Why do so few designs use opamps working in class A? - Page 2 - diyAudio
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Old 22nd May 2002, 10:41 PM   #11
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Quote:
Originally posted by morsel
[
The two FETs and resistor are actually cheaper and a lot better than a CRD. We discuss this in gory detail here:

http://www.vishay.com/document/70596/70596.pdf [/B]
-vishay seems to imply that their J50x range is two FETs plus resistor, or am I misreading? Much better than a resistor.
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Old 22nd May 2002, 11:19 PM   #12
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Default J50x range

You are misreading.
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Old 22nd May 2002, 11:35 PM   #13
mlloyd1 is offline mlloyd1  United States
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WHAT?
Quote:
Originally posted by alvaius
.....Idle current determines how much overlap there will be between two drive transistors, but does not determine where class A and AB operation happens......
Please check some texts!
Oh, I see HH responded to this already.
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Old 23rd May 2002, 01:45 AM   #14
alvaius is offline alvaius  Canada
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Back to my original response to Harry.... I fully understand biasing output stages with a current source in order to ensure that only one of the transistors remains on all the time.

The problem I have found with this is that the output power that the transistor in the op-amp must supply can be quite high as the voltage drop across the transistor can now be twice the supply voltage and being in class-A, it is never less than a supply rail. Hence, you need to be careful that you don't overheat your op-amp. You could lightly bias the output to maintain class-A operation and then put a power class-A buffer after that. That is probably one reason why it is not done very often.

The other issue I have found is that with heating, etc., the output transistor that is in theory biased off, has a tendency to turn on as you move through the voltage swing. This did not happen on all op-amps and if memory serves me correctly was more a problem with FET op-amps than bipolar and was definately a factor of the output bias current. One day when I have time I will look more into the mechanism. The net effect was an increase in distortion.

But back to the original statement from Harry about an op-amp being in class A as long as the load is small. Assuming that the op-amp is running from bi-polar supplies and the load is connected to ground on one side, then the two output transistor are likely to be switching on and off dependant on what the voltage level is and hence not operating in Class-A. If there is some other mechanism in play, I would like to hear about it.

.... and see, I did not need to take any personal attack on anyone and their ability to read engineering texts, etc. to make my point.
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Old 23rd May 2002, 02:23 AM   #15
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Question I fully understand biasing output stages with a current source

You are as confused as a barking cat and I don't know where to start or how to explain this. I will agree that the power dissappation for one of the output transistors is increased and the complementary transistor is turned off with sufficient current from the external current source. Voltage has nothing to do with Class A bias per say and the rest of your discourse lost me completely. Does this make sense to anyone else?

"One day when I have time I will look more into the mechanism."
We too will wait for that day.


H.H.
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Old 23rd May 2002, 02:29 AM   #16
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Default Re: FET cascode current source

OK Harry, I'll bite.

Why does "cascode" freak people here?
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Old 23rd May 2002, 02:43 AM   #17
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Unhappy Cascode

Beats me.... Jocko and I love them. Do a search on the forum for all the fun and frolic on this subject.

H.H.
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Old 23rd May 2002, 02:48 AM   #18
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Some people around here are totally against all forms of cascodes. They complain about it affecting the sound or some such thing... probably does. Others admit to using cascodes everywhere in everything saying either they dont remember how it sounds without them (it's been so long since they built a circuit without a cascode in it) or that cascodes are the only way.

I personally would like to remain somewhere in the middle believing that cascodes have a place in the tool box but that they are not the be all and end all. I try to take this (and many other things) into consideration when designing a circuit.

Harry likes cascodes but dont mention folded cascodes to him.
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Old 23rd May 2002, 02:59 AM   #19
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Thumbs up folded cascodes

Go ahead mention them. I used my first folded cascode in an audio circuit over ten years ago. AudioFreak will be happy to explain the differences in folded cascodes and regular cascodes and do a much better job than I can.

H.H.
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Old 23rd May 2002, 03:25 AM   #20
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Just didnt get that impression from your comments regarding the use of one in the SOD.
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