Samuel Hunter Christie was a British scientist and mathematician.He was particularly interested in magnetism, studying the earth's magnetic field and designing improvements to the magnetic compass. Some of his magnetic research was done in collaboration with Peter Barlow. He became a Fellow of the Royal Society in 1826, delivered their Bakerian Lecture in 1833 and served as their Secretary from 1837 to 1853. In 1833 he published his 'diamond' method, the forerunner of the Wheatstone bridge, in a paper on the magnetic and electrical properties of metals, as a method for comparing the resistances of wires of different thicknesses. However, the method went unrecognised until 1843, when Charles Wheatstone proposed it, in another paper for the Royal Society, for measuring resistance in electrical circuits. Although Wheatstone presented it as Christie's invention, it is his name, rather than Christie's, that is now associated with the device.
First, to achieve voltage variable resistance operation, we must operate in the linear region. Otherwise, the current is either a constant regardless of drain voltage (saturation region) or is approximately zero (cutoff due to the capacitor being in either accumulation and depletion). Jan 23, 2019 FETs in (JFETS, MOSFETs, whatever) general can be used as variable resistors. One of the tricks to doing this over a wide range is to use a pair of FETs with one correcting for the non-linear Vgs versus Rds of the other. A small amount of positive feedback from the drain to the gate may be used to improve nonlinearity. Oct 13, 2017 The answer is yes (e.g., VCR11), but it turns out that virtually any other FET (e.g., JFET and MOSFET) can be used as a voltage controlled resistor. Basic voltage controlled resistor (VCR) circuits. One of the simplest uses of a voltage controlled resistor is an electronically controlled attenuator or “volume control”. The field effect transistor as a voltage controlled resistor We consider the use of a n-channel FET as a voltage controlled resistor where the resistance between the drain and source is controlled by the gate-source voltage. There are two distinct regions. In the Ohmic region1, the drain-to-source current, I DS, depends on the gate-to-source. In the picture shown, can 'Output' be controlled to be 0V or 12V based on 'Control'?Will be Drain and Source the way it is connected be a problem?
Variable resistor
Adjustable resistors
A resistor may have one or more fixed tapping points so that the resistance can be changed by moving the connecting wires to different terminals. Some wirewound power resistors have a tapping point that can slide along the resistance element, allowing a larger or smaller part of the resistance to be used.
Where continuous adjustment of the resistance value during operation of equipment is required, the sliding resistance tap can be connected to a knob accessible to an operator. Such a device is called a rheostat and has two terminals.
Potentiometers
A common element in electronic devices is a three-terminal resistor with a continuously adjustable tapping point controlled by rotation of a shaft or knob. These variable resistors are known as potentiometers when all three terminals are present, since they act as a continuously adjustable voltage divider. A common example is a volume control for a radio receiver.
Accurate, high-resolution panel-mounted potentiometers (or 'pots') have resistance elements typically wirewound on a helical mandrel, although some include a conductive-plastic resistance coating over the wire to improve resolution. These typically offer ten turns of their shafts to cover their full range. They are usually set with dials that include a simple turns counter and a graduated dial. Electronic analog computers used them in quantity for setting coefficients, and delayed-sweep oscilloscopes of recent decades included one on their panels.
Best Mosfet As A Variable Resistor
Resistance decade boxes
Variable Resistor Purpose
A resistance decade box or resistor substitution box is a unit containing resistors of many values, with one or more mechanical switches which allow any one of various discrete resistances offered by the box to be dialed in. Usually the resistance is accurate to high precision, ranging from laboratory/calibration grade accuracy of 20 parts per million, to field grade at 1%. Inexpensive boxes with lesser accuracy are also available. All types offer a convenient way of selecting and quickly changing a resistance in laboratory, experimental and development work without needing to attach resistors one by one, or even stock each value. The range of resistance provided, the maximum resolution, and the accuracy characterize the box. For example, one box offers resistances from 0 to 100 megohms, maximum resolution 0.1 ohm, accuracy 0.1%.
Special devices
Mosfet As Variable Resistor
There are various devices whose resistance changes with various quantities. The resistance of NTC thermistors exhibit a strong negative temperature coefficient, making them useful for measuring temperatures. Since their resistance can be large until they are allowed to heat up due to the passage of current, they are also commonly used to prevent excessive current surges when equipment is powered on. Similarly, the resistance of a humistor varies with humidity. One sort of photodetector, the photoresistor, has a resistance which varies with illumination.
The strain gauge, invented by Edward E. Simmons and Arthur C. Ruge in 1938, is a type of resistor that changes value with applied strain. A single resistor may be used, or a pair (half bridge), or four resistors connected in a Wheatstone bridge configuration. The strain resistor is bonded with adhesive to an object that will be subjected to mechanical strain. With the strain gauge and a filter, amplifier, and analog/digital converter, the strain on an object can be measured.
A related but more recent invention uses a Quantum Tunnelling Composite to sense mechanical stress. It passes a current whose magnitude can vary by a factor of 1012 in response to changes in applied pressure.
JFET Variable Resistor
CIRCUIT
JFET_VARIABLE_RES1.CR Download the SPICE file
JFET_VARIABLE_ATTEN1.CR Download the SPICE file
Some designs call for a variable resistor such as variable gain amplifiers, attenuators and some guitar effect boxes. The main goal here is device whose resistance is some function of bias voltage. One such device is your basic JFET. For small signals, where the device operates in the linear (or ohmic) region, the resistance between the Drain and Source terminals (Rds) decreases as the gate voltage increases.
IV CURVES
How can you determine the JFET resistance versus bias voltage? Nothing tells the story like the slope of the IV curve. Here's the basic circuit to develop these insightful graphs.
* GATE AND DRAIN VOLTAGE
VG 1 0 DC 0
VD 2 0 DC 0
J1 2 1 0 J2N5952
*
* Nested DC sweep
.DC VD -0.05 0.05 0.1 VG -1.75 0 0.25
The file includes an essential line - the nested DC sweep command.
.DC V1 Start Stop Incr V2 Start Stop Incr
You define how the test voltages V1 and V2 will start, stop and increment. Then, for each V1 voltage point, V2 will sweep through its entire range from start to stop. This comes in handy where the drain (VD) is swept through its range for every gate bias voltage (VG).
CIRCUIT INSIGHT Run a DC analysis of the 2N5952 JFET in the JFET_VARIABLE_RES1.CIR and plot the drain current ID(J1). The drain is only swept from -50mV to +50mV. You should see a nice family of curves representing the Id as a function of Vd for each value of Vg. The slope of each VI curve represents the Drain-Source resistance. What is the minimum resistance? The smallest slope (resistance) can be approximated by Rds = 50mV/30uA = 1.66k ohms. The largest slope is Rds = 50mV/250uA = 200 ohms. This shows the range of resistance possible when sweeping the gate from -1.75 to 0V.
As warned earlier, this resistor-like operation applies only to small signals. What about larger signals? Increase the Vd sweep parameters to Start = -0.5V and Stop = +0.5V. What happens to the linearity of the IV plots?
VARIABLE ATTENUATOR
Here's an application of a JFET taken straight out of a National app note. The JFET implements a voltage attenuator as one leg in a voltage divider.
The voltage divider ratio is defined by Vo/Vs = R2'/(R1+R2') where R2' is actually the parallel combo of R2 and Rds. For this example, setting R1=10k and R2=100k defines a small attenuation, even when the JFET is OFF (very high resistance)
CIRCUIT INSIGHT The gate voltage VG ramps from -2V to 0V while a 100mV (1kHz) sine wave is applied to the divider input. Run a TRANSIENT ANALYSISs of JFET_VARIABLE_ATTEN1.CIR and plot both the gate voltage (V3) and the output voltage V(2). At what gate voltage does Rds begin conducting and attenuating the signal? Where does the max attenuation occur? You can estimate Rds by measuring V(2) peaks and solving the divider equation for R2.
UPCOMING TOPICS
In a future topic, we'll use the variable resistor as part of a popular guitar effects pedal called the phase shifter.
SPICE FILES
Variable Resistor
Download the file or copy this netlist into a text file with the *.cir extension.
Download the file or copy this netlist into a text file with the *.cir extension.
Variable Attenuator
© 2012 eCircuit Center