Sziklai and darlinton pairs

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boylesg

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Can some one please explain to me what open loop and closed loop followers are in relation to these?

I found this (https://www.audioasylum.com/forums/prophead/messages/4692.html) but it doesn't really explain what they mean exactly.

I have a pretty good idea of what a 'follower' is, e.g. a class A amplifier with no voltage or current gain where the input current and voltage equal the output voltage and current, generally used as a buffer between entities with mismatching input and output impedance.

And also can anyone suggest a practical sziklai and darling pair using BD139/BD140 and BC547/BC548 for example with an appropriate bypass resistor value?

Or else point me in the direction of a website that details construction of specific example pairs rather than just theoretical stuff?
 

Sziklai pairs, also known as Complementary Feedback Pair (CFP) have better thermal stability than emitter followers. In this configuration the output devices sit inside a local negative feedback loop. It also produces better linearity when utilized inside this loop. Overall, this is a better option than emitter followers, but can suffer from parasitic oscillations, which may explain why it is not used more often.

CFP stage can be designed with or without gain.
 

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Boylesg, here is my advice: Forget the text as given in the provided link. It contains a LOT of errors (it`s really "garbage")

*The author speaks about "gain" without any difference between current gain and voltage gain.
* He speaks about "followers" without adding any resistor to the transistor combination. In contrast,both combinations can be seen as a new transistor with modified properties - but NOT as "followers". Instead, both combinations can be used as common collector or common emitter stages - as you want.
* There is no negative feedback at all within the two-transistor combinations (without an external resistor)
 


Consider it disregarded then.

What about the resistor value? How do you determine what resistor value you should use to create a darlington pair?
 

boylesg said:
Consider it disregarded then.

What about the resistor value? How do you determine what resistor value you should use to create a darlington pair?
Click to expand...

As I have mentioned - the combination of both transistors alone is equivalent to a "new" single transistor (called "Darlington" transistor) with other properties as each one of this pair.
That means - no resistor is needed to form the Darlington combination. You treat this pair as a single transistor, which can be used - together with external resistors - to realize common collector or common emitter amplifiers.
 

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