differential amplifier

kell · 2006-12-28T18:48:05+00:00

Hi guysI have to amplify a small differential voltage near the top rail.This voltage has some indeterminate impedance (like between 100k and 1Meg),represented by R1 and R2 on the diagram.I plan to use the dual jfet op amp TL082: __________ | + Vx - | |__________| | | R1 R2 | | __ | | /+ |--' | ,--< | | | \_-|--, | | | | +---------' | | | R3 | | | +-------------, | | | | | __ | | p- '-| / -|--' | channel ||---< | | mosfet ,-| \_+|-----' | +--v out | R4 | gndV out goes to a voltage-controlled oscillator, and Iwant Vout to get within 200 mV of the rails. In the application Vx isgoing to be very small (millivolts) and I'll use a lot of gain, sooffset has to be eliminated.Now the op amp doesn't have any nulling pins. I wouldn't want toinstall two pots for two amps anyway. So I was thinking aboutnulling it like this:R5=R3 __________ | + Vx - | |__________| | | R1 R2 | | __ | | /+ |--' | ,-- \ | R3 | / | | | \ | | gnd / | | | | +-------------+ | | | | | __ | | p- '-| / -|--' | ...

Dec 28, 2006 22 Replies

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kell 19 years ago

At startup, the plus end of Vx will be right on the rail. Running, there will be at least a few tens of millivolts headroom. I may run the op amps off a charge pump. That will get rid of trying to squeeze in a voltage follower against the upper rail.

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Jim Thompson 19 years ago

How about a "flying capacitor" to transfer Vx to some more usable operating point?

...Jim Thompson

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kell 19 years ago

I could do the switching using an IR2153 to drive four nmos.

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