Variable Attenuator The 2N3685 acts as a voltage variable resistor with an RDS(ON) of 800X max. The 2N3685 JFET will have linear resistance over several decades of resistance providing an excellent electronic gain control. Negative to Positive Supply Logic Level Shifter This simple circuit provides for level shifting from any logic.

1. Experiment with the JFET as a variable resistor by using it instead of a regular resistor in a two resistor voltage divider. Apply a small sinewave signal (about 0.2 V) to the input and observe variation of the output amplitude while changing V GS (negative voltage must be used!).
2. JFETVARIABLEATTEN1.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.
3. JFET is operated in the constant current portion of its output characteristics for the linear applications. In the region before pinch off, where V D S is small the drain to source resistance rd can be controlled by the bias voltage V G S.The FET is useful as a voltage variable resistor (VVR) or Voltage Dependent resistor.
4. The JFET (an MMBF5484) is acting as R1 of a voltage divider (with a 100 ohm resistor in series before it), and our load would be R2 of the divider. I'm using 2x 1Meg feedback resistors as described in various sources on the subject to linearize the JFET. The control voltage is a negatively rectified input signal which is averaged by R2 and C1.

OBJECTIVES

Familiarity with basic characteristics and parameters of the J-FET.

Applications of J-FET as a current source and a variable resistor.

PRELAB

Draw a circuit for measurements of characteristics of a depletion mode, n-channel JFET, described in part 1 of the Laboratory (below). Sketch basic characteristics of a n-channel J-FET (I_D vs. V_DS and I_D vs. V_GS) and explain why it may be used as a constant current source and a voltage controlled resistor. Indicate the parts of the characteristics where these functions can be realized.

Jfet As Variable Resistor Code

LABORATORY

Equipment needed from the stockroom: ECE 392 parts kit, analog universal meter, resistance substitution box, leads.

1. JFET CHARACTERISTICS; V_P AND I_DSS.

1. 1. Insert a JFET into the protoboard, connect the source to ground and the drain to a 15 V power supply through an ammeter, which will measure the drain current (I_D). Measure this current for different voltage values between the gate and the source (V_GS). Use only negative voltage on the gate. Determine the pinch-off voltage (V_P), i.e. the gate voltage at which the drain current is (practically) zero. Get a few measurements at low current, with V_GS close to V_P so that you have enough points on the logI_D vs. V_GS graph to determine V_P. (see description of the report, below). Measure also I_DSS, the drain current with V_GS = 0. This current flows through the transistor when the gate connected to the source. Repeat measurements of V_P and I_DSS values for another transistor of the same type in your kit and see if there is significant difference between the two transistors. If so, make sure that you can identify these transistors when you use them in other measurements.

1. 2. Next, measure I_D(V_DS) characteristics of one of the transistors for V_GS = 0 and two different negative values. Note the linear part of the characteristics, where I_D is proportional to V_DS (behaves like a resistor) and the saturation part, where current is (almost) independent of the voltage.

You will explore saturation range of the JFET transistor characteristic in part 2 and the linear range in part 3, below.

2. FET AS A CURRENT SOURCE.

The flat parts of the I_D vs. V_DS characteristics of the FET allow to use this device as a simple constant current sources because the current is (almost) independent of the voltage across it. Test this idea with two transistors. Measure the current with different values of the load resistor R_L (100 Ω - 100 kΩ)chosen from the resistance substitution box.

How good is this current source? Determine the range of the load resistor values which allows the current to stay constant within a given interval (say 2 % or 5%). What is the range of voltage across the transistor operating as a current source.

You can buy JFETs with the gate connected to the source, so called current regulator diodes. These two terminal devices, calibrated for different current values, are current equivalents of Zener diodes which provide a constant voltage.

A variation of JFET current source, with self-biasing, is shown on the next schematic. One of its advantages is that you can obtain different current values by adjusting the resistor R (a few k). Try this simple circuit and again determine the range of load resistor R_L which allows you to keep the current constant.

Is this a better current source than the one without a resistor? How does it work? Do you see feedback in this circuit? What does the voltmeter here show?

3. JFET AS A VARIABLE RESISTOR.

Variable Resistor Purpose

In the linear part of the JFET I_D vs. V_DS characteristics, the current through the transistor is (roughly) proportional to the voltage across it, like in a resistor. Moreover, the slope of these characteristics depends on V_GS so that changing the latter changes the value of the 'resistance'. This effect can be used in many 'voltage controlled circuits'.

Experiment with the JFET as a variable resistor by using it instead of a regular resistor in a two resistor voltage divider.
Chose R = 10 k.

Fet As Variable Resistor

Apply a small sinewave signal (about 0.2 V) to the input and observe variation of the output amplitude while changing V_GS (negative voltage must be used!). To see if the transistor really behaves as a resistor, switch the waveform generator to a triangular wave. Nonlinear dependence of voltage on current will show as a distortion of the straight lines of the waveform. A resistor has a linear I-V characteristic and will not distort a triangular wave.

From observation of the output waveform with a triangular wave at the input estimate in what range of input voltage the transistor behaves approximately as a resistor? Explain your observation.

The circuit shown below is an improved version of a two resistor voltage divider, with R a regular resistor and the transistor being an adjustable resistor. The divider ratio can be adjusted by the control voltage V_C. A compensation circuit (between the output and the transistor gate) greatly improves the circuit linearity as a part of output voltage (what fraction?) is added to V_GS. Check that this circuit behaves much better as a voltage controlled resistive divider.

Jfet As A Variable Resistor

Compare the range of V_in with undistorted triangular waveform with the previous case of the uncompensated circuit. Explain.

REPORT

Describe briefly the measurements. Include all schematics. Show all results with proper units. Do not forget to include the frequency used in ac measurements. For part 1, make a graph of I_D vs. V_GS characteristic and indicate the values of I_DSS and V_P on the graph. V_P is best determined from a plot of logI_D vs. V_GS. If you have data for two transistors, plot them on the same graph. For part 2, you may plot I_D vs. log R_L to cover a wide range of resistance. In the discussion, comment whether the parameters I_DSS and V_P are the same for a given transistor type.. Address the topics and answer the questions printed in bold letters in the manual. Add any observations or conclusions you wish to make.

A PROJECT IDEA (OPTIONAL): ONE TRANSISTOR AM TRANSMITTER.

Jfet As Variable Resistor

You could use the last circuit for amplitude modulation of a high frequency carrier signal, just as it is done in AM radio transmission. Supply the input with a high frequency sinewave (about 1 MHz) and modulate its amplitude by feeding a low frequency signal (in the kilohertz range) through a capacitor (~ 1 µF) to the slider of the potentiometer. The low frequency signal may be picked up by an AM radio tuned to the appropriate frequency (in this case about 1 MHz). If you supply an amplified signal from a microphone you may hear your voice 'on the air'. A piece of wire attached to the drain may serve as a transmitter antenna, extending the reception distance.