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I need some clarification regarding the open loop gain of Op Amp. I tried simulating this active BPF on LTspice. enter image description here

The Avol of the Op Amp U6 (UniversalOpamp) is 1meg and its GBW is 10meg. My understanding is that the gain at around DC should be 120 dB (10^6) and will reach unity gain (0 dB) at 10 MHz, but the bode plot that I got above shows that the open loop gain is 20 dB lower than what it should be, i.e. the gain starts at 100 dB instead of 120 dB and reaches unity gain at 1 MHz instead of 10 MHz.

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2 Answers 2

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That's because you have a DC feedback factor of 10. You're not measuring "open-loop" gain this way, you're measuring loop gain. $$ A\beta = 10^6\frac{1}{10}= 10^5 = 100dB $$

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The Avol of the Op Amp U6 (UniversalOpamp) is 1meg

My understanding is that the gain at around DC should be 120 dB (10^6)

This applies to an "open loop" configuration. You have a closed loop system so no, the DC gain will not be 120 dB in your case. The expected gain is calculated in Designalog's answer so I'll not repeat here.

Here's how you should measure the open loop response:

enter image description here

I kept the output low enough in case the output may saturate.

As you can see, the gain at DC and low frequencies is 120 dB, as expected. And the gain drops to 0 dB (1 V/V) at 10 MHz which is the GBW of the op amp model.

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  • \$\begingroup\$ Nice answer. You could also lose the feedback resistors, but keep it as a unity gain buffer and also measure the equivalent open-loop gain. \$\endgroup\$
    – Designalog
    Apr 3 at 8:26

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