Sziklai and darlinton pairs

[boylesg]

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?

 

[E-design]

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.

 

[LvW]

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?

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)

 

[boylesg]

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?

 

[E-design]

Have a look at this site https://www.renardson-audio.com/

 

[LvW]

Consider it disregarded then.

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

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.