Op-amp Buffer

[johnferrier]

I'm interested in a emitter follower with active current source design to buffer op-amps with differential outputs. I've sketched a design:



If this seems reasonable, I probably will parallel the NPNs (4 per side) to obtain 80mA class A for each output.

Has anyone seen anything like this? I'm mostly interested in a transistor design non-complementary.

I don't mind dealing with heat issues. This amplifier is to drive headphones.

Comments welcome.


JF

[Christer]

To me it looks basically like two half diamond buffers in
bridge coupling. Have you considered using two full diamond
buffers instead? In theory and in measurements the diamond
buffer has excellent distorsion figures. I am sure different
people have different opinions on which sounds best. I have
no opinion as of yet, but I am currently working on a board
design with opams followed by discrete diamond buffers, so
I expect testing it in the near future.

[Bricolo]

since he wants class A, why should he bother with the other half of the diamond buffer?

I still don't understant the benefict of push pull class A compares to SE class A

[Christer]

Because a full diamond buffer is also class A, if properly biased
and, at least in theory, it lacks even-order distorsion when
the output transistors are matched, while a half diamond
buffer has quite a lot of second-order distorsion. I do not
know which of the two has the lowest amount of odd-order
distorsion, but the full diamond buffer performs excellently
in this respect as can be confirmed by calculations or SPICE
simulations, and has also been confirmed by measurements
published by Walt Jung.

[Bricolo]

hod did you measure distortion with spice?

[johnferrier]

Originally posted by Christer
Because a full diamond buffer is also class A, if properly biased
and, at least in theory, it lacks even-order distorsion when
the output transistors are matched, while a half diamond
buffer has quite a lot of second-order distorsion.

I'd like to know why half a diamond would have much higher second order distortion, then a full diamond. I wonder if it is because it slews on faster than off. Of course, I expect some distortion cancellation (especially, with second order) because I'm using a differential bridged circuit.

I don't see much "half diamond" circuitry. The old BUF-03 used more of a non-complementery emitter follower out. Of course, newer buffers use complementery diamond topology.

Well, igonoring the fact that non-comp emitter followers are rare (only because of inefficiency?), I'd like to find a reference that indicates that the full diamond performs better than the half diamond. Again, efficiency aside.

Thanks for the replies.


JF

[Christer]

Originally posted by Bricolo
hod did you measure distortion with spice?

I let my CPU sweat and give me some value for the money it
cost me.

Basically, I run a transient analysis of desired source amplitude
and frequency and then do an FFT. I always make sure to do
an FFT also on the source signal as a reference, to see that
the simulation parameters chosen give none, or sufficiently
low distorsion on the source signal. Empirically, but testing
various trade-offs I have settled on the following parameters
in most cases. I run 21 cycles, starting to collect data after the
first one, ie. 20 cycles are analysed in the subsequent FFT.
I also set the minimum step-length to 10E-5 times the cycle
length, ie. for a 10 kHz source signal, the step length is 1ns.
Then I FFT those signal, including the source, that interest me,
choosing a large number of data points, usually 1Meg. I have
found this to produce a noise floor of around -120dB or better,
which is usually sufficient. you can improve of that, but it takes
quite some time already with these parameter values, so it
is hardly worth it.

Just don't forget, no simulation is better than the models used
and the simulator may produce strange effects in certain odd
cases.

Originally posted by johnferrier


I'd like to know why half a diamond would have much higher second order distortion, then a full diamond. I wonder if it is because it slews on faster than off. Of course, I expect some distortion cancellation (especially, with second order) because I'm using a differential bridged circuit.

I don't see much "half diamond" circuitry. The old BUF-03 used more of a non-complementery emitter follower out. Of course, newer buffers use complementery diamond topology.

Well, igonoring the fact that non-comp emitter followers are rare (only because of inefficiency?), I'd like to find a reference that indicates that the full diamond performs better than the half diamond. Again, efficiency aside.

Thanks for the replies.


JF

The exponential transfer characteristic of a BJT causes a lot
of distorsion, both odd and even. In the SE (non-balanced)
case, you get all of this. With complementary output BJTs you
get a distorsion cancellation. I have done simulations on the
diamond buffer, and with perfectly matched BJTs the even
order distorsion seems to cancel out completely, but there is
still odd order distorsion. Changing either IS or BF of output
device, which corresponds to mismatch in Vbe or hfe respectively
will introduce also even order distorsion.
In the simulation I run, where I used a
rather high input signal and heavy load to see distorsion effects
clearly, I got -81dB 3rd order and no 2nd order distorsion.
mismatching BF by a factor 4 between the output devices gave
the same 3rd order distorsion, but introduced -86 dB 2nd order.
Mismatching IS by a factor 2 also gave the same 3rd order but
introduced -114dB 2nd order. BTW, these tests were done at
10kHz and using the procedure explained above.

Unfortunately, I do not remember the exact load conditions etc.
Since I was only interested in the effects of mismatching when
I did that analysis, I only jotted down the distorsion figures
on a paper.

After your recent question, I decided to try the SE (non-balanced)
version. I am not sure if the bias current and load resistance
are the same as in the previous simulation, but I think they
are about the same. The result I got was -60dB 2nd order
distorsion and still -80dB 3rd order distorsion. It thus seems
that the diamond buffer cancels even order distorsion only.
I also tried your balanced setup, but for some reason I cannot
figure out right now it didn't work. I think the reason is
that I took your variant with only series resistors on the inputs,
which does not bias the amp properly if the input signal is
differential. I would expect this setup to cancel distorsion
also, perhaps better in practice than a single, non-balanced
diamond buffer set-up, since you have two NPNs cancelling
each other, not complementary devices. That's just a guess
though.

As for referring to it as a half diamond buffer. Well, I thought
is sounded appropriate, but others may disagree. It is no
important issue what to call it, and that easily just leads to
a philosophical discussion.

[djk]

Can you re-wire your headphones for a four-wire plug instead of the common three-wire plug?

He will still get second harmonic reduction because of the bridge connection.

[Christer]

Originally posted by djk
He will still get second harmonic reduction because of the bridge connection.

Actually I said that I believed this to be the case and the
reduction possibly even better than for a (non-balanced)
diamond buffer.

OTOH, what I originally meant in my first post was bridging
two diamond buffers, which I suspect would be even better
than bridging to SE buffers.

[johnferrier]

Christer,

Thanks for taking the extra time to do this. And I like the idea of a bridged pair of diamond buffers--a double diamond--as being the perferred topology. I need to spend more time with your post to make sure I understand what you wrote.


JF