Mosfet As Variable Resistor

Variable resistor basics

Mosfet As Variable Resistor Formula
Resistor
Variable Resistor Connections
Potentiometer
Use Mosfet As Variable Resistor

variable resistors are used as volume control and also for other controls in radio and TV sets.They are also referred as 'pots'.For high power application fixed wire wound resistor and for low power application fixed carbon resistor of highvalue is used.

Oct 31, 2006 mosfet variable resistor As the gate is reverse biased relative to the channel (the conductor between the gate and source leads) the current is pinched off, making an increase in effective resistance. If the gate voltage is varied relative to the source lead, you will get approximately the effect you are looking for. FET is a device that is usually operated in the constant-current portion of its output characteristics. But if it is operated on the region prior to pinch-off (that is where VDS is small, say below. UT3006, datasheet for UT3006 - 55A, 30V N-CHANNEL ENHANCEMENT MODE POWER MOSFET provided by Unisonic Technologies. UT3006 pdf documentation and UT3006 application notes, selection guide.

As shown in the figure, variable resistor will have three tags viz. two outer one and one at the center.A wiper makes contact with the track which is connected with center tag. Rotation of the spindle moves the wiper over the track which changes theresistance between the center tag and the end ones.slide type variable resistors will have straight track.

There are different types viz. linear and log.In linear type, change in resistance is proportional to angle. In log type, change in resistance at one end of trackis less than at the other for equal angular positions.

Variable resistors are available with typical maximum values which include 10KOhm, 50KOhm, 100KOhm, 500KOhm and 1MOhm.These devices often referred as potentiometers are tuned as per resitance value needed using screwdriver.

Variable resistor is used for following functions:
• As rheostat to control the current in the electronic circuit.
• As voltage divider to obtain any voltage from zero to maximum of supply voltage.

Variable resistor manufacturers

Following are few of the reputed variable resistor manufacturers also referred as trimmers, potentiometers and rheostats.
1. vishay
2. Hindustan Electronic Components
3. Jameco Electronics
4. Mouser Electronics
5. Ohmite
6. CTS
7. NTE Electronics Inc.
8. Murata electronics
9. ACP
10. C & K Components
11. Nobel
12. Omite
13. TOCOS America
14. P3 America

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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

Mosfet As Variable Resistor Formula

The file includes an essential line - the nested DC sweep command.

.DC V1 Start Stop Incr V2 Start Stop Incr

Resistor

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.

Variable Resistor Connections

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) How to mineshafter on mac.

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.

Variable Attenuator

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