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I would like to request some help in analyzing the op-amp circuit used by SparkFun here. I'm particularly interested in the pre-amplifier portion of this design.

The write-up included by SparkFun mentions an arithmetic gain of 100. When I simulate this circuit with LTSpice and provide a 10mV 8kHz AC sine wave signal as the input, the output I am getting is ~0.55V 8kHz sine wave. How can this be? I would expect an output of ~1V 8kHz sine wave.

A screenshot of my spice setup is below, I'm using standard part values from the built-in libraries on LTSpice:

LTSpice Simulation

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    \$\begingroup\$ The op amp you are using in your simulation is not the one used by Sparkfun. What happens when you use the correct op amp? \$\endgroup\$ – Elliot Alderson Feb 1 '19 at 2:15
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OP-07 does not work with 3.3V supply. Datasheet shows it can work at 6V supply though.

The SparkFun device datasheet also mentions it needing 3.5V or more. It uses a low voltage op-amp that is said to operate down to 2.7V (the op-amp itself, of course the surrounding circuitry must as well).

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  • \$\begingroup\$ Thank you! Supplying 6V yielded the gain as expected. In terms of doing circuit analysis, can you clarify how this configuration yields 100V/V? It looks like a non-inverting op-amp configuration but R1 does not connect to GND as one would expect in a non-inverting configuration. \$\endgroup\$ – athedcha Feb 6 '19 at 0:10
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A gain of 100 at 8 kHz is a gain-bandwidth product (GBW) of 800,000. The OP-07 has a typical GBW of 500,000, so a typical part will not be able to make the calculated output amplitude.

This is a common problem when using older or low-power amplifiers in audio-ish applications. The solution is to use an "audio opamp" an opamp with the extra gain and bandwidth needed for full-spectrum audio. The NE5523/34 also is a fairly old part, but designed from the ground up for low noise, high gain audio applications.

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  • \$\begingroup\$ Is this all that's happening? If I change the input frequency to 3kHz, I'm still only seeing ~0.7V 3kHz sine wave at the output. \$\endgroup\$ – athedcha Feb 1 '19 at 16:55

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