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In Reply to: Re: OPA2134 vs OPA2604 vs OPA637 posted by Ehien on January 30, 2002 at 18:29:43:
doubt that top grade, i.e. lowest offset voltage and/or military tempareature will do any good in audio applicationsany FET input op amp will have problems when used in non-inverting mode due to common-mode-input-voltage-dependend input transistor capacity causing distortion
the 6x7 may be less prone due to alleviating action of cascode circuit
the 627 and 637 are the same except for minimum noise gain - in applications above noise gain of 5, the 637 will offer higher bandwidth which helps to reduce hf and intermodulation distortion
look at D. Self web site - he tested op amp performance with an audio precision one unit - not the most meaningful test, but still, it is interesting to observe discrepancies to manufacuturers' data sheets! the OP-275 turns out to be pretty poor btw!
i have had good experience unsing a 627 in inverting mode and with a diamond transistor output buffer that keeps thermal influence from output load outside of the chip - minimizing thermal coupling and hence LF distortion
for non-inverting mode I recomment a fully complementary discrete op-amp
greetings,
Eric
Follow Ups:
Hi,> any FET input op amp will have problems when used in non-inverting
> mode due to common-mode-input-voltage-dependend input transistor
> capacity causing distortionI would re-phrase this tro include ALL Op-Amps with "long tailed pair" type inputs, be they discrete, monolithic, Fet or BJT input. All suffer from various common mode induced problems, though FET OPA's do less so then BJT ones.
Only OPA's build on the current feedback complementray technology (AD811, LM6181/82/71/72 and other similar ones) which operates without the long tailed pair input is free from those effects.
Later T
Hi Thorsten,
> any FET input op amp will have problems when used in non-inverting
> mode due to common-mode-input-voltage-dependend input transistor
> capacity causing distortionYou seem to be so sure to emphasise the above line.
Maybe I am totally wrong but with a cascode connected input stage the above effect will be nill. Or are you not referring to the Miller effect of the input stage?.
ELSO
Hi,> You seem to be so sure to emphasise the above line.
Actually, I was commenting on it.
> Maybe I am totally wrong but with a cascode connected input
> stage the above effect will be nill.Not so.
> Or are you not referring to the Miller effect of the input
> stage?.I was refering to common mode effects in general. The original poster referred to common mode modulation of the Input Fets by the signal, unless the cascode elements directly reference the common point of the long tailed pair cascoding is not effective in this case, though the overall magnitude of the effect would be larger if miller amplified.
I'm much more concerend about common mode induced distortion in circuits with a long tailed pair input (BJT, MOSFET, J-Fet - all are affected) if the circuit is operated non inverting.
Ciao T
./
ELSO
Hi Eric,Thanks for some interesting notes. I do have some questions and hope you don't mind answering...
> any FET input op amp will have problems when used in non-inverting mode due to common-mode-input-voltage-dependend input transistor capacity causing distortion
By "non-inverting" do you mean signle-ended signal sending into "+" input? Or do you actually mean "non-differential"?
Also in above sentence you mean "capacitance" instead of "capacity", right? Can you elaborate a bit more on the mechanisms (I already understand Cgd, or Cbc for bipolar does depends on bias conditions) and problems associated with the effect you mentioned above? And why such effect doesn't happen when the opamp is not operated under "non-inverting" mode?
I visited D. Self's site before, pretty interesting. Will visit again to correlate his results with what I heard.
-Ehien
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