Op amp gain in db formula

[calico]

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?

Thanks in advance.

 

[Christer]

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).

 

[bigparsnip]

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.

 

[calico]

Thank you very much .