Small signal behaviour of Darlington pair

seanvn · 20th March 2011, 12:01 AM

I created an audio filter circuit using an Darlington pair emitter follower for signals in the microvolt to millivolt range. The circuit didn't work as intended. At that signal level all I got was complete attenuation. Increasing the input signal to the hundreds of millivolts range the filter gave the expected unity gain response in the band pass. It is not a noise issue, so what is going on?
Simulation using LTSpice 4 didn't show any problem at the microvolt range but obviously something does change down in those regions. Would changing to a CFP configuration improve matters?

sofaspud · 20th March 2011, 12:07 AM

More info needed.
I'm wondering why you chose a Darlington, but it'd certainly help if we knew what transistors were used and how you have it biased.

seanvn · 20th March 2011, 12:48 AM

Darlington speech filter circuit

20th March 2011, 12:01 AM	#1
seanvn diyAudio Member Join Date: Sep 2010	Small signal behaviour of Darlington pair I created an audio filter circuit using an Darlington pair emitter follower for signals in the microvolt to millivolt range. The circuit didn't work as intended. At that signal level all I got was complete attenuation. Increasing the input signal to the hundreds of millivolts range the filter gave the expected unity gain response in the band pass. It is not a noise issue, so what is going on? Simulation using LTSpice 4 didn't show any problem at the microvolt range but obviously something does change down in those regions. Would changing to a CFP configuration improve matters?

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20th March 2011, 12:07 AM	#2
sofaspud diyAudio Member Join Date: Nov 2005 Location: San Antonio	More info needed. I'm wondering why you chose a Darlington, but it'd certainly help if we knew what transistors were used and how you have it biased.