 11, 2008.

  1. coffwf04

    Thread Starter New Member

    May 11, 2008
    Hi, everyone

    I am completely stuck with this question and the assignment is due on Wednesday. I am studying Electrical Circuits at Curtin University as part of my Mechatronic Engineering course and need help with Op-Amps.

    Q1.The op Amp in the following circuit is supposed to be ideal. There is no initial energy stored in the capacitors.


    • Find the expression for the transfer function:
    H(s) = Vout(s)/Vin(s)

    • Let R1 = R2 = 1 kΩ, R3 = 500 kΩ, R4 = 270 kΩ and C1 = C2 = 316 pF, plot the amplitude of H(j2πf) with respect to the frequency f. What could we say about this circuit?

    • If Vin(t) = 100cos(1000*pi*t) mV; find and plot the expression of Vout.

    • ZL in the circuit is an AC motor, which it can be modulated by a resistor Rm = 10Ω in series with an inductor Lm = 500mH, (i.e. ZL = Rm + j w Lm ) .

    • Calculate the total power transferred to ZL.

    • Find the power developed by Vin. Comment the results obtained in last two questions.

    • The power supply of the op Amp is Vcc = 10 V. If the amplitude of Vin has been increased by a factor ten, replot Vout.

    • Using passive RLC circuits (in series and in parallel), find two equivalent circuits to H(s).

    I have looked everywhere for explanations on how to get the transfer function but cant find anything useful. Our lecture has not covered the topic yet and i cant afford the textbook. Could you please help me out.

    Thanks in advance,

  2. mik3

    Senior Member

    Feb 4, 2008
    Do you know how to derive the equation for the output for an inverting amplifier for example? If yes, its the same procedure but use the capacitors as an impedance Xc=1/(jωC). Finally, rearrange the equation to get Vout/Vin on one side and something else on the other side. Vout/Vin is the transfer function.
  3. Distort10n

    Active Member

    Dec 25, 2006
    Two words:

  4. hgmjr

    Retired Moderator

    Jan 28, 2005
    Two more words:

  5. coffwf04

    Thread Starter New Member

    May 11, 2008
    Hi thanks for your replies. I realise that it is a low pass Sallen-Key circuit but I still cant figure out the transfer function. I have been using nodal and substituting the equations so that i end up with Vout/Vin. This is the equation that i get:

    Vout/Vin = C1.(R3+R4).C2/(R4.C2.C1+R4.C1.R2+C1.R1.R4+R4.R2.R1-R1.C2.R3)

    Does this look right too you? If so, how do i convert this to the s domain?


  6. The Electrician

    AAC Fanatic!

    Oct 9, 2007
  7. gaisandreas

    New Member

    May 5, 2008
    Im not sure about this,but isnt H(s) like this:
    R1=R2=R , C1=C2=C
    H(s)= G/((sRC)^2+s(3-G)RC+1)
    (3-G)RC=1.414 (from butterworth table)
  8. cazz226

    New Member

    May 20, 2008
    Hey with regards to that question - there's two main ways I know of to derive the Transfer function, use one or both of KCL/KVL to get Vout in terms of Vin plus one node voltage/mesh current using the fact that Vn = Vp for our ideal opamp, then apply Cramer's rule. Borrow the textbook if you need help with this, then check the butterworth filter circuit and adjust for the sallen key.
    For the second way look up sallen key low pass filter and you'll find lots of results - of particular interest to you will be the Texas Instruments pdf about designing sallen key filters but you'll need to learn about gain and quality factor to have much hope of understanding it.

    These assignments are pretty terrible considering what they teach us - not much but they can be done, just requires a lot of individual learning. If you have any ideas on the other two questions let me know.