A push-pull differential stage- why not?

[darkfenriz]

hallo

Have you ever used (or considered ) something like this ?

seems to cancel some distortion compared to CFB, doesn't it?

[PRR]

> something like this?

Never seen it done like that.

Without the stack-o-diodes, and with two long-tail current-sources for the input pairs, it is an old idea sometimes attributed to Dan Meyer.

Such ideas keep coming up. Some excellent designs are like this. And the symmetric elegance is seductive. Yet many (even most) very fine amps are built without push-pull differential.

The killer issue is setting a stable current in the second stage, your Q10 Q9, with variations in parts and temperature. At a glance, I suspect yours works better in a simulator than it will with real transistors of various Betas. (The fact that R19/R35 is about the same as a typical Beta suggests that if you change transistor, you have to change the R19/R35 ratio, which is unproducable.)

What do R46 R47 do for you?

I see C12 C13 and wonder if that is enough frequency compensation to be stable, especially when current and Ft in Q10-Q19 is wandering all over the place. It is customary to take some capacitor feedback around the 2nd and buffer stages to keep their phase response predictable in the face of changes. Your system may well work, or may work with an ideal resistor.

[Mr Evil]

I have seen that done before. In particular, I remember reading an article in Electronics and Wireless World some years ago with such a differential in it as part of an amp with extremely high slew rate. I think it was one of a number of alternative methods given to set the current in the differentials rather than the normal current sources.

[jcx]

many modern high speed voltage op amps are derived from their current mode cousins by adding a push-pull buffer to the low impedance negative input resulting in a balanced push-pull differential stage

i believe a japanese researcher created a variation he called a "Diamond differential" input stage in a mid 80's jaes article

LT1794 op amp simplified circuit:

[Steven]

Originally posted by jcx

i believe a japanese researcher created a variation he called a "Diamond differential" input stage in a mid 80's jaes article

Yes, I've seen it from Sansui in the eighties. They called it a diamond differential. They used it as a second stage in a fully balanced amplifier (BTL), using all 4 collector outputs. The first stage was a simple JFET differential, driving the diamond. I think I have an AES preprint somewhere describing it.

Steven

[Steven]

Found it back: "Fully balanced bridge amplifier", Susumu Takahashi and Susumu Tanaka, Sansui Electric Company, Ltd., Tokyo, Japan. AES preprint 1918, presented at 72nd Convention 1982, Anaheim ,California.

I did not scan the complete preprint, only the actual diagram. In the text it is called "diamond differential circuit".

Steven

[jwb]

What's the advantage supposed to be? The node at the center of the four emitters is (should be) constant voltage, so why bother connecting the diffs together?

[MikeB]

Hi !

I would say, this one is very sensitive to fluctuations from PSU, as
current through diodes changes with supplyvoltage, changing the
bias through the diffamps. (You could replace the resistors with ccs)

You could replace the bjts in the diffamps with jfets, not needing
the bias-stuff with the diodes. I already tried that, works very fine !
(With amazingly stable currents, giving high psrr)
The jfets work without the diodes because of their ability to work
with negative Vgs for currents below Idss. So you get pefectly
balanced symetrical input !

Mike

[anli]

Originally posted by MikeB
I already tried that, works very fine !

Mike, is it possible to look at this your schematics?
Does "tried" mean that you have rejected such amp later?

Andrew

[MikeB]

Originally posted by anli

Mike, is it possible to look at this your schematics?
Does "tried" mean that you have rejected such amp later?

Andrew

Yes, i rejected it later, but this was before i knew how to "cure"
the problem with bright sounding amps... That was the reason
why i rejected the topology, not knowing better.
Except the bright sounding everything else was "perfect".

Attached is a circuit how i would build it today, but i have not
exactly built this one, the vas is different now...
The vas in the rejected one was without rloads to the vas and
without cascodes. Also i no longer use cdoms in my latest amps...
I think i will try this one later. It's seems the only way to get perfect
symetrical currents in a LTP. In sims, DC-offset for openloop (!!!) is
30mv, closed loop down to 6.4uv. (Without DC-blocking cap)
In real world of course it will be impossible to match the jfets this
closely...
The current through the LTP is determined by the 150ohm and the
idss of the choosen jfets. For higher power the jfets needs to be
cascoded of course...

The THD of this circuit is ~0.008% with 20khz swing ~12volts.

Mike