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  • Author Author: Jan Cumps
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Discrete difference amplifier - DC analysis

Jan Cumps

This is the DC analysis of the difference amplifier module that  designed for a Direct Conversion Receiver.

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Before going into the small signal AC analysis, it's interesting to see how the amplifier behaves in steady state (DC).
I measured all nodes on the amp, with the inputs shorted. The output was left open*.

Voltages (annotated in black on the schematic) were measured. transistor voltage bias (orange) and currents (brown) were calculated based on the voltages measured.

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While it doesn't show the dynamic behaviour, it shows how each of the submodules of this design is set up.

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

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*except for an oscilloscope probe.

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  • first signal measurements.

    I used a 100 mVpp, 1 kHz sinus. Output loaded with a 27K resistor.

    1. signal at in1, in2 to ground
    2. signal at in2, in1 to ground
    3. same signal at both

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    the amp does what it needs to do. Amplifies differences, reject the same signal

    AI says the gain is 7.96 dB.

    > what is the gain in dB if I input 100 mV peak-to-peak and I get 250 mV peak-to-peak out?

    image

  • in reply to Jan Cumps

    Glad it's working! Although I had simulated it, my stripboard prototype had not worked fully, I had some mistake in my connections but there were enough parts that it was hard to troubleshoot. Looking forward to assembling this in the intercom circuit, since it plays a key role there.

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