Diamond buffers w. CFP outputs

[Christer]

This thread is a continuation of a discussion going on in the thread
about Per-Anders' CFB headphone amp. Since this started to turn
into a long threadjacking, I decided it was appropriate to do as
pavel suggested and start a new thread. Maybe the moderators
will try to move some of the old discussion over here.

The topic that arose and is the topic of this thread is the suggstion
to use CFP outputs in Diamond buffers instead of ordinary followers.
Maybe the topic is about CFP outputs in general too, we'll see.

[Christer]

I have now done some more simulations to compare the two types
of buffers. I did a number of experiments yesterday, but it turned
out that in both cases the distorsion is so low that I had to increase
the resolution of the transient analysis to get a reasonable noise
floor. The simulations now take about two hours each on my machine,
so I only have two basic cases to present for now.

Since the main point was CFPs in the output stage vs. followers as
in the standard version of the diamond buffer, I have started with
a rather idealized case to try pinpointing the effects of this difference
only. I use ideal BJT models with only IS and BF specified and I use
ideal current sources in the input stage. The results seem to correlate
rather well with PMAs claim of a tenfold improvement in distorsion
when using CFPs.

Below follows the circuits I simulated. First the standard buffer

Note that the BJT models used for the input stage are intended
to be similar to small-signal devices and those for the output stage
similar to power devices.

[Christer]

And here is the CFP version. I use the small-signal type of BJT for
the "drivers" of the CFP pairs, and the power type for the "boosters"
(or output devices).

Also note that for both buffers, the pairs are perfectly complementary
by using ideal models.

Further note that I had to increase the emitter resistors in the input
stage to get approximately the same bias current in the output stage
for both cases. Both buffers run in class A with approximately 34mA
quiescent current through the output emitter resistors.

[Christer]

I used an input source of 5V pk, 10kHz in a 100 Ohm load (or 110
Ohm if one counts also the 10 Ohm "isolation" resistor) in order to
get rather high distorsion figures. The simulation was done using
a 21 cycle transient analysis, discarding the first cycle, and with a
mnimum time step of 100ps. I then ran a FFT on the input and
output signals using 0.5Mpoints and no windowing. There is no
trace of even-order distorsion at all when using perfectly
complementary transistor pairs, which concurs with earlier
experiments I have done on the effects of mismatching transistors.

The odd-order distorsion for the standard buffer is approximately
as follows:
3rd: -79dB,
5th: -106dB,
7 th -132dB

and the spectrum looks like this

[Christer]

For the CFP version the distorsion figures are
3rd: -96dB,
5th: -112dB,
7th: -128dB

and the spectrum looks as below. There is more noise in this case.
For some reason the noise is present already in the input signal
which I don't quite understand since Spice uses ideal voltage
sources. Maybe it is some roundoff errors or some FFT artifact.
Hence, the higher order products are less reliable here, but we
clearly have a lower 3rd order distorsion than in the standard buffer,
but maybe there is a tendency towards slightly higher values for
high-order harmonics. The latter could be due to the noise artifacts
too, though.

Furthermore, please note that these are not to be treated as
absolute figures reflecting a real circuit, but only an attempt to
study the relative effects of followers vs. CFP outputs.

[pjacobi]

Christer,

Is the effect you've seen class A vs class B?

I'd suggest running diamond alone case with higher Ibias so that it also stays in class A.

Then both versions should be more close together, especially using the ideal transistors.

Regards,
Peter Jacobi

[Christer]

They both run in class A. The load current swings about 45mA pp,
the current through the emitter resistors stay above 12mA all the
time and for the CFP case, the Ic of the power transistors stay
well above 5mA all the time.

[PMA]

Christer,

nice work. Probably noise about -150dB level is not so important. Have you tried to increase R1 and R2 to get higher quiescent current? I would guess that this could lower THD values. And maybe decrease R3 and R4 values. This might be pretty tricky work to find an optimum.

Regards,
Pavel

[Christer]

Originally posted by PMA
Christer,

nice work. Probably noise about -150dB level is not so important. Have you tried to increase R1 and R2 to get higher quiescent current? I would guess that this could lower THD values. And maybe decrease R3 and R4 values. This might be pretty tricky work to find an optimum.

Regards,
Pavel

Maybe I have on some earlier occasion, with less resolution in
the simulation, but I think not. Both buffers seem to stay well
within class A, but maybe some more margin
would help. I might try that and maybe some other things, but it
is a bit tedious to do too many experiments when so long simulations are required to get useful data.

I did some experiments with R9 and R10 yesterday, but in shorter
simulations, where it was harder to distinguish noise and distorsion.
I tried 10, 100, 1k and 1G and out these values, 100 Ohms seemed
best, but the noise made those simulations less reliable for
comparisons.

Another question is also whether even these two types of
ideal BJTs are a reaonable combination.? Maybe the IS differs
too much to correspond to a realistic combinations? Well, anything
close to realistic would have to include a lot more parameters
anyway.

[ojg]

Thanks Christer, very interesting!

Maybe you can find the time to post some AC-plots with gain and phase also? You would need to use "real" transistor models for this though.

I don't think you should worry to much about the high-frequency noise. This is an artifact of the FFT. In the FFT window try switching between "Using extent of simulation data" and "Specify time range". In the text-box change the 4.999998ms to 5.0ms (numbers for example only, I think you see what I mean).