Darlington Bipolars as switches

rtarbell · 4th February 2006, 12:57 AM

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 · 4th February 2006, 02:07 AM

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 · 5th February 2006, 06:33 PM

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 · 5th February 2006, 06:47 PM

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.

4th February 2006, 12:57 AM	#1
rtarbell diyAudio Member Join Date: Sep 2005	Darlington Bipolars as switches 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

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4th February 2006, 02:07 AM	#2
I_Forgot diyAudio Member Join Date: Jan 2005 Location: Phoenix, Az.	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

5th February 2006, 06:33 PM	#3
Curmudgeon diyAudio Member Join Date: Nov 2005	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.

5th February 2006, 06:47 PM	#4
poobah diyAudio Member Join Date: Nov 2005	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.

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