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Ultrafast NoCap-ClassAB
Ultrafast NoCap-ClassAB
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Old 27th February 2005, 01:46 AM   #11
Eva is offline Eva  Spain
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I own a Hameg HM407-2 [40Mhz] and I've succesfully detected up to 65Mhz waveforms with it [undamped diode ringing from SMPS]

At such high frequencies you can no longer rely on amplitude or phase, but you know that there is a RF waveform here

I would start gathering money for an oscilloscope if I was you
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Old 27th February 2005, 01:51 AM   #12
Tom2 is offline Tom2  United States
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Default re: Ultrafast NoCap-ClassAB

re: Ultrafast NoCap-ClassAB


Does the circuit need a resistor or current source from the
negative node of the green led to ground (or neg rail)?

This would provide bias current for the led and bias (base current) for
the two associated pnp 2n5401 transistors.


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Old 27th February 2005, 03:13 AM   #13
jcx is offline jcx  United States
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resistance in parallel with output L provides damping of reasoance, cable complex Z and inductor L can have multiple high frequency reasonances and a low value R can soak up some of the energy - ideally the rising Z of the output L "disconnects" the amp output from cable C at high frequencies and the amp only sees the low value R ("grounded" by the cable C) as its high frequency load - making high frequency stability easier to assure
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Old 27th February 2005, 08:21 AM   #14
Alme is offline Alme  Ukraine
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Uh, sorry Mike, I didn't notice from the first sight that they are EMITTER resistors to 2N5401, not BASE. Now it looks even more impressive to me!
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Old 27th February 2005, 08:36 AM   #15
sajti is offline sajti  Hungary
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Default Re: Ultrafast NoCap-ClassAB


I working something same. But I think to use current source in the second stage as well. It's improve the balance, and the PSRR in the second stage too.
I think, there is capacitor in Your amplifier. But this cap built into the transistors. And if You use higher power transistors in the second stage, instead of 2N5401s, You will get more phase margin.
You can get more phase margin, if You increase the local feedbacks, and decrease the global.

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Old 27th February 2005, 11:11 AM   #16
MikeB is offline MikeB  Germany
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Hi guys !

Eva, as i make amps only as hobby it would be too much money
to buy a scope, i will try to borrow one somewhere...

Tom2, i just skipped this resistor in sim, the voltagesupply
in sims is 100% stable. On prototype the green led is of course
fed with a resistor.

Alme, it seems impossible to get a "standard" topology stable
without feedbackcompensation, with folded cascode it's different.

jcx, thanks for the explanation, i will add a 10ohm...

Sajti, welcome to the FC-club !
But for the case of the folded cascode, "slower" transistors reduce
phasemargin as they introduce delay. But the inputcapacitance
of the drivers connected to the vas do in fact "replace" these caps.
As i switched to doubledarlington these caps got very big, this has
the advantage that the vas follows exactly the voltage inside the
drivers, beeing syncron with the outputstage. This result in perfect
behaviour for fast transients, means squarewaves are reproduced
perfect, not a single sign of overshooting/ringing in the whole circuit.
As i use sligthly higher current through the FC than the diffamp
puts out, the vas-devices can never saturate because of slewlimits.
So, only diffamp saturates, but it does not enter reversebias to
have recover from...
I was inspired by the JLH/DoZ, also having no caps at all, and i had
the thought that this could be the reason for their sound.
I seem to be right, this amp shows the same sonic reproduction
for voices and dynamics.

I checked with a version using ccs instead of this, but it showed
far inferior transient speed (slewrate), using same currents.
A 200khz signal was fully triangulated, where the other version
was still sinus with reasonable thd. As i use diffampoutput
balanced followed by a currentmirror, this amp is fully balanced.

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Old 27th February 2005, 11:36 AM   #17
MikeB is offline MikeB  Germany
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Default updated schematic

Here is updated schematic using doubledarlington.
The current in vas is a bit small for this, but the amp sounds
slightly better this way...
The 2 100k in vas to gnd were necessary to extend openloopbandwidth
again to cover audioband again. Without, the sound was too dry...

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Old 27th February 2005, 12:11 PM   #18
jan.didden is offline jan.didden  Europe
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Interesting circuit. I have a question: I think your oscillation-fredom (in the sim) is very much due to the fact that you have set the gain quite high, some 45 times. What happens when you set the gain to 26 or 28, which is more common, it may be different. All feedback amps get more stable if you increase the closed loop gain.

How does the freq response look? Can you post that? That would also give an indication of the stability margin. You wouldn't want it to burst into oscillations when you connect a diffferent speaker!

What happens when you use a LED + resistor in the sim instead of the 1.8-something battery, that may be also more realistic also.

Jan Didden
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Old 27th February 2005, 01:35 PM   #19
MikeB is offline MikeB  Germany
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Hi Jan !

I checked your suggestion, i replaced the 1.87 voltage with 3 diodes
in series fed with 15k, (I had no adequate LED-model)
there are only subtile changes.

I checked with different closedloopgains, with gain of 26 results
in acsweep look identical, with gain 1:10 the acsweep gets critical,
but no oscillation yet. I decreased timestep from 10ns to 2ns to
be sure.

Attached is acsweep, blue is output before coil, green left base
of diffamp, red right base of diffamp. Feedback keeps always
below input, so should be safe.

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Old 27th February 2005, 01:37 PM   #20
MikeB is offline MikeB  Germany
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And the logarithmic view...
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