Bias chain current for SAP16

cs · 19th November 2008, 03:11 PM

The datasheet for the SAP16 temperature compensated darlington transistors recommends a bias current of 2.5mA down the diode biasing chain. In a practical power amp, this current would also flow through the VAS stage. But 2.5mA is quite low for the VAS, and I would normally use 2 or 3 times that.

Has anyone tried the SAP devices at higher bias chain currents, and if so, does the temperature compensation still work OK ?

Chris.

AndrewT · 19th November 2008, 03:18 PM

the bias chain of diodes is ~ 5diodes + a variable resistor.
What voltage do you expect across this combination? Maybe 2.5V?
what parallel resistor would be required to pass the excess current through the VAS? Maybe 470r.
Leave a couple of extra pads to add one or two parallel resistors to trim the diode current once you get it running.

cs · 19th November 2008, 03:21 PM

Hi Andrew,

Yes, that did occur to me, although I was concerned that the compensation effectiveness would be reduced.

Chris.

19th November 2008, 03:11 PM	#1
cs diyAudio Member Join Date: Jun 2005 Location: .	Bias chain current for SAP16 The datasheet for the SAP16 temperature compensated darlington transistors recommends a bias current of 2.5mA down the diode biasing chain. In a practical power amp, this current would also flow through the VAS stage. But 2.5mA is quite low for the VAS, and I would normally use 2 or 3 times that. Has anyone tried the SAP devices at higher bias chain currents, and if so, does the temperature compensation still work OK ? Chris.

19th November 2008, 03:18 PM	#2
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	the bias chain of diodes is ~ 5diodes + a variable resistor. What voltage do you expect across this combination? Maybe 2.5V? what parallel resistor would be required to pass the excess current through the VAS? Maybe 470r. Leave a couple of extra pads to add one or two parallel resistors to trim the diode current once you get it running. __________________ regards Andrew T.

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19th November 2008, 03:21 PM	#3
cs diyAudio Member Join Date: Jun 2005 Location: .	Hi Andrew, Yes, that did occur to me, although I was concerned that the compensation effectiveness would be reduced. Chris.

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