Sziklai emitter follower as voltage stabilizer - diyAudio
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Old 17th May 2004, 09:08 AM   #1
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Default Sziklai emitter follower as voltage stabilizer

It has been hinted at least by Elso Kwak that you could use a Sziklai pair emitter follower as stabilizer.

Is this a common practice? Quick simulations indicate that there is a bit lower output impedance at the cost of a few dB worse PSRR at the high end.

Circuit:
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Old 17th May 2004, 09:10 AM   #2
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Default Simulations

Output impedance:
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Old 17th May 2004, 09:12 AM   #3
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Lower output Z compared to what? I would expect it to be one or two orders of magnitude better than the single emitter follower.

Lower PSRR compared to what? This should be also (much) better. What are the resistor values? How did you define/measure the PSRR

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Old 17th May 2004, 09:16 AM   #4
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Sorry if i was unclear. Versus the single emitter follower that is.

PSRR:
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Old 17th May 2004, 09:17 AM   #5
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R1-2 100ohm
R1-4 1k
It was merely to set up an example but you think trimming the values will help?
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Old 17th May 2004, 09:43 AM   #6
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Hjelm,

Increasing R1-4 would help the PSRR as this is influenced by the ratio R1-4 and the zener dynamic impedance. Increasing R1-2 to say 1k would lower the Zout.

But final values can only bedetermined if you know the max load current, because that determines the Q-type for the pass element and the requirements for base current. But at any rate it should be a large improvement from a simple emitter follower.

Look at it this way: it's like a black box with 3 terminals. Now inside the bb you have an element that comes much closer to an ideal xsistor. Naturally, everything improves, generally speaking.

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Old 17th May 2004, 10:24 AM   #7
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Jan,

Thanks for the quick replies.
Tried increasing the R1-4 and got a broader dip in the PSRR but the maximum level was unchanged.

If i increased the R1-2 though the PSRR got worse at the high end and the output impedance decreased.

My quick conclusion is that there is,as always, a tradeoff.

Question is it seems worth doing since it is a simple change from the emitter follower why isn't it used more, is there a sonic problem with it? I see one nice feature in the output impedance which seems to be in the 300mohm range and constant in the audio band for these small signal transistors.
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Old 17th May 2004, 10:59 AM   #8
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Yes, there always is a trade-off, as with everything in life.

How do you test the PSRR? Where/what is the input, what the measuring point?

It IS done often, like in using an integrated regulator like the 317 series with an external pass xsistor. Same concept.

Don't ask me about the sonic implecations, I have tin ears.

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Old 17th May 2004, 11:27 AM   #9
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Jan,
I do not know if it is correct but i did a AC sweep at 1 (input) and read it out at three (output) and and did a 20*log10(Voltage_ratio)

I am not that experienced with spice but intuitively i thought it should say something.

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Old 18th May 2004, 08:43 AM   #10
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Post Sziklai

Hi Hjelm,
I found the Ziklai in the powersupply of the Rotel preamp.
Never tried it actually.
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