Op amp gain in db formula
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calico
diyAudio Member

Join Date: Jun 2003
Location: USA
Op amp gain in db formula

I've built several op amp microphone pre-amps through the years but lost my copy of the Jung Op Amp cookbook.

Anyway...
Can someone please give me the formula for the gain in db of the difference op amp pictured? I have formulas for the voltage gain.

In my circuits
r2=r4
r1=r3

For example:
r2= 10k
r4= 10k
r1= 1k
r3= 1k
gain is 20 db

Can someone tell me:
r2= 2.31k
r4= 2.31k
r1= 1k
r3= 1k
gain is ?db

I've got to make a low gain microphone pre-amp and can't find the correct formula on line.

What are the correct values for r2 and r4 for a gaijn of 10db?

Attached Images
 diffopamp.gif (8.3 KB, 241 views)

 15th June 2003, 10:53 AM #2 Christer   diyAudio Member   Join Date: Sep 2002 Location: Sweden If the voltage gain is Av then it is 20logAv dB. In the first case (10k and 1k resistors) you have a voltage gain of 10, which is 20*log(10) = 20 dB In the second case (2.31k and 1k resistors) you have a voltage gain of 2.31, which is 20*log(2.31) = 3.6 dB Generally the difference between two voltages v1 and v2 is 20log(v1/v2) dB and the difference between two currents i1 and i2 is 20log(i1/i2).
 15th June 2003, 01:20 PM #3 bigparsnip   diyAudio Member     Join Date: Apr 2003 Location: Cambridge To actualy work out the gain in dB, just use 20Log (R2/R1), and the voltage gain woul just be R2/R1 still, as for a normal circuit.
 15th June 2003, 03:51 PM #4 calico   diyAudio Member   Join Date: Jun 2003 Location: USA Thank you very much .

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