The cookbook opamp phase stage

[lightningfingers]

...the one with a resistor to the inverting input, a cap to the noninverting input, a resistive element from the noninverting input to Vb (usually a divider set by 2 resistors and a cap), and a feedback resistor from the output to the inverting input. So the cap affects the tone, moreso when there are multiple stages involved...but what about the resistors? How does one determine the value of those resistors? Would I be right in thinking that, like any opamp gain stage increasing the feedback resistor will give more gain? Could someone clear this up for me?

Thanks,

[davebungo]

The basic gain of this circuit at low frequencies ignoring the effect of C and Rb (connecting the non-inverting input to the reference bias voltage) is :
              -Rf
              ----
               Ri

At whatever frequency you choose, increasing Rf will always increase the gain obtained at that frequency.  I had a go at working out the transfer function of this circuit - a picture would be handy here but assuming I have not misinterpreted your description this is it:

Vo(s)/Vi(s)=-(Rf/Ri).(1/(1 + sCRf/Av + (1+Rf/Ri).(1+sCRb)/Av))

Av is the open loop gain of the op-amp

This isn't for the faint hearted and I haven't tested this in PSpice, but what I think it tells us generally is that a larger C and/or Rb will result in phase changes at a lower frequency.  Obviously, if you take C to an extremely small limiting value, then the circuit becomes a simple inverting amplifier.

I hope I haven't confused the hell out of anyone 'cause I think I have done just that to myself.  Please feel free to check out my maths as I have probably got something wrong.  It was fun anyway.

[lightningfingers]

Thanks  its kind of confusing but i think i understand it now :mrgreen: