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Old 9th March 2012, 03:17 AM   #1
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Default Another of Ken's bizarre circuits

This could be a front end with inverting or non-inverting VAS...
Distortion is really really low, given the simplicity and nothing
fancy parts. Second harmonic -120db down for smaller signals.

Anyways, mo' crazy abuse of shunt tail pairs. Resistor provides
an excess of unregulated the tail current. And CFP shunt regs
divert away all the low class electrons, leaving only the tastiest
regulated currents. Or some high scientifical nonsense like that...

Adds some gain in the process. Or more truthfully, pushing the
balance of existing gain unfairly toward the output side of the
pair. Thus shunt tail pair output current gain appears doubled.

I am kinda preferring the inverting version as shown. Holds
input transistors at constant voltage and current. No need for
cascode. And you get to push rails a little further apart for the
same VCE max. This is all because comparison to GND keeps
the input stage stationary between the rails.

Don't worry .models, its all in there. Just cropped the picture...
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File Type: gif Draft76_1V.gif (35.3 KB, 1226 views)
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Last edited by kenpeter; 9th March 2012 at 03:34 AM.
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Old 9th March 2012, 12:46 PM   #2
Elvee is offline Elvee  Belgium
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Interesting, but did you notice that CFPing also Q3 and Q7 improves the linearity, and more importantly makes it much more load-tolerant?
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Old 10th March 2012, 04:44 PM   #3
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Nah: maybe this is load tolerance, 400WRMS into 2ohms?
Mind you, won't go lower than 5Hz without violating SOA.
Brands might differ where the 100mS line theoretically is.
I am not recommending 2ohm load for this transistor.

Lets look the schematic for a sec: I got rid the 8K2 on
the GND side. Why? Cause it might double offset instead
of cancelling, issue of inverting. I could be wrong here,
its not something I've thought long enough to be sure...

Getting rid the 8K2 makes a little unstable, not sure why.
But adding a 47u fixes that, and generally helps in every
way that I can determine. Even if you leave the 8K2, the
new 47u should probably stay.

If you bias an output for Class B, you can Muntz out a
great many emitter resistors all around. The feedback
loop assures nothing will run away, as long the output
doesn't short. If output short happens, game over...

Warn: This sim includes some noise on the power rails
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Last edited by kenpeter; 10th March 2012 at 04:47 PM.
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Old 10th March 2012, 05:05 PM   #4
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As for Elvee's mod, violates some of my pet concerns that may not matter.
I would just add a diode to the input side to keep collector voltages same.
Then again, I am also about to claim that imbalance was to our advantage.

Certainly more linear open loop in CFP vs CFP balance, but half the useful
gain of that stage is pushed back. Open loop gain improves only because
a transistor got added. I suspect we could add this same gain without re-
balancing the shunted pair and throwing half of it away. But we close the
big loop and this extra gain is going to be thrown away anyhow, so the
distinction of where gain gets sacrificed to linearity is a mere preference.

If closed linearity ends same, I'd just go whichever way used least parts.

Last edited by kenpeter; 10th March 2012 at 05:25 PM.
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Old 10th March 2012, 08:20 PM   #5
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by kenpeter View Post
Nah: maybe this is load tolerance, 400WRMS into 2ohms?
Mind you, won't go lower than 5Hz without violating SOA.
Brands might differ where the 100mS line theoretically is.
I am not recommending 2ohm load for this transistor.

Lets look the schematic for a sec: I got rid the 8K2 on
the GND side. Why? Cause it might double offset instead
of cancelling, issue of inverting. I could be wrong here,
its not something I've thought long enough to be sure...

Getting rid the 8K2 makes a little unstable, not sure why.
Probably acts as a base stopper: generally, such a resistor interact with B-C capacitance of the transistor and acts as a self compensation.
Except of course with "funny" circuits, like the Xquad, where it has the opposite effect (BTW, I stumbled upon the prime originator of this delightful circuit: it's a guy called Raimondo Caprio, and he invented it back in 1973).
Quote:
But adding a 47u fixes that, and generally helps in every
way that I can determine. Even if you leave the 8K2, the
new 47u should probably stay.
That's strange: it probably prevents some kind of side mode mixing/conversion, but it shouldn't have a dominant effect on stability.
It could have an impact on linearity though, particularly on even order harmonics.

Quote:
If you bias an output for Class B, you can Muntz out a
great many emitter resistors all around. The feedback
loop assures nothing will run away, as long the output
doesn't short. If output short happens, game over...
I think it could be a nice class A driver, but a 10K load limitation for good performances is too restrictive. 1K or 600 ohm is more realistic
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Old 10th March 2012, 11:14 PM   #6
Bonsai is offline Bonsai  Taiwan
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What's the performance at 20k say 12 or 13v pk into 600 Ohms?
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Old 10th March 2012, 11:54 PM   #7
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Quote:
Originally Posted by Bonsai View Post
What's the performance at 20k say 12 or 13v pk into 600 Ohms?
Circuit of Post #3 with clean power rails: 20K sine, 26VPP out, into 600R.
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Last edited by kenpeter; 11th March 2012 at 12:02 AM.
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Old 11th March 2012, 05:42 PM   #8
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by kenpeter View Post
Circuit of Post #3 with clean power rails: 20K sine, 26VPP out, into 600R.
The 1µs timestep doesn't do justice to the circuit's performance.

But on the other hand, when it is shortened under 500ns, it unveils instabilities.
It can be cured by reintroducing the 8K2 resistor.
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Old 11th March 2012, 07:21 PM   #9
MiiB is online now MiiB  Denmark
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nope it svings in a rather spiky way....made some feedback into the input transistors and reduced gain to 8 db with at supply of 18 V.. inserted current-sources for instead of rail-resistors...magnificent Pre-amp with distortion at -140dB...at 4 Volts out PP...but it inverts...
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Old 11th March 2012, 09:19 PM   #10
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by kenpeter View Post
Second harmonic -120db down for smaller signals.
Wow, that's really tiny - somebody gotta build it....
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