Transistor as a variable resistor

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Discussion Overview

The discussion revolves around designing a circuit that utilizes a transistor, specifically as a voltage-controlled rheostat. The focus is on achieving a resistance range of 200 to 5500 ohms, with a controlling voltage from 0 to 3.3 volts, while maintaining high input impedance suitable for a DAC.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant seeks a circuit design for a voltage-controlled rheostat using a transistor, specifying the desired resistance range and input impedance.
  • Another participant suggests prototyping the circuit on a breadboard to test its functionality, implying that practical experimentation is necessary.
  • A suggestion is made to search for using FETs as voltage-controlled resistors.
  • Some participants propose that a FET with negative feedback from drain to gate can function as a linear resistor, noting that while the impedance may not be extremely high, it should be adequate for driving analog ICs.
  • There is mention of integrated circuits (ICs) that can serve as digitally controlled potentiometers, although one participant admits to not having used them.
  • A repeated point emphasizes the effectiveness of FETs with negative feedback for achieving a linear resistance, with a request for a circuit diagram illustrating this configuration.

Areas of Agreement / Disagreement

Participants express multiple approaches and suggestions regarding the use of transistors and FETs for the desired application, indicating that there is no consensus on a single solution or design method.

Contextual Notes

There are limitations regarding the specific circuit configurations and the assumptions about component behavior, particularly concerning the feedback mechanisms and their impact on impedance.

Who May Find This Useful

Individuals interested in circuit design, particularly those exploring voltage-controlled components and transistor applications in electronic circuits.

JerryG
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I am looking to design a circuit, most likely with a transistor, to act as a voltage controlled rheostat. The controlling voltage can range from 0 - 3.3 v and I am trying to get a resistance range of about 200 - 5500 ohms. The circuit should have fairly high input impedance since it will be controlled by a DAC. Does anyone know of such a design?
 
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I have not tried yet but you can try yourself on the bread board. Either your circuit works for this design or not or need some extra component for that. So better way is go ahead with your design and check the output.
 
Search for "FET as voltage controlled resistor."
 
Yes, the FET with negative feedback from drain to gate makes an excellent linear resistor with gate voltage. The impedance will not be super high due to tge negative feedback network but it's plenty high enough to be driven by analog ICs. The ohmic range will extend from near zero to near open circuit.
 
I haven't used them, but there are ICs that act as digitally controlled potentiometers.
 
Antiphon said:
Yes, the FET with negative feedback from drain to gate makes an excellent linear resistor with gate voltage. The impedance will not be super high due to tge negative feedback network but it's plenty high enough to be driven by analog ICs. The ohmic range will extend from near zero to near open circuit.

Antiphon, do you have a circuit diagram w/negative feedback?
 

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