Transistor Used as a Resistor #2

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PaulEngineer

Joined Dec 21, 2016
172
The name transistor is a contraction of "transfer resistor".

Of course the transistor can be used as a resistor that is exactly what it is - a controllable resistance.

The resistance is between the collector and emitter - we control it by applying suitable signals to the base.

However there is no requirement for that resistance to be high or low, it is up to us to set its value. And yess transistors can be cheaper to fabricate on chip than resistors.

One question for you

The transistor has two regions of operation the active region and the saturated region, which do you think applies in this case?
Can i ask? Doesn't it have 3 operation regions? Its a cut-off, an active one, and the saturation one... Well as an electronic enthusiast, and still a long way to go, i guess its active region!

Moderator edit: New thread created from old thread:
https://forum.allaboutcircuits.com/threads/transistor-used-as-a-resistor.89256/
 

WBahn

Joined Mar 31, 2012
26,398

WBahn

Joined Mar 31, 2012
26,398
I guess you could say that cut off isn’t really “operation”, since it’s not doing anything when it’s cut off.
It's a perfectly valid region of operation, which merely describes regions in which the behavioral characteristics are reasonably described by a certain set of equations (or a certain reasonably simple verbal description).
 

sparky 1

Joined Nov 3, 2018
401
The original term was semiconductor triode. The terms transconductance and varistor were closest as a descriptive name.
Bell labs formed a committee in order to standardize the terminology. It was less confusing to adopt a general term and have subclasses of the general term. It is a descriptive name as a general terminology to which sub classification could be made.

Not to confuse a subclass such as an Field Effect Transistor that can form a voltage controlled resistor. When the drain current is reduced to a point where the FET is no longer conductive, the maximum resistance is reached. The voltage at this point is referred to as the pinchoff or cutoff voltage and is symbolized by VGS = VGS(off). Thus the device functions as a voltage- controlled resistor. The resistive part of that region is just prior to the cut-off voltage.
 
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LvW

Joined Jun 13, 2013
1,160
Quote: Of course the transistor can be used as a resistor that is exactly what it is - a controllable resistance.
The resistance is between the collector and emitter - we control it by applying suitable signals to the base.


No - that is wrong! The resistance between C and E does not fulfill Ohms law. It does not behave like a resistor. This applies also to the saturation region - in contrast to FETs where this "first" region (for small Vds voltages) can be used as a voltage -controlled resistor.
 

vanderghast

Joined Jun 14, 2018
48
To the opposite of a diode where the voltage become almost independant of the current (past a threshold), providing an almost vertical line on a graph I vs V, a BJT can be seen as a current (IC) being almost independant of the voltage VCE (neglecting Early effect, or at "low" VCE), producing an almost horizontal line on the graph IC vs VCE. But while the threshold voltage of a diode is fixed, the constant current of a transistor is "programmable" through VBE. As already pointed out, this is not as for a MOSFET where we can really observe V = R I with a variable R, but since it is VBE, not VCE, which controls IC, or I from C to E, and if you assume a small signal, for which the exponential can be approximated with a straight line, you may consider something like delta_VBE =R_pi times delta_IC. (Some author define R_pi differently, beware). And with that, you can see the "transfer of resistance", since it is not VBE which supply the power behind IC.
 
Do a Google search on "Muting Transistor". A dinosaur technology from analog audio days, for switching/selecting low-level audio channels. They do a reasonable job of behaving like a resistor, provided the Vce remains very low.
 
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