Fully differential I/O amplifier

[ojg]

So according to TI, these new fully differential I/O amplifiers, like OPA1632 is the best thing since sliced bread, and when I look at their datasheet I tend to agree.

The deal is this: "Differential output reduces even-order harmonics and minimizes common-mode noise interference."

So I start thinking, why haven't we seen this before? Smart people must have thought of this long ago. And if so why aren't there any discrete power amplifiers out there based on this? Or maybe there is but I haven't found them yet.

Anyone knows? Is it something worth pursuing?

Here is the diagram of the THS4131 amp:

[sss]

Originally posted by ojg
So I start thinking, why haven't we seen this before?

i guess we all have seen this before , its good for balanced inputs etc but thats the only application i can think off

[anatech]

Hi ojg,
Push pull amplifiers (tube & transistor) do this. Tube types have been doing this since before many of us were born. Of course, as sss pointed out, the studio standard balanced out does this too. There are only some situations where this is a benefit, like long cable runs or really noisy (electrically) enviroments.
-Chris

[ojg]

Originally posted by anatech
There are only some situations where this is a benefit, like long cable runs or really noisy (electrically) enviroments.

Why is "reduction of even-order harmonics and minimized common-mode noise interference" only a good thing when you have long cable runs?

Back to my point. These amplifiers seem (to me at least) different from the regular balanced output drivers such as
DRV135 (see attachment)

The THS4131 looks rather normal at the input, a regular diff input stage with folded cascode. The peculiarity is that there are two output buffers in opposite phase. There are two sets of feedback networks one from each buffer to each side of the diff input stage. In addition there is a "servo" to cancel common-mode errors. This is also different from a normal bridged amplifier.

If this has been used in tube circuits before I am interested in seeing a schematic (even though I know nothing about tube design...)

[anatech]

Actually, the even order harmonics are cancelled when the opposite phases are combined ... like in an output transformer of a tube amp. The same occurs in the balanced input which could be a transformer or differential pair.
Otherwise, you have the same distortion spectrum (no cancellation).
A long cable run is an application of the noise cancelling property of a balanced output. I should have made that more clear.
-Chris

[thanh]

This technique probadly can only reduce and not cancel distortion. Its THD is 0.000003%
I love zeroTHD.I think perhaps phase problem cause aliving distortion.Can you understand me? My english is not good!

[Petter]

The basic topology is very similar to Pass X as I have posted here before.

Petter

[x-pro]

I've already posted on this subject a couple of weeks ago:

OPA1632 and THS4131

Cheers

x-pro

[ojg]

x-pro:
I saw your post earlier, but thought the subject was worthy of more discussion!

Petter:
I need to study the X circuit closer, but I thought it used two amplifiers in a "sort of" bridged mode? Similiar but not quite the same? There's been many X threads in the Pass forum but which one discussed the theory behind this?

anatech:
I know the even harmonics are cancelled when you combine the opposite phases. If that is all there is to it then the DRV135 and other bridged amplifiers should have equally impressive specs, but they don't.

Also noteworthy is that the OPA1632 has only 78dB of open-loop gain, very low by audio standards. (OPA134 has 120dB!) So the low distortion is not due to large amounts of NFB.

I have a feeling that the trick is having the feedback from each output buffer return to a single differential input pair, but I can't prove it...

[x-pro]

Originally posted by ojg
Also noteworthy is that the OPA1632 has only 78dB of open-loop gain, very low by audio standards. (OPA134 has 120dB!) So the low distortion is not due to large amounts of NFB.

More important that this 78 dB gain is the same almost over all audio frequency band, but "120 dB of gain" in OPA134 is only valid for very low frequencies- at 10 kHz it has only about 60 dB gain - almost 20 dB lower than THS4131 (or OPA1632)

x-pro