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Old 20th March 2011, 01:01 AM   #1
seanvn is offline seanvn  Viet Nam
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Default 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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Old 20th March 2011, 01:07 AM   #2
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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.
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Old 20th March 2011, 01:48 AM   #3
seanvn is offline seanvn  Viet Nam
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Darlington speech filter circuit
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