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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 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