Electrical – Output impedance in a circuit with Operational Amplifier



How I can obtain output impedance of this circuit? I would like to know in the ideal case and the LM741 case.

Best Answer

It is difficult to get good information about output impedance on this ancient (and largely crappy) beast but I'd start here by looking at the equivalent circuit: -

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And what I would note are the two resistors inside the red circle I've drawn. If the 741 is operated open-loop these two resistors are the output impedance so then you have to work out what the output impedance is when you have negative feedback.

To do this you need to consider frequency and the open-loop bode plot of the 741: -

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At DC, the open-loop gain is really high (106 dB or 200,000) and this will mean that for a unity gain amplifier the output impedance is dragged from the 50 ohm open-loop value to something like 50/200,000 ohms i.e. 0.25 milli ohms.

At 10 kHz, the open-loop gain is about 40 dB or 100 and this means the closed-loop output impedance will be about 0.5 ohms. At 100 kHz, the closed loop output impedance will be about 5 ohms.

Remember this is a rough approximation. It's also worth noting that the Fairchild version of the 741 has a graph showing open loop output impedance as being about 75 ohms up to 100 kHz then it rises to about 300 ohms at 1 MHz.

I would like to know in the ideal case

The ideal case would be zero ohms but it all comes down to what you define as ideal.