Darlington Resistor

Steve Eddy · 7th January 2009, 05:48 AM

According to H&H, the base-emitter resistor that's sometimes used in Darlington pairs is intended to address two issues. One, to allow the driving device to turn off the output device much more quickly, and two, to prevent the driving device's leakage current from biasing the output device on.

That being the case, if the pair is going to be used class A and would never be turned off during normal operation, is there any particular need for the resistor?

In other words, is there anything more to the resistor than the two issues outlined by H&H?

se

latala · 7th January 2009, 05:22 PM

Yes It will also speed up the output transistor and also improve the vceo rating if you look up tables on power transister you will find the vceo rating increases when a low value resistor is placed between base and collector
regards Trev

jcx · 7th January 2009, 05:27 PM

you could add that the R establishes a fairly constant current bias for the driver Q which seems to be a good idea for better driver operation even with Class A output bias

without the R the bias for the driver is the base current of the output Q with all of the output Q Hfe sensitivities multiplying the output Q current modulation

with complementary EF "Darlington" output discrete stages a single driver bias R is often connected directly between driver emitters to provide them with a nearly constant current bias

mjf · 7th January 2009, 05:34 PM

hello.
perhaps for the reason of more thermally stability..........
greetings.....

7th January 2009, 05:48 AM	#1
Steve Eddy diyAudio Member Join Date: Sep 2002 Location: Sacramento, CA	Darlington Resistor According to H&H, the base-emitter resistor that's sometimes used in Darlington pairs is intended to address two issues. One, to allow the driving device to turn off the output device much more quickly, and two, to prevent the driving device's leakage current from biasing the output device on. That being the case, if the pair is going to be used class A and would never be turned off during normal operation, is there any particular need for the resistor? In other words, is there anything more to the resistor than the two issues outlined by H&H? se

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7th January 2009, 05:22 PM	#2
latala diyAudio Member Join Date: Feb 2006 Location: staffordshire	Yes It will also speed up the output transistor and also improve the vceo rating if you look up tables on power transister you will find the vceo rating increases when a low value resistor is placed between base and collector regards Trev

7th January 2009, 05:27 PM	#3
jcx diyAudio Member Join Date: Feb 2003 Location: ..	you could add that the R establishes a fairly constant current bias for the driver Q which seems to be a good idea for better driver operation even with Class A output bias without the R the bias for the driver is the base current of the output Q with all of the output Q Hfe sensitivities multiplying the output Q current modulation with complementary EF "Darlington" output discrete stages a single driver bias R is often connected directly between driver emitters to provide them with a nearly constant current bias

7th January 2009, 05:34 PM	#4
mjf diyAudio Member Join Date: Jan 2009 Location: upper austria/near linz	hello. perhaps for the reason of more thermally stability.......... greetings.....

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