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Old 5th October 2002, 08:40 AM   #31
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Default opamp inverting

Nelson,

You are right in your earlier post on opamps often being on the edge of oscillation. Especially for opamps that are supposed to be unity gain stable, the manufacturer tries to squeeze the last drop of gain-bandwidth out of it.
As Jonathan says, creative use of compensation often can get you a bit further.
If you want to throw away exta open loop gain by putting a resistor from the inverting input to ground (in the inverting stage case) be aware that you increase the noise gain of the circuit equally. If you decrease the open loop gain by 20dB, you get 20dB extra noise from the opamp input stage, plus 20dB less THD reduction because of less loop gain.
If you don't want to mess with the external compensation, I would get an opamp that is not so hot, but in the end gives better overall results. The above is the reason that I turned away from the 797, for instance.

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Old 5th October 2002, 01:33 PM   #32
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Default Re: offset errors

Quote:
Originally posted by Alice Liddell
But isn't the bias current very low and the driving inpedance small?

What about AC coupling, very often used?

What about bias current on the positve input?



`I have asked three questions, and that is enough,'
Said his father; `don't give yourself airs!
Do you think I can listen all day to such stuff?
Be off, or I'll kick you down stairs!'

Alice
Alice,

The bias current can be very small, but if the
IP resistor is large enough and the OPA closed
loop gain large enough, then output offset
can be significant.

There are ways to get around this by balancing
impedances of + & - inputs so the offset cancels.
I have done this in balanced pre amps with gain
of 40dB (100) and managed to get offset to 1mV
or less. In this case the balanced topology also helps.

As you stated, generally there is no offset
due to bias current into driving impedance which
is usually low enough.

Having said all of the above, using a precision
FET OPA such as OPA627 is offset heaven. I have
driven these with megs and still were hard pressed
to measure any offset.


Terry
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Old 5th October 2002, 02:47 PM   #33
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Default Stability

Jonathan and Jan,

It is not quite clear to me whether you mean only that we must
make sure that the op amp is stable under worst-case load
conditions (which we do want, of course) or if you are asking
for something more? That is, do you also mean that the sound
could benefit from improving a phase margin that is already
sufficient for stability?
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Old 5th October 2002, 06:18 PM   #34
JohnG is offline JohnG  United States
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If there is benefit to be gained from throwing away some of the open loop gain of the op-amp, can you throw it away at the output, by a resistive divider, for example? How does this affect the noise of the circuit, compared to throwing away gain at the input?

John
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Old 5th October 2002, 08:08 PM   #35
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The output is not usually a good place to throw away your
gain, as you also throw away the maximum output. Depending
on the circuit, you will probably get greater distortion than
the "gain toss" figure.
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Old 5th October 2002, 09:12 PM   #36
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Opamps in inverting circuit usually have lower distortion. The reason is that both inverting and non-inverting input are kept at zero voltage (virtual ground) and there is no so called "common mode distortion" encountered at non-inverting circuit.
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Old 6th October 2002, 03:54 AM   #37
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Anything you can do to reduce noise at the front end of an amp will give you better returns than an equivalent reduction at the back end. Noise from the front end is multiplied by all the stages later (and added to, of course). A reduction in noise at the front end keeps this snowballing effect from occuring. Reduction at the back end, after the fact, will work but not be as efficient.
As an admittedly oversimplified view of things, assume an arbitrary number of identical gain stages. The noise figure of the circuit is grossly determined by the noise figure of the very first stage. Let's say that each stage amplifies by a factor of 100 (40dB), a not unrealistic number for solid state circuits, although it's stretching things for tubes. If there's, say, 1mV of noise injected per stage, then the noise from the first stage is amplified by the second stage to become 100mV. The second stage then contributes its 1mV of noise, so now we stand at 101mV of total noise. You can see that the first stage's noise swamps that of all following stages. If you cut that first 1mV of noise in half, you've only got 51mV of noise coming out of the second stage.
It's easier just not to put it in there in the first place. Trying to get it out later is a royal pain. Not to mention more subtle factors like the noise intermodulating with the signal, etc.

Grey
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Old 6th October 2002, 07:45 AM   #38
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Quote:
Originally posted by GRollins
.....amplified by the second stage to become 100mV. The second stage then contributes its 1mV of noise, so now we stand at 101mV of total noise......
Grey, I must correct you little bit. Hope you don't mind.

You can't add noise and distortion in the way you describe.

Total noise will be SQR(100^2+1^2) = 100,004999875 (*)

Conclusion: It's even more important to have a low noise first stage. All of the noise is generated there for most amps.

(*) What is this type of addition called? Is it algebraical addition?
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Old 7th October 2002, 05:47 AM   #39
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Of course noise in the input diff pair and its biasing
current source can't be removed by feedback, but feedback
will reduce the noise that follows.
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Old 7th October 2002, 06:16 AM   #40
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dorkus

If you want to have high input resistance AND high gain with an inverting OP-AMP circuit, then you can avoid high-value feedback resistors by a simple trick: Instead of using a simple feedback resistor you can use a voltage divider between output and feedback resistor.

Per

What you are talking of is geometrical addition (it's at least called like that in German).

Regards

Charles
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