wasserkasten
Newbie level 6
definition of "resistance looking into"
Hello,
in the book "Microelectronic Circuits" (Sedra Smith, 5th Edition, page 446) the emitter resistance is defined as "... the small-signal resistance between base and emitter, looking into the emitter...".
The same formulation is used for the other direction "... the small-signal input resistance between base and emitter, looking into the base...".
For the MOS T-modell (page 296) "... the resistance between gate and source looking into the source is 1/gm" and the resistance looking into the gate would be infinity.
Why is the current different (once when looking into the gate/base and once when looking into source/emitter)?
Doesn't the voltage source (which is connected between the two corresponding pins) form a loop where the same current flows into/out of the same pins (similar to the port condition in **broken link removed**
If the port condition would hold reverting the signs of the voltage source would also revert the sign of the current and the resistance would be the same (?).
Or is "resistance looking into" defined differently (only one pin is used for current measurement)?
Thank you in advance.
- - - Updated - - -
OK, I think I got it by myself:
The definition of base resistance or emitter resistance is: incremental_voltage divided by incremental_current of the corresponding pin (base or emitter)
I mixed it up with the "classic Thevenin like approach" of connecting a voltage source and measuring the current.
- - - Updated - - -
OK, I think I got it by myself:
The definition of base resistance or emitter resistance is: incremental_voltage divided by incremental_current of the corresponding pin (base or emitter)
I mixed it up with the "classic Thevenin like approach" of connecting a voltage source and measuring the current.
Hello,
in the book "Microelectronic Circuits" (Sedra Smith, 5th Edition, page 446) the emitter resistance is defined as "... the small-signal resistance between base and emitter, looking into the emitter...".
The same formulation is used for the other direction "... the small-signal input resistance between base and emitter, looking into the base...".
For the MOS T-modell (page 296) "... the resistance between gate and source looking into the source is 1/gm" and the resistance looking into the gate would be infinity.
Why is the current different (once when looking into the gate/base and once when looking into source/emitter)?
Doesn't the voltage source (which is connected between the two corresponding pins) form a loop where the same current flows into/out of the same pins (similar to the port condition in **broken link removed**
If the port condition would hold reverting the signs of the voltage source would also revert the sign of the current and the resistance would be the same (?).
Or is "resistance looking into" defined differently (only one pin is used for current measurement)?
Thank you in advance.
- - - Updated - - -
OK, I think I got it by myself:
The definition of base resistance or emitter resistance is: incremental_voltage divided by incremental_current of the corresponding pin (base or emitter)
I mixed it up with the "classic Thevenin like approach" of connecting a voltage source and measuring the current.
- - - Updated - - -
OK, I think I got it by myself:
The definition of base resistance or emitter resistance is: incremental_voltage divided by incremental_current of the corresponding pin (base or emitter)
I mixed it up with the "classic Thevenin like approach" of connecting a voltage source and measuring the current.
