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Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
That schematic shows the basic "virtual earth" summing amplifier with multiple inputs.
However I'm not happy with it showing polarised coupling capacitors with a split-rail power supply. In practice, you would probably get away with it as long as your inputs and outputs didn't have DC on them. Both input and output have DC continuity to ground. The amplifier input is within an offset of ground, and the amplifier output is only offset * gain (which is small) from ground. Having said that, why would you need coupling (also called "DC blocking") capacitors in a split rail circuit like this at all?
As shown, the circuit will tolerate +ve input bias and -ve output bias only. You would need non-polarised caps to tolerate either polarity of input/output bias.
Often, the amplifier will be supplied from a single rail. Then you have an AC ground at mid rail, and polarised caps would be appropriate to shift the signal from around ground at the I/O to the mid-rail false ground.
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Multiple Analog inputs to one Analog output (for an analog synthesiser)
