Why the 220K resistor?

[wnichol]

Hello,

I have noticed that almost everyone is using a 220K resistor as their feedback resistor. Why don't people use larger resistors and increase the gain (Av=Rf/Ri)? Does going past this gain setting degrade the quality? Thank you for your help.

[hugonot]

Less gain - less thd. Gain have to be min. about 10 times for LMXXXX to keep them stable.
Less gain - less problem with hum. If you get gain about 100 times ( 40db ) then noise and hum which from input is 100 times greater on output.

You could use any resistors values. Only criteria is the feedback formula. You could use 220k and 10k, 22k and 1k, 2k2 and 100R, or 2k2 and 10R - for high gain. ( 390k and 1k6 are allowed too, be free.)

There is a lot of laws to choice a feedback resistors in op amps ( for egz. to keep 0V on output ). Literature - " The art of electronics " for egz.

I recommend you a national semiconductors site, they know what to do with these amps.