leave it out if you don't
have the data. The value is
hre. I assumed the feedback
is 180 phase, so I swapped
the control wires. That
completes the model.
■ FIGURE C.
SwitcherCad designated as f; you
will have to tell it (right-click on the
symbol) the value which is hfe at
10 mA, and the voltage source that
provides the reference current.
Take a look at Figure C. I read hfe
to be 180. The output admittance is
hoe which I read to be 70 µohm.
The inverse is a resistance .0143
megohms = 14.3K. The voltage
controlled source, E1, models
internal feedback in the transistor.
The effect is small, so you can
I chose a load resistor,
R6, to give 15 volts drop at
10 mA and R7 = 150 ohms
for a gain of 10. The input
is one volt peak and the
output is close to 10 volts
peak ( 20 volts peak to
peak). The comparison
circuit (Figure D) using the
If you don't have a datasheet
but you have the transistor, you
can calculate and measure the
For input impedance, use:
hie = .026*hfe/Ie where Ie = Ib+Ic and
hfe = Ic/Ib. For output impedance,
set the collector current at the
desired value, then vary the
collector supply voltage and
measure the collector voltage and
QUESTIONS & ANSWERS
current. The change in collector
voltage (∆V) divided by the change
in collector current (∆Ic) is the
output resistance. I don't know
how to measure hre so leave
■ FIGURE D.
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