Inverting op amp gain stage question

[Kipper4]

Looking over at Groovenuts Sixty 8 drive (members only thread)
It was pointed out by Sam that inverting op amp gain stages are under used and that its hard to make them High input impedance hence why groovenut made an input buffer to compensate (part of the reason)
Q1
why is it hard to make them high impedance?
Q2
How do you make a high input impedance gain stage?
Q3
Why is this under used?
Q4
Any other drive circuits you can think of that use inverting gain stages off the top of your head?

Other comments welcome on the subject too.

Thanks
Rich

[PRR]

The input impedance is the input resistor.

You can make the resistor as big as you want BUT in the inverter this resistance adds hiss (not so much in the follower).

Also if you want gain you need an even bigger resistor for the feedback, which you may not have in stock.

Inverters are good for inverting polarity.

They make dandy adders (mixers) for high-level signals.

Inverters USED to be the only standard op-amp (in days of tubes when you needed to trim the "+" input for DC accuracy) so there are many classic circuits for inverters (notably some less-used filters).

Some low-low-cost gain blocks (simple BJT, CMOS) only invert and you have to harvest from the inverter designs.

An inverter can take signal from outside the DC supply voltage. A follower would need an attenuator and DC bucking reference in front, why is about the same but maybe more work.

[Kipper4]

I read that somewhere, that the input cap is essentially eqaul to the max input impedance.
Thanks for confirming that.

Thanks for the reply Paul. I understood all of it except this which I'm a bit sketchy about

"An inverter can take signal from outside the DC supply voltage. A follower would need an attenuator and DC bucking reference in front, why is about the same but maybe more work"

Perhaps someone can elaborate for me
Cheers

[samhay]

> I read that somewhere, that the input cap is essentially eqaul to the max input impedance.
Thanks for confirming that.

Not input cap, but the resistor(s) in series to the (-) input.

> "An inverter can take signal from outside the DC supply voltage. A follower would need an attenuator and DC bucking reference in front, why is about the same but maybe more work"

You run your op-amp on a 9V supply. If you don't add an input capacitor, you can take signal that is not biased within the 0-9V rails (i.e. it could be swinging either side of 10V (but probably not 100V) ) and output something as long as the gain is < 1. This works (with some caveats) as the (-) input is a virtual ground / low impedance and you drop most of the voltage across the series resistor, much as you would if the input resistor was connected to ground.

Which brings me to the more relevant point regarding gain / clipping.
The gain of an inverting op-amp is R(feedback)/R(in). If the feedback resistor is shorted then you go to zero gain (no output) as opposed to unity gain in a non-inverting op-amp setup.
This makes clipping in the feedback loop different as the diodes will clamp the voltage to +/- the diode forward voltage (Vf) rather than input signal + diode Vf in the non-inverting case.

[Kipper4]

Thanks Sam
I did indeed mean resistor not cap.

[Kipper4]

"Which brings me to the more relevant point regarding gain / clipping.
The gain of an inverting op-amp is R(feedback)/R(in). If the feedback resistor is shorted then you go to zero gain (no output) as opposed to unity gain in a non-inverting op-amp setup. "


Then I guess I could always put a series resistor in the feed back loop  and a gain pot to make it unity at the very least ?
Or adjust said resistor to just below clipping threshold or less than unity.
The world is my resistor.

[samhay]

> Then I guess I could always put a series resistor in the feed back loop  and a gain pot to make it unity at the very least ?
Or adjust said resistor to just below clipping threshold or less than unity.
The world is my resistor.

I think you are getting it now.
Have fun.

[Pojo]

Check out the Marshall Bluesbreaker or one of it's derivatives (King of Tone, JHS Morning Glory, etc). It uses a non-inverting stage at the input to set the input impedance, pre-clipping tone shaping, and an easy source of gain which is then followed by an inverting stage with 2 pairs of clippers in the feedback loop. The gain pot straddles the 2 stages effectively setting the gain of both stages simultaneously.

Schematic can be found here: http://www.generalguitargadgets.com/pdf/ggg_mbb_sc2.pdf

And like Samhay said the diode clipping is different within an inverting vs non-inverting stage. You don't get that fake sounding 'clean-bleed' mixed in with the clipped signal that happens in a Tube Screamer or similar.

I've recently had the honor to be contracted by a fairly well known and successful guitarist to design and produce a new signature overdrive based on that circuit. So I've been messing around with it a lot lately and am liking it quite a bit so far!

[Gus]

Something to think about is the IC little muff

When you set up a passive guitar/bass to  an inverting gain stage(IC based, FF without the gain control maxxed, input of a BMP) the guitar/bass and cable become part of the input(add to the input resistor)

This can be fun to adjust

[Groovenut]

And like Samhay said the diode clipping is different within an inverting vs non-inverting stage. You don't get that fake sounding 'clean-bleed' mixed in with the clipped signal that happens in a Tube Screamer or similar.

This is because the diodes in the feedback loop of an inverting gain stage "see" the inverting input as ground (well virtual ground).

[Kipper4]

Some great ideas there.
Thanks Gus I'll more than likely have a go with some differant ideas I've had,nothing revolutionary but it's all been done before anyway so.
I'm thinking jfet buffer or op amp BMP or non inverting op amp buffer
Inverting gain stage/clipping amps or amps
Active tone stack.
Wish me luck here I go again.
Rich