Q about R/C networks and OpAmp gain.

[dan5150]

I am trying to wrap my head around this, and hopefully one of the smarter folks here can help:

I am working on a breadboarding an op-amp based distortion. To set the gain of the opamp, I am using an R/C combination. (per the schematic)

It looks like the higher value resistor I use, the higher the gain is. Does that mean that less current = higher gain? That seems a bit counter intuative that something that is supposed to resist current means more gain.

[iccaros]

I am trying to wrap my head around this, and hopefully one of the smarter folks here can help:

I am working on a breadboarding an op-amp based distortion. To set the gain of the opamp, I am using an R/C combination. (per the schematic)

It looks like the higher value resistor I use, the higher the gain is. Does that mean that less current = higher gain? That seems a bit counter intuative that something that is supposed to resist current means more gain.

the resistor is setting how much negative feedback is returning to the feed. The more negative feedback the less gain, as feedback cross cancels signal, so more resistance blocks more feedback which allows more signal through to the point that the resistance blocks all feedback, which can make the opamp unstable. 



some math here but good overview --> http://www.physics.arizona.edu/~varnes/Teaching/405-505Spring2006/Notes/Lecture4.pdf
or lighter reading --> http://en.wikipedia.org/wiki/Operational_amplifier#Negative_feedback_applications

[newfish]

I am trying to wrap my head around this, and hopefully one of the smarter folks here can help:

I am working on a breadboarding an op-amp based distortion. To set the gain of the opamp, I am using an R/C combination. (per the schematic)

It looks like the higher value resistor I use, the higher the gain is. Does that mean that less current = higher gain? That seems a bit counter intuative that something that is supposed to resist current means more gain.

the resistor is setting how much negative feedback is returning to the feed. The more negative feedback the less gain, as feedback cross cancels signal, so more resistance blocks more feedback which allows more signal through to the point that the resistance blocks all feedback, which can make the opamp unstable. 



some math here but good overview --> http://www.physics.arizona.edu/~varnes/Teaching/405-505Spring2006/Notes/Lecture4.pdf
or lighter reading --> http://en.wikipedia.org/wiki/Operational_amplifier#Negative_feedback_applications

+1

...and again, if you want to skim over the maths (page one), R2 (Feedback resistance) / R1 (input resistance) = gain, or thereabouts.
It's a good place to start - so an inverting Op-Amp set-up with R1 as 10K, and R2 as 100K, would yield a gain of (100 / 10) = 10.

Have a 'google' for BOSS DS-1 / SD-1 schematics - or the humble Tubescreamer.  This configuration is used time after time to control the gain of Op-Amp circuits - and needs no akward, hard to find reverse log pots (as used in the MXR Distortion +).

Cheers!

[PRR]

> the higher value resistor I use

There's TWO resistors, right?

Like a lever, it is the ratio that matters.