Diamond buffers w. CFP outputs

[Christer]

Andy_c,

how did you get .fourier to work in LTSpice? It is not available in
the GUI, and explicitly adding it as a command in the schematic
didn't do anything. It is not long ago since I upgraded my version,
but maybe they have just included it??

BTW, I tried to rerun with a larger time step and switch off
compression. That made quite an improvement. It didn't give quite
the same figures, but close. It must obvioulsy be a lossy compression.
I'll make some more experiments with the other suggestions. My
PC shouldn't be that much slower than yours, so it seems worthwhile
experiimenting with.

[pjacobi]

Originally posted by Christer
[...]
how did you get .fourier to work in LTSpice? It is not available in
the GUI, and explicitly adding it as a command in the schematic
didn't do anything.[...]

View->Spice Error Log

This command displays the output of the SPICE engine itself, as would running a command line version would do.

Also usefull when running .op => gives table of active devices parameters at operating point.

Regards,
Peter Jacobi

[Christer]

Sounds almost a bit Microsoftish that one should have to view the
errorlog to get ordinary runtime output.

Edit:
Yes, I just tried and it worked fine.

BTW, I just tried to run with a rather large time step, but without
compression and suddenly it made a lot of difference to use
windowing in the FFT. I got almost the same result as before using
a 1us time step and a Hann window. Previously windowing has
made little of no difference, That compression must be terribly lossy.
That makes an old question much more relevant, how to know
which window to use? Empirical tries only, or is there some
rule of thumb on this?

[andy_c]

Kind of like exiting via the "start" button?

If you still can't duplicate my results I can post my .asc file if you'd like.

[Christer]

Originally posted by andy_c
Kind of like exiting via the "start" button?

Kind of what I was thinking of.

If you still can't duplicate my results I can post my .asc file if you'd like.

I just made a quick try as pjacobi explained and it worked to get
the fourier results that way. Too late do any more simulations
right now, but thanks for the help. I would never have have got
the idea of looking in the error log, especially since LTSPice gives
me the impression of being written by someone used to real OS's.

[Pedja]

Originally posted by Christer
That makes an old question much more relevant, how to know
which window to use? Empirical tries only, or is there some
rule of thumb on this?

For harmonic distortion measurements probably Hanning (Hann). Blackman will work too.

Pedja

[AKSA]

Christer & Guys,

This is very interesting. Can you pretty much specify the input impedance by simply setting the bias resistor? Could you achieve a 200K Zin, for example?

Christer, you say Class A. I presume by this you mean that none of the devices ever switches off; certainly the idle current is much lower than the 45mA you say it will swing into a load with 5Vpp.

Do you see application for the output stage of a power amplifier? For myself, having tried CFPs some time ago, I found that instability precluded this, but of course high idle Class A should be fine.

Cheers,

Hugh

[pjacobi]

Hi Aksa, All,

Originally posted by AKSA
This is very interesting. Can you pretty much specify the input impedance by simply setting the bias resistor? Could you achieve a 200K Zin, for example?

In the variants having current source biasing, Zin is simply "very high", with Rs you get 0.5 * beta * Rbias

Originally posted by AKSA
Christer, you say Class A. I presume by this you mean that none of the devices ever switches off; certainly the idle current is much lower than the 45mA you say it will swing into a load with 5Vpp.

The circuit in post #3 has Iq 30mA in the 'boosters'. As this is push-pull, up to 60mA output current is available, 45mA will have 7.5mA in one device and 52.5mA in the other.

Originally posted by AKSA
Do you see application for the output stage of a power amplifier? For myself, having tried CFPs some time ago, I found that instability precluded this, but of course high idle Class A should be fine.

Sure (for me at last). The idea is to build I giant current feedback OpAmp, input diamond, current mirrors, output diamond with CFP boosters.

But the instability wouldn't go away because of the nice looking topology alone. I've got a simulated amplifier stable with a outbuf buffer like that one:
http://www.linearaudio.de/scratch/DARL-BUFFER.pdf
http://www.linearaudio.de/scratch/DARL-BUFFER.asc

Note that the boosters became darlingtons, to have fast, high-beta BJTs in the diamond itself, so the whole would qualify for 'output triples'. Also complications arose to have some gain in the buffer, for the goal of getting the highest possible voltage swing.

Regards,
Peter Jacobi

[Christer]

Originally posted by pjacobi

In the variants having current source biasing, Zin is simply "very high", with Rs you get 0.5 * beta * Rbias

Maybe I am too tired to think right now, but wont the CCS be
in parallel with beta*Re for the output stage (assuming the
follower version). This shuld be the limiting factor, so very high
input Z seems somewhat diffiult to achieve for low Re values.
I might be wrong though, considering the late hour.

The circuit in post #3 has Iq 30mA in the 'boosters'. As this is push-pull, up to 60mA output current is available, 45mA will have 7.5mA in one device and 52.5mA in the other.

Agree, although, right now, I don't remember the exact figures
I got. I think I posted them earlier, though.


For the third question I am not so sure it would work without
some temp. comp. for the output bias, but maybe the buffer
actually does this withouth explicit temp. comp. I will have to think more about that.

[pjacobi]

Originally posted by Christer
Maybe I am too tired to think right now, but wont the CCS be
in parallel with beta*Re for the output stage (assuming the
follower version). This shuld be the limiting factor, so very high
input Z seems somewhat diffiult to achieve for low Re values.
I might be wrong though, considering the late hour.

You are right for then pure diamond.

For the #3 circuit, most of the output conductance is hidden from the diamond by the booster BJTs, which take almost all of the current swing. So the diamond's output stage is effectively also driving current sources. (Of value Vbe(Q6)/R10 = about 7mA).

Count this on the 'pro' side for using boosters. Without them, we would indeed be at beta1*beta2*Rload.

Regards,
Peter Jacobi