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Old 5th July 2004, 07:46 AM   #21
sajti is online now sajti  Hungary
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Default Re: Thanks for the tip Danny

Quote:
Originally posted by mashaffer
I did as you suggested and googled around a bit. It looks like the load resistor would need to be on the emitter of the first transistor in the pair (I was trying to put it on the emitter of the last transistor).

So are we talking about something like this? The attached file is my stab at a Sziklai output. If I understand it right hFE of the pair is hFEq1 X hFEq2 so that Q1 could be a small signal device and only Q2 would actually pull large current.

Does that look like a workable design? I am not entirely sure about the stability of the bias currents.
I think You have to change the whole topolgy. In Your amplifier the VAS stage has very low output. You can apply other type output stage, with voltage gain, and some feedback, as well.

sajti
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Old 5th July 2004, 11:37 AM   #22
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Quote:
Originally posted by sajti


Try to add some small resistor between the emitter of the 2N3055, and the ground. It will set some local feedback, and increase the linearity.

sajti

I am not sure. the amp has a compounded pair configuration in the output stage and the "emitter" resistor for the pair is the 7.5ohm resistor sitting on the collector of 2n3055.

adding an emitter resistor to the 2n3055 may limited the voltage swing and adding distortion.
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Old 5th July 2004, 01:42 PM   #23
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from Millwood:
Quote:
And distortion is very sensitive to the bias of the first stage: a couple of kohm out and you will immediately see distortion.
Are you talking about changes in the value of the top resistor in the voltage divider bias network? I am also planning to try running higher current in the bias network and using a bootstrap resistor to raise the input impedence.

from Sajti:
Quote:
I think You have to change the whole topolgy. In Your amplifier the VAS stage has very low output. You can apply other type output stage, with voltage gain, and some feedback, as well.
Possibly. I also have some options for increasing the gain of the VAS stage. My plan is to try to get the best performance I can before adding interstage NFB and then add only what is necessary. I also plan to look at bootstrapping Rcq1 as well as current source or cascode on Q1.

Quote:
Try to add some small resistor between the emitter of the 2N3055, and the ground. It will set some local feedback, and increase the linearity.
That sounds like a good idea. I imagine that a fractional ohm power resistor would be about right.

As always all input is appreciated. I will look at that other sim software as well. It never hurts to have more than one similar tool on hand.
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Old 5th July 2004, 04:15 PM   #24
sajti is online now sajti  Hungary
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Quote:
Originally posted by millwood



I am not sure. the amp has a compounded pair configuration in the output stage and the "emitter" resistor for the pair is the 7.5ohm resistor sitting on the collector of 2n3055.

adding an emitter resistor to the 2n3055 may limited the voltage swing and adding distortion.

You have right. Sorry. This was my mistake....

sajti
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Old 6th July 2004, 12:35 AM   #25
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Default Simulation

I was able to get LTspice working first so I did some sims with it. Using the circuit attached I got the DC operating point sorted out and ran a transient response on which I did an FFT (it took quite a while for me to figure out that this was the way to do it).

On the next two posts I will attach the resulting FFT and THD calculations.
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File Type: jpg circuit1sch.jpg (97.2 KB, 181 views)
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Old 6th July 2004, 12:38 AM   #26
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Default The FFT

This is the FFT. HD2 is about 30dB below the fundamental and HD3 is about 44dB down.
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File Type: jpg circuit1fft.jpg (50.4 KB, 109 views)
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Old 6th July 2004, 12:44 AM   #27
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Default THD Spreadsheet

I downloaded a spreadsheet from the net which is supposed to calculate THD from the dB output of an FFT. The results are attached. This shows a bit over 3%. This is about what one would expect but is about 1/2 of what Millwood got so I am suspicious that I have done something wrong.

The interesting thing is that I tried using various layouts including GNFB, NFB around VAS only, Current source load for VAS, Cascode on VAS and standard darlington output. In no case was the distortion reduced by more than about 1% (NFB) and whenever HD2 was reduced (max 3dB) HD3 actually increased.

Got to do some more thinkin'.
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File Type: jpg circuit1thd.jpg (47.2 KB, 111 views)
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Old 6th July 2004, 01:52 AM   #28
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Just remember guys, the map is not the territory.

By that I mean that predicting distortion via simulations
is a very iffy business. Spice was developed for
IC designers, and I doubt that even they rely on it
too much for this sort of thing.
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Old 6th July 2004, 02:07 AM   #29
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the fft calculation in ltspice (and others I guess) seems to be quite sensitive to how long you let the simulation run, and windowing. You may want to let it run a little bit longer (I did 100ms and got something that looks different from yours).

I also used perfect transistors (too lazy to use real ones).

Anyway, it should be directionally accurate but not exactly accurate. so take it with a grain of salt.
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Old 6th July 2004, 03:05 AM   #30
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Default Nelson Pass and Mashaffer

Mr. Nelson.... the one and only is too much lonely, sorry if i capture the damn destiny of the special people that have more knowledge than a most of us.... lonely is an unavoided consequence...if wrong, better...if rigth... sorry to remember you that.

Mr. Mashaffer, i know how you can have Multisim 2001 free.

write directly to me:

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