Op Amps: Bandwidth question

Discussion in 'General Electronics Chat' started by kimoni, Dec 31, 2004.

  1. kimoni

    Thread Starter New Member

    Dec 3, 2004
    Is anyone any good with Op amps and their frequency response?
    I've got written in my notes that

    BW = GBP / A0

    GBP=gain bandwith product

    but what is A0?
    I've got "DC or low frequency gain" written down, but what does that refer to, the open loop or closed loop gain? I've seen it reffered to both and it's very confusing, surely this can't be?

    Thanks to anyone who can enlighten me
  2. Dave

    Retired Moderator

    Nov 17, 2003
    If we looked at an ideal op amp we would expect it to have infinite bandwidth, i.e. it would amplify all signals of frequency zero upto infinity with the same open-loop gain Av. However in real op amps this is not the case.

    Ao is the maximum gain of the amplifier in an open-loop configuration, or correctly as you state, the low frequency gain and the open-loop gain Av begins to fall off from the max Ao as the frequency increases, this characteristic is known as the frequency response of the amplifier.
  3. Brandon

    Senior Member

    Dec 14, 2004

    A0 is gain. Look at the formula.

    BW = GBP / A0

    Bandwidth = Gain Bandwidth Product / A0
    Bandwidth = Gain * Bandwidth / A0

    only way to get Bandwidth from Gain * Bandwidth is to divide by the Gain.

    Its better to look at expanding frequency response the same as a transformer.
    Say we have a certain GBP for a circuit that is designed. The GBP is not going to change (theoretically) for the circuit. It is a constant. Therefore as you increase your gain, your band width decrees and vice versa.

    Lets say your GBP for a circuit is 100,000 when the gain is 100. Therefore we have a bandwidth of 1,000 Hz. But from the design specs, that is too small. Lets say we are trying to amplify music. Musical range is ideally up to 22,000 Hz, but we'll settle at 20,000 Hz to make it easy math.

    GBP1 = GBP2 (GBPs must remain the same for both circuits)
    100 Gain1 * 1,000 Hz = X Gain2 * 20,000 Hz
    X Gain2 = 100,000 / 20,000 Hz
    Gain2 = 5

    This works both ways. Say we knew the new gain of the circuit was to be 10. That is the A0. Therefore the new bandwidth woudl be 100,000 / 10 = 10,000 Hz.

    The gain they are talking about is what ever gain was originally used for the circuit. If you used open loop to find the GBP then you use open loop for A0. More likely than not, the gain used in the circuit is closed loop because your trying expand the bandwidth of your circuit by sacrificing gain.