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Old 28th February 2012, 12:34 AM   #1
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Default Gain Formula for Opamp Circuit

So, this one is killing me. I see that the first stage is 1+2*R2/R7 (or 1+2*R4/R7). The second stage is 2, but is that from R9/(R14+R4)?
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Old 28th February 2012, 06:33 PM   #2
jcx is offline jcx  United States
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look up 3 op amp instrurmentaion amp - plenty of articles online Google

the 4 equal resistor positive and negative feedback around the output op amps are differential to single ended converters - the "one op amp" instrumentation amp

this version connects a 2nd diff to SE converter in opposite polarity for balanced output

so you get +3 and -3 x at the outputs for 6x diff gain
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Old 28th February 2012, 06:58 PM   #3
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I see. So the later half is operating in unity gain (R9/14) and the front end is 3x gain (1+2*R2/R7). However, the second diff converter is added in order to allow the input signal to be single ended and in the process, the result is 3 for the top and -3 for the bottom or 6 total differential gain. The lowest possible gain in this circuit is then 2, when R7 is open.

One more question. When R7 is open though, what's the common mode input voltage to any given opamp here, V1?
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Old 29th February 2012, 05:23 AM   #4
jcx is offline jcx  United States
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your sim source is floating, diff V only, the 100K provide a DC path for op amp input bias

if you had a source with common mode V content the min, max V (diff + cm) still has to fit in the op amp's input common mode input V range

with gain you have to check that the outputs of the 1st 2 op amps don't hit the rails too when the amplified diff V is added to the (1x gain) common mode
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Old 20th November 2012, 02:29 PM   #5
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One more question involving this circuit. What I'd really like is to be able to support -inf to 0 attenuation and say 0-6dB gain (or continuously variable in that region).

Can I change the 100K to a potentiometer to get attenuation and then add a pot and series resistor for R7 for the gain?

What things should I watch for when simulating this? I have figured out how to step through variables in LTSpice, I should be able to do this quite easily.
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