Input impedance of a non-inverting op amp?

[glennb]

Should be: Input impedance of an inverting op amp?

That's correct. And to complete the picture, consider that, if you like, the op-amp output will "move" to ensure that there is always 0V at the inverting input. An input signal will attempt to pull the - input one way, and the output swings the other way to pull the input back to zero.

Yes. Let's remove the 47K Rin from the circuit in the very first posting. Say if a DC offset appears on the input socket (faulty preamp or whatever), the input cap Cin charges up through Ri, and the charge current is precisely balanced by the opamp output pumping a current through Rf. The - input remains at ground potential and no current flows into or out of it, except its normally minute input offset current.

However, if the incoming DC offset offset voltage is very high ( > Ri / Rf * MaxOpampOutputVoltageSwing ), the op amp won't be able to keep up with it, and the - input will swing above ground for a while until the cap is charged up. This is not so bad if Rb is present and the - / + inputs have a pair of reversed diodes across them on the chip, but it could destroy the opamp if they aren't there. This same risk applies to nearly all cap input coupled opamp circuits.

 

[moamps]

Re: Should be: Input impedance of an inverting op amp

Cin1 & Rin1 are isolated from interacting with Cin2 & Rin2 because the Audio Input A is driven by an AC low impedance source.......Therefore the 47K at Rin1 does not influence the AC gain.......

Hi,
if source isn't connected (my post 12), Rin1 has influence on gain (loop gain) of second opamp (-RF2/(Rin1+Rin2)) If this loop gain is too small, opamp will oscillate.
Regards

 

[mhennessy]

Re: Should be: Input impedance of an inverting op amp?

However, if the incoming DC offset offset voltage is very high ( > Ri / Rf * MaxOpampOutputVoltageSwing ), the op amp won't be able to keep up with it, and the - input will swing above ground for a while until the cap is charged up. This is not so bad if Rb is present and the - / + inputs have a pair of reversed diodes across them on the chip, but it could destroy the opamp if they aren't there. This same risk applies to nearly all cap input coupled opamp circuits.

Yes - this is a very good reason to include RB. Also, it helps protect the chip during power-up/down, and also during massive input signals that drive the op-amp into clipping.

I remember being quite astounded that most people omitted the resistor when inverting GainClones were all the rage last year

Cheers,

Mark

 

[carlosfm]

I've always used a multi-turn pot instead of Rb to adjust DC.
On some cases I removed the MT-pots and replaced by a resistor of the value the pot was measuring (which BTW was never the same value on both channels).
Anyway, I'm not so sure about this NI movement...
I've done almost everything with GCs (IGC, NIGC, BIGC, BNIGC), and the best sound I have is my BIGC power amp (with OPA627), and that's why I have this one on my main system.

 

[glennb]

Should be: Input impedance of an inverting op amp

Hi, if source isn't connected (my post 12), Rin1 has influence on gain (loop gain) of second opamp (-RF2/(Rin1+Rin2)) If this loop gain is too small, opamp will oscillate.
Regards

OK, I missed the bit about 'source not connected'.