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  • Author Author: Jan Cumps
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Discrete difference amplifier - AC analysis: gain and bandwidth

Jan Cumps

This is the AC gain and BW analysis of the difference amplifier module that  designed for a Direct Conversion Receiver.

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I'm searching for these points:

  • gain at 1 kHz (approx. mid of bandwidth)
  • -3 dB frequencies

I use Vrms values in the post - common for audio circuits. When measuring one of the differential channels, I tie the other to ground.

image

Specs:

  • Gain: ~10 dB
  • Bandwidth (-3 dB): ~30 Hz to 40 kHz (approximate)

Measurements:

image

IN1: gain (1 kHz): 9.2 dB, bw (-3 dB): 30 Hz - 80 kHz

IN2: gain (1 kHz): 9.3 dB, bw (-3 dB): 30 Hz - 80 kHz

image

  published the KiCad project on GitHub, including simulation. You could load it into KiCad and check if the small signal simulation and my measurements match ...

For all blog posts in this series, click here

spreadsheet: differenceamp.zip

  • in reply to jc2048

    In LTSpice, with NPX BC547C model:

    image

    ... and the zoom:

    image

  • in reply to Jan Cumps

    I'd do it like this with Tina-TI. Here I've asked for a DC Transfer Analysis where it sweeps a generator between two voltages and plots the result. I imagine you'll be able to do something equivalent, because it's almost certain to be fundamental to the spice simulator.

    image

    The graph then shows the compliance of the source. Not very useful below 2V, but fine above, with a reasonably flat top.

    Below 0.5V the current goes negative, because there's a path back through the base-collector diode from the potential divider, but you can ignore that.

    This is with the vertical scale magnified so that you can see the slope better.

    image

    Keep in mind that that's coming from simplified transistor models in a simulator - so the real slope may be different, and may not even be a precise straight line like that.

    1.  IN1
    2.  out
    3.  Q4 collector

    conditions:

    • V+: 9V
    • IN1: sinus 1 kHz, 100 mVrms 
    • IN2: ground

    image

  • in reply to jc2048

    First attempt to simulate the current source / sink:

    image

  • in reply to shabaz

    "...since there are two inputs and I'd have to keep changing the signals..."

    Something that's handy for these kind of situations are the controlled sources in spice. Here I've got a voltage-controlled voltage source on the second input controlled by the generator on the first. If I set the controlled source gain to -1, then the second input is simply the inverse of whatever the generator is set to, and if I set the gain to 1, both inputs are the same and the second tracks the generator.

    image