Class-A Opamps

[Workhorse]

Hi everyone,

Has anybody came across Class-A opamps?.
Do they really exist.
Does there is a way in which a regular class-AB opamp could be converted to Class-A opamp.

waiting for replies.......
Regards,
Kanwar

[shusha]

I was asking myself same question and this article gave me an answer. Must read!

http://tangentsoft.net/audio/opamp-bias.html

[richie00boy]

kanwar,

You can force any op-amp to flow current in one side of the output stage therefore that part is always class-a, by simply connecting a resistor or constant-current device from the output to the opposite supply rail. It seems to be more common to make the upper transistor the class-a one but I think really it depends on the PSRR for that rail and the intrinsic linearity, i.e. a lot of op-amps use quasi output.

[Workhorse]

Originally posted by shusha
I was asking myself same question and this article gave me an answer. Must read!

http://tangentsoft.net/audio/opamp-bias.html

Hi shusha,

Thanks very much for the help.

Originally posted by richie00boy
kanwar,

You can force any op-amp to flow current in one side of the output stage therefore that part is always class-a, by simply connecting a resistor or constant-current device from the output to the opposite supply rail. It seems to be more common to make the upper transistor the class-a one but I think really it depends on the PSRR for that rail and the intrinsic linearity, i.e. a lot of op-amps use quasi output.

Ritchie you are simply great man. yes you are right in case of some quasi-opamps.
I want to convert OPA2134 best professional high audio opamp from texas instrument.but i dont know that iis it comp. or quasi.
Now i will do the job on it and give the results soon.
here is the link to opamp http://focus.ti.com/docs/prod/folder...t/opa2134.html
Regards,
kanwar

[jacco vermeulen]

I've been forcing opamps in class A since the late 80s.

For opamp preamp's, opamp driven Mosfet class A amplifiers, currently in rebuilt partly class A 4-channel car power amplifiers with OPA627 and sanken1216/2922, and in the Stokes SDS class A headphone amplifier with RFP12N20/RFP12P10 Mosfet outputs.
With many different opamps from 5532/5534, OP27, LT1028, now OPA627 and OPA637.
Also the 134, and 2134.
Difference was staggering on every attempt.
Not surprising as mosfets only sound any good driven in class A(imo), think of the number of fets in an opamp.

Think of glueing a heatsink on top of the opamp, if you are planning to stretch it.

[Workhorse]

Originally posted by jacco vermeulen
I've been forcing opamps in class A since the late 80s.

For opamp preamp's, opamp driven Mosfet class A amplifiers, currently in rebuilt partly class A 4-channel car power amplifiers with OPA627 and sanken1216/2922, and in the Stokes SDS class A headphone amplifier with RFP Mosfet outputs.
With many different opamps from 5532/5534, OP27, LT1028, now OPA627 and OPA637.
Also the 134, and 2134.
Difference was staggering on every attempt.

Think of glueing a heatsink on top of the opamp, if you are planning to stretch it.

hi jacco,

nice to see you.
Thanks for the comments on your expertise in Class-A opamps.
I will definately gain much knowledge from you.

Thanks again,
regards
kanwar

[richie00boy]

kanwar,

I use the OPA134 and OPA2134 myself as it's a really nice chip. I believe it has a quasi-complementary output as the max swing available to each rail is not symmetrical (see output voltage swing vs output current graph). Because it can swing closer to the lower rail I believe that one is the quasi side. No PSRR data is given for the individual rails so I have to base my choice on the output stage symmetry alone. Therefore I hang a current source made from 2N3819 JFET and resistor off +ve rail and down to the output as I believe the cfp/quasi side works better in class-a and also there is more swing available.

I hope this brings you success and nice sound

[sam9]

Extrapolating from experience with discrete amps, I think the asymetic swing is more likely to from use of a single differential stage and non-symetric intermediate stage. I.e., non-"mirror image" topology. I don't think quasi-comp output stage contribute to this.

My thoughts are definately non-expert and extrapolations at that so give them no more than their due. Others with more specific knowledge may care to correct this.

[jackinnj]

i ran an SSM2019 in class "A" inadvertently -- eventually it pooped out due to heat but the distortion was not measurable on my TEK DA4084 or on the Boonton 1120.

btw, this is a really nice chip if you need balanced input and switchable gain.

[Leolabs]

I do know one opamp that's not a Class A but definitely with a single -ended output stage,the old NE5532/5534.

One famous US amp maker(can't remember which one)did use this opamp for their power amp with forced-bias arrangement.