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[Amps] bipolar input

To: AMPS <>
Subject: [Amps] bipolar input
From: Bill Fuqua <>
Date: Wed, 10 Jun 2009 12:05:54 -0400
List-post: <">>
I don't think you can model the input as a diode due to the Miller-Effect 
capacitance caused by the voltage gain of the
amplifier and the base-collector capacitance.
      Usually this is swamped out either by negative feedback or low 
driving impedance.
Unfortunately with transistors this capacitance can't be neutralized our 
tuned out because it
changes. In tubes this feedback capacitance is fixed due to the fixed 
    I often wonder about the "diode threshold voltage" and now may problems 
this oversimplification has
caused designers.  This simplification is OK for instances where the 
forward diode current is near the
current specified for the particular diode for the forward voltage drop spec.
     Great for power supply calculations and such but for other things not 
so great. PN diode junctions do not
have a "threshold" it is a continuous  down to zero volts. Silicon PN 
junction diodes can detect voltages down to
to millivolts.

Bill wa4lav


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