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EXPERIMENT 4-2 THE JFET AMPLIFIER
Because of its high input impedance, the JFET is very useful as a small signal
amplifier. You will now test a simple single-stage amplifier using a JFET. Here you'll
see how to bias a JFET, even when you do not have a set of characteristic curves.
EQUIPMENT
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12-VDC power supply, oscilloscope, VOM, and signal generator
(1) N-channel JFET (Use MPF 102 if available)
(1) 10-KΩ pot
(1) 1-M Ω, 1/2-W ± 5% resistor
(1) 2-K Ω, 1/2-W ± 5% resistor
(1) 20-µF capacitor
(1) 5-µF capacitor
E4-2A Circuit for experiment 4-2 with picture.
PROCEDURE
1. Use the WEB to find the pin-out for the transistor then build the circuit of Fig.
E4-2A. With a voltmeter connected from drain to ground, apply power and adjust RS
until VD = 10 V. This makes ID = 1 mA.
2. Measure the voltage from source to ground and the voltage from gate to
90 Electronic Troubleshooting - Modified for CSN’s ET289B
Field Effect Transistors 91
ground. If your measurements differ greatly from the ones given (over + 10%)
recheck the circuit.
vs = 2.3 V, ________________
VG = 0.00 V, ______________
If your circuit is working properly, the gate-to-ground voltage should equal
zero, so the source-to-ground voltage is the same as the gate-to-source bias
voltage.
3. Next readjust RS until VD reads 8 V. What is the
value of ID now?
ID = ____________
4. Measure V S again
VS = _______________
5. You learned in the text that the transconductance gm =Δ ID/ Δ VGS. Using the
measured values of steps 1 through 4, determine gm for your transistor.
gm =
E4-2B Circuit for experiment 4-2 with picture.
6.
Add
capacitors CC and
CS to your circuit,
as shown in Fig.
E4-2B. Then apply
an input signal of
0.5 Vp-p at 1 KHz.
Measure the output
voltage vo with an
oscilloscope.
vo = ___________
Av= vo / vgs = ______
Calculate the voltage gain (should be 5 or
greater – if not troubleshoot).
E4-2C Circuit for experiment 4-2 with picture.
NOTE: CC = 5uF,
CS = 20uF
7. Now let's measure the input impedance of the circuit. Put a 1-M Ω, pot in series with
the input coupling capacitor, as shown in Fig. E4-2C. Keep lead lengths short to minimize
noise pick up. Keep all of the other circuit components as in step 6, and keep VS at 0.5
Vp-p. Now while watching vo with an oscilloscope, adjust potentiometer, until vo reads
half the value obtained in step 6. Since no amplifier circuit values were changed, the
gain of the circuit is the same as before. What did change was the amount of input signal
vgs. Since vO is half its previous value, vgs must be half of generator supplied signal. The
rest of generator supplied signal must then be dropped across R. That means that the
resistance of pot (RX) must be equal to the input resistance of the amplifier (assuming
that the internal resistance of the signal generator is very small compared to RX).
8. Remove RX (the 1 M ohm pot) from the circuit,
and measure it with an ohmmeter. R X = r in =
____________
QUIZ
1. If you wanted to replace the pot in Fig. E4-2A with a fixed resistor so that ID = 1
mA, what value of RS would you use? (Hint: Use your measurements of steps 1 and
2.)
RS = ________________
2. If you wanted to replace the pot with a fixed resistor in Fig. E4-2A so that ID = 2
mA, what value of Rs would you use?
RS = _______________
3. Estimate a suitable value for RS in Fig. E4-2A to make .1D = 1.5 mA.
RS = ________________
4. Calculate the theoretical value of voltage gain using equation 4-5B and your calculated
value of gm from step 4.
AV = ________
5. Determine the percentage of error between the theoretical value of gain and the
measured value, by using
% error = (theoretical value — measured value)/theoretical value x 100 =
If the percentage of error is within the tolerance of your parts and test equipment
capabilities, your results are good.
6. Suppose you decided to measure the input impedance of the amplifier by connecting an
oscilloscope from gate to ground, and adjusting R, until vg, = 0.25 V. If the oscilloscope's
input impedance were 1 MΩ, would it affect your measurements as performed in step
7?
(a) Yes. (b) No.
92
Electronic Troubleshooting - Modified for CSN’s ET289B