Why do so few designs use opamps working in class A?

[fmak]

Can anyone comment pl. Surely its easy to draw a few mA with a CRD.

[HarryHaller]

For resonable and typical values of load impedance ie. greater than 10K ohms, op amps are operating Class A.

H.H.

[alvaius]

How do you figure that Harry? Am I missing something? The output stages of most are AB, which would mean the class of operation and which transistors are actually being driven should be dependant purely on the output voltage and have nothing to do with the load impedance. Am I missing something?

[peranders]

alvaius, you are really missing something...:-)

It is purely the current which is determinating wether it's class A or B. Heavy load reduces class A operation. If the output stage is running at 10 mA you can only take out MAX 10 mA peak value.

[alvaius]

I disagree. Idle current determines how much overlap there will be between two drive transistors, but does not determine where class A and AB operation happens. A class A-B output stage will only have overlap and essentially class-A operation for a very small region. The outputs of most op-amps are class AB. The output transistors are essentially emitter followers with a couple diode drops between the PNP and NPN assuming bipolar. Hence, the output current should not matter, only the output voltage.

I think you are thinking about a class-A output stage where the output current can be no more than the bias current.

[Nelson Pass]

Bob Widlar, the legendary fountainhead of IC op amp
design did in fact do at least one Class A gain circuit,
the LM110 (and 310), which is a high performance
voltage follower. Deconstructing its schematic is an
education, and is recommended to topology geeks.

You can, of course, externally bias an op amp into Class A
with a current source on the output, or even a resistor to
one of the supply lines. I did this many years ago on the
Forte preamp, and I have seen it elsewhere.

[HarryHaller]

"Idle current determines how much overlap there will be between two drive transistors, but does not determine where class A and AB operation happens." Oh really....and what does?

"Hence, the output current should not matter, only the output voltage." OH REALLY! You are just making all this stuff up aren't you?

You need to go read some Data sheets for some op-amps. If the bias current in the output stage of an op-amp is greater than the peak current swing it is Class A by definition. Now if you want to learn what putting an external current sink or source on the output of an Op-Amp does, that is another matter....

[Nelson Pass]

In a push pull stage, the output leaves Class A
at about twice the bias current.

[morsel]

Tangent and I are almost finished with our headphone amp project over on Headwize. The amp uses a FET cascode current source suggested by PPL to bias the opamp into class A. Note that the output is buffered, the opamp is not driving headphones directly:

http://tangentsoft.net/audio/meta42/misc/schematic3.pdf

The two FETs and resistor are actually cheaper and a lot better than a CRD. We discuss this in gory detail here:

http://headwize.powerpill.org/ubb/sh...num=3&tid=2672

Here is an old Siliconix paper on FET constant current sources:

http://www.vishay.com/document/70596/70596.pdf

[HarryHaller]

I love it when you talk techno. Especially the cascode word which will get your mouth washed out with soap around here!

H.H.