SPICE BJT model parameter question

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.

Reply to
dakupoto
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"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
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Reply to
John Larkin

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

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Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

Look up the hybrid-pi model.

Tim

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Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
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Reply to
Tim Williams

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