Feedback circuit modelling

Discussion in 'Homework Help' started by u-will-neva-no, Nov 28, 2011.

  1. u-will-neva-no

    Thread Starter Member

    Mar 22, 2011
    Hi everyone!

    So I have a laboratory session that requires me to do some circuit modelling in LT spice (or multisim if possible). The aim is to find the expected behaviour of a closed loop system. The issue is that the actual multistage amplifier is just presented as a black box.

    I have never simulated before so any help in general would be great!

  2. crutschow


    Mar 14, 2008
    So what are the characteristics of the black box and the loop? Can't help you if we don't know that.
    u-will-neva-no likes this.
  3. u-will-neva-no

    Thread Starter Member

    Mar 22, 2011
    Looking at the appendix of my lab report, the black box contains two stages that are both created from an operational amplifier and an RC circuit.

    It says that "...the measured open loop characteristic shows that the amplifier has a cross-over frequency and is therefore capable of oscillating at a frequency equal to the cross-over frequency." From that sentence, it deduces that the circuit is higher than 2nd order.

    A closed loop circuit diagram is shown but none that include a capacitor configuration. I also have a closed loop gain formula relating Rf, Ra where Rf is the feedback resistor and Ra is the resistor connected to the input side of the circuit. Ra was changed to a range of values and R f was fixed, so I know them values. The closed loop formula also has A symbol, which represents the gain. (Im not sure what that value should be).

    What I would like to know is:

    Does the above information help to simulate a circuit?
    I would just like a general discussion on how to simulate also. Are there any decent videos I could watch that may help?