opamp matching +/- input impedance necessary for buffer?

[HiFiNutNut]

I have read a lot of articles which emphasise the importance of matching the +/- input impedance of opamps in order to achieve low distortion.

My question is, is it necessary to match the +/- impedance for opamp buffers? An opamp buffer is one where the - input is directly connected to the output with a piece of wire, not a resistor.

I found it hard to match the impedance, DC or AC, especially AC impedance in circuits like Sallen-Key filters.

Regards,
Bill

[MINUX75]

hello

audio is NOT high frequence, matching input and output is not necessary.
Idealy the OUT should be O,OOOOOOOOOOO Ohm like a wire and the input should be 1 0000000000 M Ohm like plastic or dry wood.

[jcx]

op amp inputs show nonlinear input impedance if you look closely enough

can give measureable distortion - matching impedance seen by +,- inputs can help - sugested for fet inputs particularly since filter circuit impedances may be higher and their problem is nonlinear device-substrate parasitic C

it is possible to match filter component impedance with duplicate parts but not often done

don't really need to worry unless low order, smooth distortion charateristic at <0.01% levels bother you

[abraxalito]

I use a series resistor in the connection between output and -ve in. Not just for distortion reduction but also for improved DC offset - so the input bias currents give rise to the same voltage at each. I put around 100pF in parallel with this resistor so the input capacitance doesn't degrade the phase margin.

[HiFiNutNut]

Originally Posted by jcx

don't really need to worry unless low order, smooth distortion charateristic at <0.01% levels bother you

The lower the distortion the better it is.

In my BJT input power amp, when I slightly mismatched the differential input AC impedance seen from the input and the feedback I found the sound degraded.

In my 4 way active crossover with 14 stages, the ones with matching +/- input impedance show low DC offset of 0mV - 0.2mV, while those with large impedance mismatch show as much as 6mV!

I am trying to match the impedance at DC and don't worry about it at AC because it is too difficult.

The problem I had was that I don't know how to match the impedance of a buffer, i.e. the - input connects to the output.

[HiFiNutNut]

Originally Posted by abraxalito

I use a series resistor in the connection between output and -ve in. Not just for distortion reduction but also for improved DC offset - so the input bias currents give rise to the same voltage at each. I put around 100pF in parallel with this resistor so the input capacitance doesn't degrade the phase margin.

Interesting. I have never seen anybody published any circuits using this method.

I quickly drew it in LTSpice and it showed the same response, at least within the audioband.

When I get home, I can post a picture.

[HiFiNutNut]

Originally Posted by abraxalito

I use a series resistor in the connection between output and -ve in. Not just for distortion reduction but also for improved DC offset - so the input bias currents give rise to the same voltage at each. I put around 100pF in parallel with this resistor so the input capacitance doesn't degrade the phase margin.

Is it like this?

[abraxalito]

Yep - the lower of those two arrangements was what I was thinking of. So long as its not DC coupled then both pins will see 10k. Your 22pF cap prevents an HF pole developing which would compromise stability.

[HiFiNutNut]

On top of the issue of differential input bias current causing DC offset, here is an extract taken from the opa2134 datasheet:

For lowest distortion with a source or feedback network
which has an impedance greater than 2kW, the impedance
seen by the positive and negative inputs in noninverting
applications should be matched. The p-channel JFETs in the
FET input stage exhibit a varying input capacitance with
applied common-mode input voltage. In inverting configurations
the input does not vary with input voltage since the
inverting input is held at virtual ground. However, in
noninverting applications the inputs do vary, and the gateto-
source voltage is not constant. The effect is increased
distortion due to the varying capacitance for unmatched
source impedances greater than 2kW.
To maintain low distortion, match unbalanced source impedance
with appropriate values in the feedback network as
shown in Figure 3. Of course, the unbalanced impedance
may be from gain-setting resistors in the feedback path. If
the parallel combination of R1 and R2 is greater than 2kW, a
matching impedance on the noninverting input should be
used. As always, resistor values should be minimized to
reduce the effects of thermal noise.

[HiFiNutNut]

The interesting thing is, if according to opa2134 datasheet, the buffer implementation is merely R2 = 0, which means R1 || R2 = 0, and 0 < 2k, so not matching is required.

This gave rise to my original question - do we need to match the + / - impedance of an opamp in a buffer?