This is true up to a point!
You have to load the diode VERY lightly in order to obtain a meaningful Dc
out of it and the true low level diodes is made from silicon with a slightly
different doping than regular diodes: there are the so called zero bias
detectors. With bipolars one strives to maintain as low as possible a bias -
close to zero, but not zero - at the point of minimum input of the driving
waveform. The intent is that when the input waveform is around zero, the
bias remains in force until the opposite transistor starts conducting. Under
these conditions the RF power transistors present an input impedance of a
few ohms. Being biased above the knee of the base emitter point ensures that
for most of the input cycle the transistor remains on. Besides, remember
that what really changes is the base current, not the voltage.
The Miller effect is not too troublesome, since it is dependent on the
source impedance, which is very low compared with the reverse admittance of
the transistor.
73 and all the best:
Alex 4Z5KS
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Bill Fuqua
Sent: Wednesday, June 10, 2009 7:06 PM
To: AMPS
Subject: [Amps] bipolar input
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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