Could some electronics gueu here clarify this a bit. Typical SPICE BJT models include the parameters RB, RC and RE wich add resistances at the respective terminals, and are small valued, e.g., 0.77 etc., Recently, I came across a reference in literature the input resistance of a transistor(under examination) is 1200.0 Uhms. Is it possible that the authors could have been referring to something else ? An AC analysis of a simple common emitter amplifier while measuring the voltage at the base, and current flowing into the base, gives a measure of the input resistance over that frequency rahge, but such calculated values are never that high. Any hints, suggestions would be helpful. Thanks in advance.
SPICE BJT model parameter question
Jul 28, 2018
3 Replies
"Digital transistors" have explicit series base resistors, or voltage dividers, so you can drive them directly from logic.
It's usually easier to use a mosfet.
John Larkin Highland Technology, Inc
lunatic fringe electronics
The DC input impedance is a resistance, because you can't phase-shift DC.
The input resistance of a common emitter stage is Rbb'+(beta+1)(rE+ Ree'
- RE), where Rbb' is the Spice RB parameter, Ree' is the Spice RE parameter, rE is 26mV/I_C (at room temperature), and RE is the emitter degeneration resistor.
At AC there are other important contributions, notably the input capacitance and Miller capacitance.
Cheers
Phil Hobbs
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510
http://electrooptical.net
http://hobbs-eo.com
Look up the hybrid-pi model.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website:
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