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Old 1st April 2003, 01:21 PM   #1
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Default New kind of compound emitter follower?? or - another weird circuit ;-)

Hi all. I was musing on emitter followers driven by opamps this afternoon (when I was supposed to be doing something else ) and all of a sudden this circuit idea popped into my head. Has anyone seen this before? Or have I finally invented something entirely new?
In the diagram Q1 emitter is held at approximately half rail and driven by the emitter lead. If for example Q1 emitter rose, it's base would be stationary and it's collector would rise as Q1 operates as a grounded base cct. Consequently Q2's base is driven upward pulling it's emitter upward and making an output signal and at the same time pulling Q1's base upward. This last event tends to subtract from the upward movement of Q1 emitter input so we have negative feedback. Q1 emitter moves up and it's base follows it almost exactly.
So then, we have a cct with a gain of 1 and all the internal gain is swallowed up by feedback. Late this afternoon I threw together a mosfet version first, using a 2N7000 for Q1. It is fairly fast,switching in 6 nS or so. Anyway, I used *no* gate resistors and the thing did NOT oscillate! That feedback loop must be FAST!
It worked but it's needs a bit of refining that I didn't have time to do. It may have some promise as something useful. The Vbe drift is almost entirely dependent on cold Q1, not hot Q2 so it should be easy to have stable bias in a comp symm version. And with so much gain ploughed into a tight feedback loop it should be very linear. On a square wave input the rise time was about 200nS and the fall time 50nS for a 15 volt step. That fall time is 300v per uS. I's jes' bustin' fer termorra ta come so's ah kin git orn an' try sum sterf.
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Old 1st April 2003, 02:03 PM   #2
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Hi Ciclotron!!

Why not change Q2 for a darlington...because as Q1 is a comum base stage it don't have current gain...so only Q2 is providing current gain!

The power suplly also must be well smothed as the base of Q2 is referenced to the + suplly and the emiter of Q2 is referenced to ground...so any ripple in the suplly will be amplified...

Regards

PS: I like creativ people!!
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Old 1st April 2003, 02:19 PM   #3
joensd is offline joensd  Germany
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Hey Circlotron isn´t your circuit similar with this circuit ?
If you go on like this you´ll definitely find something entirely new.

Shouldn´t you end your LM317-idea before exploring other galaxies?

BTW: I love "crazy" and simple circuit ideas; they always get me tempted to try. So keep them coming.
Where have you got your 100mH coils from?

Regards
Jens
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Old 1st April 2003, 03:56 PM   #4
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Default Similar...but diferents...

Quote:
Hey Circlotron isn´t your circuit similar with this circuit ?
The circuit of Ciclotron doesn't provid voltage gain...your circuit have voltage gain!!!

Ciclotron circuit have low output impedance...your circuit have high output impedance!!
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Old 1st April 2003, 04:44 PM   #5
jcarr is offline jcarr  United States
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Circlotron: When I need a compact voltage regulator, I occasionally incorporate something quite similar to the circuit that you posted. In my case, a constant-current circuit replaces the top resistor (or bottom resistor, for the opposite polarity). I nearly always use bipolars for Q1, but have used both MOSFETs and bipolars for Q2. Should you encounter any ringing or tendancy towards instability, I would suggest inserting a small capacitor from the Q1 collector - Q2 base node to ground. If you use a MOSFET for Q2, adding a small value gate-stopper resistor can also help control any tendancy towards oscillation.

This circuit works well and has shown itself to be completely trouble-free in practice.

best, jonathan carr
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Old 1st April 2003, 09:33 PM   #6
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Thumbs up Re: New kind of compound emitter follower?? or - another weird circuit ;-)

Quote:
Originally posted by Circlotron
Hi all. I was musing on emitter followers driven by opamps this afternoon (when I was supposed to be doing something else ) and all of a sudden this circuit idea popped into my head. Has anyone seen this before? Or have I finally invented something entirely new?
In the diagram Q1 emitter is held at approximately half rail and driven by the emitter lead. If for example Q1 emitter rose, it's base would be stationary and it's collector would rise as Q1 operates as a grounded base cct. Consequently Q2's base is driven upward pulling it's emitter upward and making an output signal and at the same time pulling Q1's base upward. This last event tends to subtract from the upward movement of Q1 emitter input so we have negative feedback. Q1 emitter moves up and it's base follows it almost exactly.
So then, we have a cct with a gain of 1 and all the internal gain is swallowed up by feedback. Late this afternoon I threw together a mosfet version first, using a 2N7000 for Q1. It is fairly fast,switching in 6 nS or so. Anyway, I used *no* gate resistors and the thing did NOT oscillate! That feedback loop must be FAST!
It worked but it's needs a bit of refining that I didn't have time to do. It may have some promise as something useful. The Vbe drift is almost entirely dependent on cold Q1, not hot Q2 so it should be easy to have stable bias in a comp symm version. And with so much gain ploughed into a tight feedback loop it should be very linear. On a square wave input the rise time was about 200nS and the fall time 50nS for a 15 volt step. That fall time is 300v per uS. I's jes' bustin' fer termorra ta come so's ah kin git orn an' try sum sterf.

...nice work circlotron..
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Old 3rd April 2003, 02:27 AM   #7
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Default Yessir, three bags full sir!

Quote:
Originally posted by joensd
Shouldn´t you end your LM317-idea before exploring other galaxies?
Nahhh. I always have several ideas on the go at the same itme so if I get a bit stale with one then I just swap to the other. Anyway, your wish is my command. I was in actual fact going to try an LM317 in this cct just for a laugh but it worked so well that I completely ditched any further attempts with discrete stuff. This cct is a killer! The supply voltage limit is set by the main mosfet and the little pnp bipolar. Another good thing is that the '317 doesn't handle the entire signal voltage swing, it only handles the difference between input and output of the main mosfet. That means the slew rate requirements of the '317 are greatly reduced. It has an internal gain of 80dB and this is multiplied by the gain of the mosfet and all of this is ploughed into linearising the mosfet.

Try it - it's a real rocket.
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Old 3rd April 2003, 11:57 AM   #8
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Another refinement that I haven't tried yet but will soon is to have a -15v negative floating rail as well, with the zero volt (+) point going to the output and the neg rail going to the collector of Q1 instead of Q1 going to ground as it is now. This way the collector-emitter voltage will be virtually constant. As the Q1 emitter goes downward for example with signal, the main fet source goes down and therefore the two floating rails go downward all by the same amount taking Q2 collector with it. The fact that the upper positive floating rail goes downward in this case by the same amount as the signal means that the mosfet gate pullup resistor voltage drop doesn't change and therefore the current in that vertical line all the way down through to Q1 is constant. Therefore Q1 Vbe stays constant during signal excursions too. See, the cct only *looks* simple. The point is, Q1 has constant current through it, constant voltage across it , and constant Vbe. Hopefully this is the recipe for the closest approach to zero distortion? I don't have a distortion meter, so could someone perhaps tell me how valid is this approach?
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Old 3rd April 2003, 12:21 PM   #9
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Default ... been there ...done that...

Hi Circlotron,

I used this configuration back in the early eighties as the input stage of the Tesserac Audio mc head amp that I designed ( with Ed Portelli). It also makes a terrific low impedance microphone pre-amp input stage.

It's a nice circuit for specific applications - I still haven't found a better one for mc input use.

ciao

James
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Old 4th April 2003, 02:08 AM   #10
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Hi James. wow! Do you think you could post the relevant part of the schematic so we could have a look-see?
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