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Sziklai pair
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Lucky-Luka:
Hi all
I've read that Sziklai is prone to high freq oscillations due to its feedback loop..
I can't figure out where this feedback loop actually is...
It should be obvious but I haven't understood this concept...
Happy new year.
xavier60:

--- Quote from: Lucky-Luka on December 31, 2019, 09:26:00 pm ---Hi all
I've read that Sziklai is prone to high freq oscillations due to its feedback loop..
I can't figure out where this feedback loop actually is...
It should be obvious but I haven't understood this concept...
Happy new year.

--- End quote ---
The feedback loop being referred to is likely to be in the other circuitry controlling the Sziklai pair.
I guess that  Sziklai pair would have a degraded frequency response compared to a single BJT, requiring some extra attention to the loop's frequency compensation.
It is good practice to put a bleeder resistor across B-E of the second transistor to improve turn off response and also frequency response.
Lucky-Luka:
I think I've read the loop I was talking about is in the picture.
This loop wasn't mentioned talking about the Darlington pair.
nfmax:
The feedback loop occurs within the Sziklai pair itself. Assume, for simplicity, that the input voltage is constant. A change in the collector current of Q3, acting across R2, causes the emitter voltage of Q6 to change. Since the input voltage is constant, this device operates in common base mode, so the change in Q6 emitter voltage causes a change in its collector current. But Q6's collector current is Q3's base current, so the change in Q6's collector current is amplified by Q3 and appears at its collector, completing the feedback loop.

At DC the sense of this feedback is negative. Q3's fT provides the dominant pole at the frequency where its beta starts to fall from its DC value. If the loop gain has not dropped to less than 0dB before the additional phase shift from Q6 reaches 90˚, the loop will oscillate. Too fast a device in the Q3 position, relative to Q6, is likely to give trouble.
Lucky-Luka:
Does the Darlington pair have this loop too?
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