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Voltage-controlled resistor
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Voltage-controlled resistor
A voltage-controlled resistor (VCR) is a three-terminal active device with one input port and two output ports. The input-port voltage controls the value of the resistor between the output ports. VCRs are most often built with field-effect transistors (FETs). Two types of FETs are often used: the JFET and the MOSFET. There are both floating voltage-controlled resistors and grounded voltage-controlled resistors. Floating VCRs can be placed between two passive or active components. Grounded VCRs, the more common and less complicated design, require that one port of the voltage-controlled resistor be grounded.
Voltage-controlled resistors are one of the most commonly used analog design blocks: adaptive analog filters, automatic gain-control circuits, clock generators, compressors, electrometers, energy harvesters, expanders, hearing aids, light dimmers, modulators (mixers), artificial neural networks, programmable-gain amplifiers, phased arrays, phase-locked loops, phase-controlled dimming circuits, phase-delay and -advance circuits, tunable filters, variable attenuators, voltage-controlled oscillators, voltage-controlled multivibrators, as well as waveform generators, all include voltage-controlled resistors.
The JFET is one of the more common active devices used for the design of voltage-controlled resistors. So much so, that JFET devices are packaged and sold as voltage-controlled resistors. Typically, JFETs when they are packaged as VCRs often have high pinch-off voltages, which result in a greater dynamic resistance range. JFETs for VCRs are often packaged in pairs, which allows VCR designs that require matched transistor parameters.
For VCR applications that involve sensor signal amplification or audio, discrete JFETs are often used. One reason is that JFETs and circuit topologies built with JFETs feature low-noise (specifically low 1/f flicker noise and low burst noise). In these applications, low-noise JFETs allow more reliable and accurate measurements and heightened levels of sound purity.
Another reason discrete JFETs are used is that JFETs are better suited for rugged environments. JFETs can withstand electrical, electromagnetic interference (EMI) and other high radiation shocks better than MOSFET circuits. JFETs can even serve as an input surge-protection device. JFETs are also less susceptible to electrostatic discharge than MOSFETs.
Two of the more common and most cost-effective designs for JFET VCR are the non-linearized and linearized VCR design. The non-linearized design only requires one JFET, The linearized design also uses one JFET, but has two linearization resistors. The linearized designs are used for VCR applications that require high input-signal voltage levels. The non-linearized designs are used in low input signal level and cost-driven DC applications.
In the circuit on the figure, a non-linearized VCR design, the voltage-controlled resistor, the LSK489C JFET, is used as a programmable voltage divider. The VGS supply sets the level of the output resistance of the JFET. The drain-to-source resistance of the JFET (RDS) and the drain resistor (R1) form the voltage-divider network. The output voltage can be determined from the equation
An LTSpice simulation of the non-linearized VCR design verifies that the JFET resistance changes with a change in gate-to-source voltage (VGS). In the simulation (below), a constant input voltage is applied (the VDC supply is set to 4 volts), and the gate-to-source voltage is reduced in steps, which increases the JFET drain-to-source resistance. The resistance between the drain to source terminals of the JFET increases as the gate-to-source voltage becomes more negative and decreases as the gate-to-source voltage approaches 0 volts. The simulation below bears this out. The output voltage is about 2.5 volts with a gate-to-source voltage of −1 volt. Conversely, the output voltage drops to about 1.6 volts when the gate-to-source voltage is 0 volts.
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Voltage-controlled resistor AI simulator
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Voltage-controlled resistor
A voltage-controlled resistor (VCR) is a three-terminal active device with one input port and two output ports. The input-port voltage controls the value of the resistor between the output ports. VCRs are most often built with field-effect transistors (FETs). Two types of FETs are often used: the JFET and the MOSFET. There are both floating voltage-controlled resistors and grounded voltage-controlled resistors. Floating VCRs can be placed between two passive or active components. Grounded VCRs, the more common and less complicated design, require that one port of the voltage-controlled resistor be grounded.
Voltage-controlled resistors are one of the most commonly used analog design blocks: adaptive analog filters, automatic gain-control circuits, clock generators, compressors, electrometers, energy harvesters, expanders, hearing aids, light dimmers, modulators (mixers), artificial neural networks, programmable-gain amplifiers, phased arrays, phase-locked loops, phase-controlled dimming circuits, phase-delay and -advance circuits, tunable filters, variable attenuators, voltage-controlled oscillators, voltage-controlled multivibrators, as well as waveform generators, all include voltage-controlled resistors.
The JFET is one of the more common active devices used for the design of voltage-controlled resistors. So much so, that JFET devices are packaged and sold as voltage-controlled resistors. Typically, JFETs when they are packaged as VCRs often have high pinch-off voltages, which result in a greater dynamic resistance range. JFETs for VCRs are often packaged in pairs, which allows VCR designs that require matched transistor parameters.
For VCR applications that involve sensor signal amplification or audio, discrete JFETs are often used. One reason is that JFETs and circuit topologies built with JFETs feature low-noise (specifically low 1/f flicker noise and low burst noise). In these applications, low-noise JFETs allow more reliable and accurate measurements and heightened levels of sound purity.
Another reason discrete JFETs are used is that JFETs are better suited for rugged environments. JFETs can withstand electrical, electromagnetic interference (EMI) and other high radiation shocks better than MOSFET circuits. JFETs can even serve as an input surge-protection device. JFETs are also less susceptible to electrostatic discharge than MOSFETs.
Two of the more common and most cost-effective designs for JFET VCR are the non-linearized and linearized VCR design. The non-linearized design only requires one JFET, The linearized design also uses one JFET, but has two linearization resistors. The linearized designs are used for VCR applications that require high input-signal voltage levels. The non-linearized designs are used in low input signal level and cost-driven DC applications.
In the circuit on the figure, a non-linearized VCR design, the voltage-controlled resistor, the LSK489C JFET, is used as a programmable voltage divider. The VGS supply sets the level of the output resistance of the JFET. The drain-to-source resistance of the JFET (RDS) and the drain resistor (R1) form the voltage-divider network. The output voltage can be determined from the equation
An LTSpice simulation of the non-linearized VCR design verifies that the JFET resistance changes with a change in gate-to-source voltage (VGS). In the simulation (below), a constant input voltage is applied (the VDC supply is set to 4 volts), and the gate-to-source voltage is reduced in steps, which increases the JFET drain-to-source resistance. The resistance between the drain to source terminals of the JFET increases as the gate-to-source voltage becomes more negative and decreases as the gate-to-source voltage approaches 0 volts. The simulation below bears this out. The output voltage is about 2.5 volts with a gate-to-source voltage of −1 volt. Conversely, the output voltage drops to about 1.6 volts when the gate-to-source voltage is 0 volts.