# op amp question

#### fran1942

Joined Jul 26, 2010
58
Hello, I am learning the basics of op-amps and have designed the attached configuration to give 25 gain. I was just wondering, if I wanted to increase the gain to 340, could I simply replace R1 with a 330k and R2 with a 10 Ohm resistor ?
Is there any reason why I could not do that ?

#### beenthere

Joined Apr 20, 2004
15,819
Use a calculator. 8200 divided by 330,000 is .0248, not 25.8. Notice that the signal out is nowhere near 25 times the input - 2.5 volts in, and about that out.

A gain of 340 is close with R1 = 68,000 ohms, and R2 = 2000. Your resistor values will give a gain of 33,000. DC gain is not 1 - try dividing any value by infinity and see what you get.

You might be interested in this part of our Ebook - http://www.allaboutcircuits.com/vol_3/chpt_8/5.html

#### eblc1388

Joined Nov 28, 2008
1,542
if I wanted to increase the gain to 340, could I simply replace R1 with a 330k and R2 with a 10 Ohm resistor ?
Is there any reason why I could not do that ?
One reason being the opamp is not fast enough to amplify signal as high as 20KHz with such gain.

Looks at the gain/frequency plot inside common opamp data sheets and you will know why. A gain of 340(50dBs) is possible at 200Hz with some opamps but when the frequency is 20KHz, few will do.

#### colinb

Joined Jun 15, 2011
351
Use a calculator. 8200 divided by 330,000 is .0248, not 25.8. Notice that the signal out is nowhere near 25 times the input - 2.5 volts in, and about that out.
I think you made a mistake there.

The circuit shown in fran1942's schematic has an 8200 ohm feedback resistor with a 330 ohm dividing resistor, so Av = 1 + (8200 ohm)/(330 ohm) ≈ 25.8, just as he stated.

(I avoid using the terms R1 and R2 since fran1942's schematic has those references swapped from the AAC ebook op-amp usage.)

#### atferrari

Joined Jan 6, 2004
3,487
(I avoid using the terms R1 and R2 since fran1942's schematic has those references swapped from the AAC ebook op-amp usage.)
I got used to Rf and Rin which are self explanatory. Hard to make a mistake with them.