Voltage-controlled resistor
Voltage-controlled resistors are one of the most commonly used analog design blocks: adaptive analog filters,[1] automatic gain-control circuits, clock generators,[2] compressors,[3] electrometers,[4] energy harvesters,[5] expanders,[6] hearing aids,[7] light dimmers,[8] modulators (mixers),[9] artificial neural networks,[10] programmable-gain amplifiers,[11] phased arrays,[12] phase-locked loops,[13] phase-controlled dimming circuits,[14] phase-delay and -advance circuits,[15] tunable filters,[16] variable attenuators,[17] voltage-controlled oscillators,[18] voltage-controlled multivibrators,[19] as well as waveform generators,[20] all include voltage-controlled resistors.
[21] 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.
In these applications, low-noise JFETs allow more reliable and accurate measurements and heightened levels of sound purity.
JFETs can withstand electrical, electromagnetic interference (EMI) and other high radiation shocks better than MOSFET circuits.
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 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.
Or in other cases, when a constant resistor value is not required (for example, in LED dimmer applications and musical pedal-effect circuits).
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).
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.
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.
In order to overcome this problem, non-linearized VCRs are simply operated at fairly low signal levels.
For example, the simulation below shows a significant amount of visual distortion when the input signal of 5 V peak-to-peak is applied to a non-linearized VCR design.
On the other hand, a simulation of a linearized VCR design shows very little distortion when a 8 V peak-to-peak input signal is applied (Figure 7).
These designs offer improvements in dynamic range, distortion, signal-to-noise ratio and sensitivity to temperature variations.
Specifically, the linear regions of the IV curves determine the input signal range where the VCR will behave as a resistor.
The mathematics behind linearization resistors is directly related to the cancellation of the second degree VDS term in the JFET triode equation.
Kleinfeld[30] applies Kirchhoff's current law to prove that the VDS non-linear term cancels with linearization resistors.
Equal valued linearization resistors divide the drain-to-source voltage by 2, effectively cancelling out the non-linear VDS term in the JFET triode equation.
VCR designs are expected to play a central role in the advancement of artificial intelligence (neural) based sensor networks.
