Question for op amp circuit jocks

[fieldwrangler]

Folks -

So, in the generally accepted topology of the (inverting) summing amplifier (in which all inputs tie to the inverting input through a series resistor and the non inverting input is tied to ground) the input impedance is confidently reported to be simply the value of the aforementioned series resistor, due to the fact that the inputs are both at virtual ground potential.

However, when the positive input has its own resistor interspersed between itself and ground in order to potentially compensate for offset voltage, is the effective input impedance then the sum of the input resistor and compensation resistor values?

Bob Palmieri

[ReadyTeddy]

No, only the input resistor connected to the -input determines input impedance. The resistor connected to the +input sees no signal, and therefore doesn't interact with the signal. The -input is still at a virtual ground with respect to the signal input. In fact, it is best to put a capacitor across the compensation resistor to avoid having the +input be a point at which noise and interference could enter the op amp, and also to short-circuit the Johnson noise of the compensation resistor.

[salvarsan]

Walt Jung's books are an excellent place to look for op amp minutiae.

Services

[fieldwrangler]

No, only the input resistor connected to the -input determines input impedance. The resistor connected to the +input sees no signal, and therefore doesn't interact with the signal. The -input is still at a virtual ground with respect to the signal input. In fact, it is best to put a capacitor across the compensation resistor to avoid having the +input be a point at which noise and interference could enter the op amp, and also to short-circuit the Johnson noise of the compensation resistor.

Should I match the value of the cap across the feedback resistor?

[fieldwrangler]

Walt Jung's books are an excellent place to look for op amp minutiae.

Services

I have the third edition of "Understanding IC Op Amps" at my workshop - I was drawing something up at home and this little issue had me scratching my head.

I do like that book.

[ReadyTeddy]

Should I match the value of the cap across the feedback resistor?

No, just make it reasonably large--all the cap is doing is shorting out the Johnson of the resistor. You're effectively low-pass filtering the resistor's noise. The filter's corner frequency is 1/(2*pi*r*c), so pick a cap that keeps the frequency below 100 Hz.

BTW, I might ask why you think you need this compensation resistor for audio? Any offset voltage, unless strangely large, is typically not a problem for audio. Also, if you use JFET-input op amps the bias current is so low, no compensation resistor is needed.

[fieldwrangler]

No, just make it reasonably large--all the cap is doing is shorting out the Johnson of the resistor. You're effectively low-pass filtering the resistor's noise. The filter's corner frequency is 1/(2*pi*r*c), so pick a cap that keeps the frequency below 100 Hz.

BTW, I might ask why you think you need this compensation resistor for audio? Any offset voltage, unless strangely large, is typically not a problem for audio. Also, if you use JFET-input op amps the bias current is so low, no compensation resistor is needed.

I usually use jfet types for the input stages (still using TLO-7x (or TLO-6x for battery powered boxes)) after all these years.

I also build boxes for upright bass, and try to eliminate the pesky 'lyrics between stages, so small offsets can create clicks when switching stages in & out. My main upright bass client (a truly world class player with at least 17 grammys on his mantle) can instantly tell if I've used coupling caps between stages.

[salvarsan]

Update:

A PDF of Walt Jung's Audio IC Op-Amp Applications (1974) is available for free IF you fill out the request form at the bottom of his eBooks page.

link:
Walt Jung Ebooks | Walt's Blog 2014


Analog Devices has a derivative version that Walt edited, Op Amp Applications Handbook (2006), also freely available by chapter or in its entirety.

link:
ADI - Analog Dialogue | Op Amp Applications Handbook