Darlington Bipolars as switches

[rtarbell]

Fundamental question here regarding BJTs used as switches (so they are in saturation mode, not active mode):

Why are darlington BJT transistors considered better than a single BJT when being used as a switch? I know that Darlingtons have a higher beta, but I thought that beta only applied in active mode, not when the transistor was being used as a switch...

Any thoughts?

-RT

 

[I_Forgot]

Beta is the ratio of collector current to base current for AC signals. At DC the ratio is MUCH lower. A transistor with a Hfe spec of 100 will often have a DC Ic/Ib that is <10 (and you won't sually find it on the data sheet). That means when using a transistor as a low frequency switch, you must drive the base very hard.

A darlington transistor configuration multiplies the Ic/Ib of the two transistors so it takes much less base drive to saturate the output.

I_F

 

[Curmudgeon]

As a general rule, Darlington BJT's are slow switchers; this might be a consideration. Nowadays, FET's are used very extensively for switching as they are fast(er) and less expensive/amp. They also are much more readily available in higher voltage ratings.

 

[poobah]

Darlingtons are consider better because they simply require less base current to achieve the same collector current. There is, as allways, a price to be paid. A single BJT will saturate at about 0.3 Volts or less (the voltage from collector to emitter), while a darlington will saturate at a higher voltage 1-1.5 Volts.