3 or more transistor darlington

ramonegumpert

Hello Experts,

My darlington pair cannot amplify the signal to the desired level.
To further amplify, is it as simple as adding another transistor (all NPN) where the Emitter of the second transistor is fed into the base of the 3rd transistor and this 3rd transistor collector is shorted to the collectors of the 1st and 2nd transistor?

And does it mean signals can be amplified like this infinitely?

regards
Ramone

ramonegumpert

Hi experts, please could you confirm if it is correct that i can amplify output from a darlington pair with a 3rd amplifer connected as described?

regards
Ramone

Averagesupernova

Lets start with a schematic.

Studiot

Yes you can add further transistors to the amplifying chain so long as you can overcome the problems.

1) Each transistor adds to the pedestal voltage.
2) The thermal and noise effects are multiplied by each added transistor.
3) You can arrange for the collector and base currents to match at each stage interface.
4) You can overcome the degradation of frequency response due to the capacitance of each additional stage.

No, the pedestal voltage increases with each additional transistor.

Having said all that, several famous manufacturers have successfully employed darlington triples in high power audio amplifiers.

go well

AlephZero

What AverageSupernova said.

The gain of the complete circuit, and the gain of the transistor, are two different things,
and only they are often intentionally orders of magnitude different from each other.

skeptic2

Also the change in bias and gain with temperature may be too much to be useful in the circuit. Out of curiosity I once designed in SPICE but never actually built a "darlington" using a FET and a BJT that seemed to work quite well.

uart

Why don't you post the details of the signal source that you need to amplify and we can make recommendations. There may be better ways to achieve it than simply increasing the gain of a composite transistor in a single stage design. Perhaps the reason (for not getting enough overall gain) is the source impedance, in which case a FET stage may be more appropriate (for example).