opamp loading query..

jeth · Jun 22, 2006

Hey all, just been lurking for a while, and still learning plenty (but still plenty to learn)..many thanks to you all.

I've had a good search here, and the web in general but my query is hard to find an answer too as any keywords I can think of tend to throw up all sorts of associated crap.

I just want to know if, when calculating the load on an op amp stage, the feedback resistor needs to be included in the sums in parallel with the calculated load. I'm assuming it does as the amp is driving the feedback R also, but I'd be pleased if someone could confirm. Or tear my theory to peices. Cheers..

NewYorkDave · Jun 22, 2006

the feedback resistor needs to be included in the sums in parallel with the calculated load

Correct.

jeth · Jun 22, 2006

Many thanks NYD,
I thought I might just be ignored for asking another newb question. I even just ran a search for a more beginner based forum, but there's nothing out there it seems.
Besides, I may be a bit behind most folks here, but I come across knowledge I'd never find on a newbie forum and I enjoy climbing the learning curve.

Thanks again, top man.. Jeth

pstamler · Jun 23, 2006

Just to amplify a bit (sorry) on what NY Dave said...

In an inverting amplifier, the feedback resistor is in parallel with the load. The - terminal of the opamp is a virtual ground. In a non-inverting amplifier, though, what's in parallel with the load is the series combination of the feedback resistor and the resistor from the - terminal to ground.

Peace,
Paul

jeth · Jun 23, 2006

Paul, thanks for the extra info.

That makes things more interesting. One of the circuits I'm looking at is a single amp eq stage with a gyrator hanging off it. The kind that puts the level pot between the input stage and the feedback network to create a voltage divider that attenuates when the pot is to the input side and boosts by creating the same attenuation in the feedback network when pot goes clockwise..

As in this circuit (but with gyrators rather than passive LRC sections)..
http://forsselltech.com/schematics/EQ1A.PDF

So would the pot (at varying value dependant on setting) or the pot AND gyrator series R be put in series with the feedback resistor then paralleled with the load? I'm also assuming the input R will vary between the value of the pot to wiper (input side) in series with the input resistor and the input resistor alone. Be grateful to know this for sure as I'm trying to put something together using the few parts I have available so getting the sums right is crucial. Thanks again, Jeth