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|28th December 2007, 01:50 AM||#11|
diyAudio Moderator Emeritus
In the thread you pointed, what do you think about this comment?
|28th December 2007, 09:33 AM||#13|
> Thank you for your work, it is outstanding.
> Cheers for that.
> Nice work
Thanks for the kind compliments... <takes deep bow>. The key element
of genius here, however, belongs to Mikhail Kulish and his very original
and brilliant adaption of the Darlington for accurate small-signal
voltage amplification. His circuit ranks with the LTP, Darlington,
Sziklai and Current-mirror as among the most fundamental building
blocks with two transistors. It still boggles my mind that this invention
took place over 50 years following the invention of the transistor!
I will, however, take credit for spotting the potential to extend the
circuit for some very practical use, in particular as a very highly
linear voltage follower/current gain stage.
> It neatly gets around the EF bugbear - Vbe compressive distortion.
Exactly. On another plane, it also allows us to use very primitive
non-linear amplification devices (say, home-brewed transistors
or products of a third-world manufacturing infrastructure, maybe
with a hfe =~ 50) and make a compound device that exceeds
the specifications of the finest 1st world semiconductor
manufacturer - that is the essence of Russian genius.
> I'm no expert in PSpice
I'd second the recommendation of Andy_C and many others on this forum
to start out with the free version of LTSpice/SwitcherCAD. It was really
painless to set up and start simulating discrete circuits with it. There's
also a lot of online fora to get help on it.
> EC was probably a concept better suited for laser trimmed chips, but not for discrete.
Generally very true - but this one seems simple enough even for a garage
tweaker of discrete circuits.
> impressive getting all those devices to sing in tune!
It's helped along a great deal by the hfe linearity of the Toshiba output
devices - it may not have been possible 20 years ago.
> would not efforts best be spent on the voltage amplifier?
For the sonics, certainly. There's far more to be gained there by
tweaking compensation, VAS & LTP characteristics, etc.
Perhaps the best approach is to use an ultra-linear voltage follower
as the output stage and forget about its sonics - and reserve the
sonic tweaking for the earlier, small-signal stages.
> what do you think about this comment?
You can certainly improve the basic emitter-follower by using a CCS to
supply emitter current. However, unless it is driving a very high impedance
load, it will show Vbe distortion due to the current swing in the load,
which causes the current through the emitter to vary, and hence Vbe
It is in this very respect that the Kulish is superior to the emitter follower -
changing the load at the corrected collector output of the Kulish alters
the voltage gain, but not the distortion characteristics. It is a pure
transconductance amplifier in this regard. This allows us to use the
Kulish as a building block in a multi-stage circuit, and obtain further
current gain through a pure current amplifier, e.g. a constant-hfe BJT.
|28th December 2007, 03:22 PM||#14|
Join Date: Jan 2003
Location: UK (south west)
do you have a reference for the Mikhail Kulish work ?
I was wodering if this koolish unit can be applied to LTP - especially with a differential VAS swinging lots of voltage
|28th December 2007, 04:01 PM||#15|
Join Date: Jul 2007
Location: Central Berlin, Germany
In LXG's first post there are the links you are after. In one thread Mr.Kulish himself gave further comment and data, additional to what Mr.Nikitin has written.
This -ish thing looks quite nice for a voltage output LTP.
I now simmed a basic Kulish stage (without a lot of tweaking, but I found interesting insights, plotting DC transfer slopes with varying parameters), gain=30, Vs=60V, Vin=200mV(p), the results are really worth further investigation:
Freq. THD 20Hz 0.000297% 200Hz 0.000328% 2kHz 0.001433% 20kHz 0.014027%
Harm.# Rel.Level Phase 1 1.000e+00 0.00° 2 1.392e-04 -180.94° 3 1.675e-05 -281.86° 4 2.138e-06 -358.77° 5 2.871e-07 -95.33° 6 4.273e-08 -183.83° 7 8.315e-09 -248.46°
Thank you, Linuxguru, for bringing this interesting topic back on table.
|28th December 2007, 04:06 PM||#16|
> do you have a reference for the Mikhail Kulish work ?
It was published in a Russian DIY/trade journal called 'Radio' in 12/2005.
The first page of the article can be downloaded here:
You'll need a djvu/djview reader to read it, and it's in Russian. The remaining
three pages are further down in the thread, but I could not read them even
with djvu-libre in Linux - it's missing something that's mandatory to render
> I was wodering if this koolish unit can be applied to LTP
My thoughts also - it can be used to replace JFET LTPs to obtain a highly
linear transconductance amp that can then drive a folded cascode or similar
I haven't simulated that yet, but I see no difficulty in principle.
Edit: Klaus, my pleasure. How do you easily read the FFT phase information in LTSpice? That H2 phase reversal of exactly pi is certainly interesting...
|29th December 2007, 11:19 PM||#17|
Join Date: Jul 2007
Location: Central Berlin, Germany
You won't see that phase stuff with the FFT feature in the waveform display, but when the FFT is done with the .FOURIER command it lists the phase relationships also -- which I find very useful, especially when you also want apply some means of error correction/cancelling, where you want to know if your distortion is compressive or expansive and what phase shift you need to cancel non-inphase/-counterphase signals.
I forgot to add the circuit I simmed, for reference (now please find it attached). I plotted transfer slopes with varying parameters and surprisingly found that the most linear transfer (constant slope) is not found with the textbook ratio of 1.039 but with a somewhat smaller value. Also the outer resistors' (R2, R4) ratio to the inner ones plays an important role as it seems, in my example it is about 0.8. The corresponding curve is the light green one, which the flattest of them all. The factors are 1.008 for the inner collector resistor and 0.8 for the outer to inner ratio.
I could improve the THD20 by a factor of more than 3 by the simple addition of a cap. This could be further reduced a bit (factor 4...5) at a cost of increased THD at LF by further imbalancing the resistor ratios. Right now THD sims in at:
As usual, to be taken with a grain of salt... but then again, any Vce never drop below 13V, so the lack of modelling the Early voltage drop a low Vce should not be too much of an issue.
A disadvantage of this "VAS" is the high supply of 90V (not 60V as I wrote wrongly in the first post). Which worries a little (Vceo of the 2N3904, but actual Vce is almost within limits with that given drive of 200mV). Spice doesn't care, for god's sake...
BTW, fist try on a LTP failed...
|30th December 2007, 09:17 AM||#18|
Klaus, thx for the info. I too had noticed that one could go a bit lower than the 3.9% in the simulation - probably a load-dependent factor.
> try on a LTP failed...
In what manner did it fail? Hard to bias, or Spice fails to coverge quickly, or is it worse than a conventional LTP under similar bias conditions?
I spent my time simulating a Class-AB emitter follower, simpler than the first one in this thread, with similar distortion numbers at small swings (<4 V). At high swings, it goes to around -55..-60 dB, which is still not too bad.
Here's the schematic:
|30th December 2007, 09:24 AM||#20|
And here's the FFT plot at 30V amplitude input (almost rail-to-rail swing), with both output transistors cutting off sharply for almost half the cycle. It's much worse than the small swing case,but still quite usable - GNFB can bring some of this under control:
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