Unipolar vs complementary input stage

[Steve Eddy]

Originally posted by john curl
I can only hope that Linear Systems makes p channel product soon. I hope very much, but I am not holding my breath.

Didn't they say sometime last year that they expected to have them early this year?

And if you don't mind my asking, what's the big advantage to using complimentary push-pull pairs as opposed to unipolar push-pull pairs? Wouldn't the latter allow for better matching, less distortion and not having to worry about complimentary devices which would also expand the choice of devices you could use?

se

[PMA]

No no. Complementary differential symmetry is really the best. The only way how to get same SR and perfectly symmetrical limitation. CCS do not make it. And even distortion cancellation, depending on part similarity, of course.

[QSerraTico_Tico]

Originally posted by PMA
No no. Complementary differential symmetry is really the best. The only way how to get same SR and perfectly symmetrical limitation. CCS do not make it. And even distortion cancellation, depending on part similarity, of course.

I am not so sure....



The 2SK389 and 2SJ109 are also not exactly each others miorror image. F. a. the 2SJ109 has much higher capacitances.

[john curl]

You people worry too much about superficial things. First of all, the advantage of complementary differential is that it creates two separate signals that are opposed in DC voltage from each other. This gives 2 times the gain, without any phase delay increase. It is the second stage that is truly improved in linearity. Also, the complementary differential fet input, with a floating resistor for self biasing, can drive more peak current under high rate of change than a simple differential stage. This can give higher slew rate for a given set of initial conditions.
I have never done a SPICE simulation of it, but I would predict that there is an additional advantage of having the complementary differential pair work in series with each other, as well as in parallel. There actually may be an optimum bias resistor value for a given set of devices. I have never modeled this, but I will, sooner or later.

[mlloyd1]

yes, the n jfets and p jfets don't match perfectly.
are you worried about the n and p channel diff amps having different characteristics?
don't forget you are usually:
1. adding source resistors
2. cascoding the jfets
these two practices help mitigate the problems mentioned.
i'd suspect there is still lots of lower hanging fruit to be addressed elsewhere in the design before worrying about this one in excruciating detail ...

... yet

mlloyd1

[Steve Eddy]

Originally posted by john curl
You people worry too much about superficial things.

Wasn't worrying about anything. Just curious as to the advantages of a complimentary pair.

First of all, the advantage of complementary differential is that it creates two separate signals that are opposed in DC voltage from each other. This gives 2 times the gain...

Yeah, assuming a single-ended source. But why limit yourself to a single-ended source?

[b]This gives 2 times the gain, without any phase delay increase.

Why would there be any phase delay increase?

Also, the complementary differential fet input, with a floating resistor for self biasing, can drive more peak current under high rate of change than a simple differential stage. This can give higher slew rate for a given set of initial conditions.

What if you were to use a transformer for your differential input with it driving a symmetrical pair of common-source fets?

se

[john curl]

One small factor that many of you might overlook is that the NONLINEAR input capacitance subtracts from its complement when it comes to nonlinearity. It is true that p channel fets still have more capacitance for a given Gm, but that doesn't matter much with drive impedances of 50Kohm or less. However, IF you are going to drive with impedances of 50Kohm or more, you should use even lower capacitance devices. They exist, but they also have lower Gm

[Bob Cordell]

Originally posted by Steve Eddy


Wasn't worrying about anything. Just curious as to the advantages of a complimentary pair.



Yeah, assuming a single-ended source. But why limit yourself to a single-ended source?



Why would there be any phase delay increase?



What if you were to use a transformer for your differential input with it driving a symmetrical pair of common-source fets?

se

Hi Steve,

You are asking some good questions, even if they may be a bit off-topic for this thread. The full-complementary differential pairs are a nice, convenient, symmetrical way to get a proper drive signal to a full-complementary VAS. I especially like they way John is able to float the tails of the complementary pairs.

However, the full complementary differential pair architecture is not the only way to skin that cat. Take a look at the front-end of my MOSFET Power Amplifier with Error Correction in the paper on my website at www.cordellaudio.com under the published papers tab. It only requires a single same-sex matched differential input JFET pair. The arrangement does not look as elegant and symmetrical on a schematic, and tends to require more transistors, however.

Cheers,
Bob

[john curl]

Bob, you make a good design, but it's topology goes back to the '60's. I first saw it in the Marantz 14 power amp, designed by Sid Smith.
In my experience, this basic topology has twice the distortion as a complementary symmetry design without cascodes in either design. I think that is has to do with thermal offset in the current mirror that made my designs at least, slightly compromised. Anyone can cascode and I have do so as well in many designs of this type.
If you could look at the line driver that I designed for Sound Technology in the early '80's, you would find cascodes everywhere with your selected input devices in front. The greatest difference between your design and mine was the output stage, although mine was complementary power mosfets as well, and the fact that I maintained a 100KHz open loop bandwidth.

[Bob Cordell]

Originally posted by john curl
Bob, you make a good design, but it's topology goes back to the '60's. I first saw it in the Marantz 14 power amp, designed by Sid Smith.
In my experience, this basic topology has twice the distortion as a complementary symmetry design without cascodes in either design. I think that is has to do with thermal offset in the current mirror that made my designs at least, slightly compromised. Anyone can cascode and I have do so as well in many designs of this type.
If you could look at the line driver that I designed for Sound Technology in the early '80's, you would find cascodes everywhere with your selected input devices in front. The greatest difference between your design and mine was the output stage, although mine was complementary power mosfets as well, and the fact that I maintained a 100KHz open loop bandwidth.

Hi John,

It is not just the cascodes. You need to look more carefully at the differential current mirror load. That part of the design is very important, and, to my knowledge, does not go back to the Sixties. I'm sure your experience is correct, but I also expect that your experience does not include this circuit.

This circuit does a near-perfect job of forcing the input differential pair into current balance, something your circuit fails to do. That significantly reduces distortion. This circuit also does a near perfect job of driving the top and bottom complementary VAS transistors with identical signals, something that any gm mismatch in your circuit between the p and n channel devices will fail to do. That will also result in distortion. This circuit also provides more gain in the input stage ahead of the VAS, greatly reducing the influence of any VAS noise.

Cheers,
Bob