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
geometry.
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.
73
Bill wa4lav
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