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March 01, 2012


george storm

This cat has been put to bed in many different ways - mostly simpler than ths.
I.e. - the function of R2 would appear to be very subtle.
Please explain (and the two-pole switch)?

Howard Edelman

What is the purpose of the lower pole of the switch?

John Dunn

When the switches are open, potentiometer R3 simply feeds the output voltage follower. In this case, R2 keeps a finite feedback loop in place around the first op-amp to keep that section from saturating to either rail. Such saturation could upset the second op-amp if they are part of the same chip.

When the switches are closed, the first op-amp is brought to very nearly unity gain since R2 gets shunted by the switch sections. Also, the upper switch's on-resistance get's "buried" in feedback which ensures that the equivalent circuit source voltage and source impedance that is presented to the second op-amp is not materially affected by either R4 or R3.

Howard Edelman

The first op amp has unity gain even without the second switch pole closed and the offset voltage due to input current can be better balanced (R1 = R2 assuming that the External Control voltage input impedance is much lower). You may wish to put a resistor in the feedback loop of the second op amp for the same reason.

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