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.

Usages

Voltage-controlled resistors are one of the most commonly used analog design blocks: adaptive analog filters,Jafaripahah, M.; Al-Hashimi, B. M.; White, N. M. (2004, May). Design Consideration and Implementation of Analog Adaptive Filters for Sensor Response Correction. Proceedings of the ICEE2004. automatic gain-control circuits, clock generators,Greason, Jeffrey K. (1983). Voltage-Controlled Resistance Element With Superior Dynamic Range. U.S. Patent US 5264785 A US 5264785 A. compressors,Sherwin, Jim (1975, August). electrometers,Wee, Keng Hoongl; Sarpeshkar, Rahul (1986) JFET Ohmic Differential Amplifier, Keithley Instruments, U.S. Patent. energy harvesters,Schneider, Leif E.; Thompson, Kevin D. (2014). Self-optimizing Energy Harvester Using Generator Having a Variable Source Voltage. Perpetua Power Source Technologies, Inc. U.S. Patent US 8664931 B2. expanders,Sherwin, Jim (1975, August). hearing aids,Madaffari, Peter L. (2000). Amplifier with Reduced Input Capacitance. Tibbetts Industries, inc. U.S. Patent US 6023194 A. light dimmers,Ballenger, Matthew; Kendrick, George (2006). Lamp With Integral Voltage Converter Having Phase-Controlled Dimming Circuit Containing a Voltage Controlled Resistor. Osram Sylvania, Inc. U.S. Patent, US 20060082320 A1. modulators (mixers),Stoffer, C. Daniel W (1971). Balanced Modulator With JFET's Voltage Controlled Resistors. Collins Radio Company, U.S. Patent US 3621473 A. artificial neural networks,Sung-Dae, Lee; Won-Hyo, Lee; Kang-Min, Chung (1998). A Highly Linear Voltage Controlled Resistor for Neural Chip. Systems, Man, and Cybernetics, 1998. 1998 IEEE International Conference. programmable-gain amplifiers,Molina, Johnnie F.; Stitt II, Mark; R., Burt, Rodney. (1994). Programmable Gain Amplifier Circuitry and Method for Biasing JFET Gain Switches Thereof Burr-Brown. U.S. Patent US 5327098 A. phased arrays,Electrosmash. MXR Phase 90 Analysis Phasers. www.electrosmash.com. phase-locked loops,Tsai, Tsung-Hsien; Hung, Tsung-Hsien, Chen, Chien-Hung; Yuan, Min-Shueh (2010) Phase Lock Loop (PLL) with Gain Control. Taiwan Semiconductor Manufacturing Company. U.S. Patent US7786771 B2. phase-controlled dimming circuits,Ballenger, Matthew B; Kendrick, George B. (2010). Lamp With Integral Voltage Converter Having Phase-Controlled Dimming Circuit Containing a Voltage Controlled Resistor. Osram Sylvania Inc., U.S. Patent US 7839095 B2. phase-delay and -advance circuits,FETs as Voltage Controlled Resistors, (1997, March). Vishay. tunable filters,Kumngern, Montree; Torteanchai, Usa; Dejhan, Kobchai (2011, April). Voltage-Controlled Floating Resistor Using DDCC, RadioEngineering. variable attenuators,McCarthy, Daniel P.; Connell, Lawrence E; Hollenbeck, Neal W. (2009) Linear Voltage Controlled Variable Attenuator With Linear db/v Gain Slope. FreeScale Semiconductor. U.S. Patent US 20090143036 A1. voltage-controlled oscillators,Griffen, Jed D. (2002). Highly Accurate Voltage Controlled Oscillator With RC Circuit. Intel Corp. U.S. Patent US 6498539 B2. voltage-controlled multivibrators,High-Voltage Electric Stimulation Apparatus for Plants (2012). 西藏农牧科学院蔬菜研究所 China Patent CN 202285631 U. as well as waveform generators,Symons, Pete (2013). Digital Waveform Generation. Cambridge University Press. P. 33. 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.VCR11 Voltage Controlled Resistor. Linear Integrated Systems. 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.Maxwell, John (1976), AN-6602Low Noise JFET – The Noise Problem Solver. Fairchild Semiconductor.

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.Levinzon, Felix (2014). Piezoelectric Accelerometers with Integral Electronics. Springer, P. 75. JFETs can even serve as an input surge-protection device.Yang, Eric; Milic, Ognjen; Zhou, Jinghai (2011, Nov), Input Surge Protection Device Using JFET, Monolithic Power Systems, Inc. U.S. Patent US 8068321 B2. JFETs are also less susceptible to electrostatic discharge than MOSFETs.Roundree, Robert Newton (2014, Nov). JFET ESD Protection Circuit for low Voltage Applications. U.S. Patent US 20140339608 A1.

Voltage-controlled resistor design

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.

Non-linearized VCR design

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

Vout = VDC · RDS / (R1 + RDS).

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.

With a 4-volt input signal and R1 of 300 ohms, the range of resistance for the JFET VCR can be calculated from the simulation results as VGS varies between −1 volt and 0 volts using the equation

RDS = V0 · R1 / (VDS − V0).

Using the above equation, at VGS = −1 V, the VCR resistance is about 500 ohms, and at VGD = 0 V, the VCR resistance is about 200 ohms.

Applying a ramp voltage to the input of a similar VCR circuit (the load resistor has been changed to 3000 ohms) allows one to determine the exact value of the resistance of the JFET as the input voltage is varied.

The ramp simulation, below, reveals that the drain-to-source resistance of the JFET is fairly constant (about 280 ohms) up until the input sweep voltage, Vsweep (Vsignal), reaches about 2 V. At this point the drain-to-source resistance starts to rise slowly until the input voltage reaches 8 V. At around 8 V, for this bias condition (VGS = 0 V and R = 3 kΩ), the JFET drain current (ID(J1)) saturates, and the resistance is no longer constant and changes with an increase in input voltage. The ramp simulation also indicates that even below 2 V, the VCR's resistance is not completely independent of the input voltage level. That is, the VCR resistance does not represent a perfectly linear resistor.

Because the resistance is not constant above 2 V, this non-linearized VCR design is most often used when the input voltage signal is below 1 V, such as in sensor applications or in applications where distortion is not a concern at higher input voltage levels. Or in other cases, when a constant resistor value is not required (for example, in LED dimmer applications and musical pedal-effect circuits).

Linearized VCR design

To increase the dynamic range of the input voltage, maintain a constant resistance over the input signal range, and to improve the signal-to-noise ratio and total harmonic distortion specifications, linearization resistors are used.

A fundamental limitation of voltage-controlled resistors is that input signal must be kept below the linearization voltage (approximately the point when the JFET enters saturation). If the linearization voltage is exceeded, the voltage control resistor value will change both with the level of the input voltage signal and the gate-to-source voltage.FETs as Voltage Controlled Resistors, (1997, March). Vishay.

For the evaluation of this design's ability to handle larger input signals, a ramp is applied to the VCR input. From the results of the ramp simulation, how closely the VCR emulates a real resistor and over what range of input voltages the VCR behaves as a resistor is determined.

The linearized VCR ramp simulation, below, indicates that the VCR resistance is constant at approximately 260 ohms for an input signal range from about −6 V to 6 V (the V(Vout)/I(R1) curve). The sweep also indicates that the VCR resistance starts to dramatically increase, as does in the non-linearized design, once the JFET enters its saturation region.

Because of the linearized VCR's wider constant resistance region, much larger input signals than the non-linearized designs can be applied to the VCR without distortion. However, it is also important to consider that the drain resistor value will slightly affect the range of drain-to-source voltages that the VCR resistance is constant.

Because of the increased linearization range, the linearized circuit is able to handle AC signals that are in the order of 8 V peak-to-peak before visual levels of distortion set in. The simulation below, which uses a 3000-ohm drain resistor, illustrates that the VCR can be successfully used at fairly high input voltage input signals. For this design, the 8 V peak-to-peak input voltage signal can be attenuated from 2.2 volts peak to 0.5 volts peak when the control voltage is varied from −2.5 volts to 0.5 volts.

What is important to note about the linearized VCR design, as opposed to the non-linearized design, is that the output signal does not have any significant offset. It stays centered at 0 V as the control voltage is changed. Simulations of the non-linearized design indicate a significant offset voltage at the output. Another important characteristic of the linearized VCR design is that it has a higher output current than the non-linearized design. The effect of the linearization resistors is to effectively increase the transconductance gain of the VCR.

Resistance range selection