combining two transistors.

[makaze808]

Hello. Can two transistors be "tied" together to create a variable gain stage?

I have some low gain 2n5133's and I was hoping to get their combined gain to a muff level.

I am sure I found a tutorial on the net a while ago but have lost the link.

Thanks.

[Sanguinicus]

I think a Darlington Pair is what you're after. Google it. From what I remember (I'm a robotics engineer so dont quote me), the emitter from the first one connects to the base of the second. Soyoure basically multiplying the gain. I've never tried this in an audio circuit though.

[boogietone]

With a darlington pair, the current gain is estimated as the product of the hFEs of the two transistors. This can easily be on the order of 10,000. The minimum hFE of a 2n5133 looks to be 50. So, a pair of these should get you at least a gain of 2500.

[amptramp]

It is true that you can make a drlington pair act like the product of the two transistor gains, but watch out for speed limitations.  The base of the second transistor has very little to sweep out minority carriers.  To get appropriate speed, put a 1 K resistor from the 2nd base/1st emitter junction to the 2nd emitter to sweep away minority carriers.  If you have a source of an even lower voltage that does not exceed the reverse breakdown of the emitter-base junction, you can use that.

[earthtonesaudio]

Tying two transistors together to make more gain is an old problem with many solutions, pros and cons to each.

Cascode or series-shunt pair for extreme speed
Long Tailed Pair for differential in/out
Common emitter with a common collector output buffer lets you get away with big collector resistors
SRPP for high gain and moderately low output Z
Two common emitter stages in series makes the math simple