calculating gain in an API type circuit

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bigugly

Well-known member
Joined
Jun 27, 2004
Messages
220
Location
Burbank, CA.
I know this is electronics 101 stuff but the formula I was shown only seems to work for max gain. Here's what I've been using:

[Rf * (Rs1+Rs2)]/[Rf + (Rs1+Rs2)] = X, then 20*Log10(X) = gain in db

When I run the numbers for max gain I get a sensible answer but when I use the numbers for min gain I get nonsense.

example:

Rf = 20000 (feedback resitor)
Rs1 = 200 (shunt resitor)
Rs2 = 20000 (rev log pot)

so at max gain Rs2 is shorted so my math is this:

[20000 * 200]/(20000 + 200) = 4000000/20200 = 198

20*Log10(198) = 45.9db of gain

min gain is:

[20000 * (20000+200)]/[20000 + (20000=200)] = 404000000/40200 = 10049

20*Log10(10049) = 80db of gain!

so i guess it's fairly obvious that I'm doing something wrong here.

Please help me. Of all the reading material I have, nothing covers this

Thanks

James Lee
Big Ugly Van Audio
 
Hey James,

I've been here before. Check this thread:

http://www.groupdiy.com/index.php?topic=1085&postdays=0&postorder=asc&highlight=algebra&start=15

Specifically, look for PRR's reply, as he simplifies things a bit.

Regards

ju
 
It's fairly straight forward:

Av=Rf/Rin+1 or in you're case Av= R3/(R2+R1) +1
Gain in dB= 20logAv

So, if you're using the typical 312 schematic the low and high gains would be:

Av= 20,000/(20,000+200) +1 = 1.99
Gain in dB = 20log(1.99) = 5.97dB

and

Av= 20,000/200 + 1 = 101
Gain in dB = 20log(101) = 40.09dB

Then if you're using the a 1:10 input transformer this would add an additional 20dB of gain at all settings. If you wire your output transformer for 1:2 it would add another 6dB to all settings.

If you're using an inverting amplifier, just drop the +1 from the equation.

Hope this helps,
Jeff
 
So, based on all this, I think it´s true to say:

Rgain = (R3 / gain-1) - R2

Rgain = resistor in ohms
gain = Vin/Vout
 
Yes. But remember an inverting amplifier's gain will not be much less than 2.

I recommend using a 25k reverse log taper gain pot (if you can find one). Rf should then be a 25.5k 1% metal film bypased with a 47pf polystyrene or polypropylene cap. Then R2 should be 470 ohm. This will translate to about 35dB maximum gain and a low gain of 6dB.

Many combinations are possible depending on the availabilty of the gain pot value you can get. Use Pspice or the trusty calculator to set the value of your feedback capacitor. A 1us R*C is a good place to be.

regards,
Jeff
 

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