THE EMITTER FOLLOWER (COMMON-COLLECTOR AMPLIFIER) Materials Needed: Power Supply: Variable regulated low-voltage dc source Equipment: Oscilloscope;
e; EVM; sine-wave generator Resistors: 3300-; 12000-; 470-k -W Capacitors: 25-F 50-V; 100-F 50-V Semiconductors: 2N2102 or equivalent Miscellaneous: Three SPST switches; 500- potentiometer Procedure: Voltage Gain 1. Connect the circuit of Fig. 18-2. Power off. Switch S1 is closed, S2 open. Set AF generator at zero output. Connect an oscilloscope across points DF and adjust it for proper viewing. The oscilloscope will be used to measure the peak-to-peak value of the signal at specified points in the circuit. 2. Power on. Slowly bring up the gain of the AF generator until a 150-mV undistorted output signal vout appears across points DF in the emitter circuit. With the scope measure vout. Record it in Table 18-1. 3. Measure and record the input signal voltage vin (points AC). Compute and record the voltage gain vout/vin. Input Impedance 4. Open switch S1. Increase AF generator output until vin is at same level as in Step 3. With the oscilloscope floating, measure, and record in Table 18-1, the signal voltage across vAB across points AB. Compute the input-base signal current iin by substituting vAB v vAB iin= AB = in the formula RAB 12000 Show your computations. Record iin in Table 18-1. 5. Compute the input resistance Rin substituting for iin and vin in the formula Rin = vin/iin. Show your computations. Record Rin in Table 18-1. Compute also the input power iin2Rin. Record it in Table 18-1. Output Impedance 6. Close switch S1. Reduce generator output until vout measures 100mV. S2 is still open. Measure, and record in Table 18-2, the output voltage vout (across DF). 7. Close S2. The load resistor RL is now in the circuit. Adjust RL until the output voltage with load is one-half the value of vout measured in step 6. Record vout/2. Open S2. Measure and record the resistance of RL. This is the value of output impedance Rout of the circuit. Compute also the output power vout2/Rout, W. Record this wattage in Table 18-2. Power Gain 8. Compute and record power gain using the formula Ap = Pout/Pin. Phase Relations 9. Following the procedure outlined in previous experiments, determine the phase relationship between the input and output waveforms. Draw the waveforms in proper time phase in Table 183. Extra Credit 10. Explain how you would determine an approximate value for the of the transistor used in this experiment. 11. Find following your procedure.
Fig. 18-2. Experimental emitter follower.
TABLE 18-1 Voltage Gain, Input Impedance, and Input Power vout, vin, Gain = vAB, iin, V V vout/vin V A
Rin,
iin2Rin, W
TABLE 18-2 Output Impedance, Output Power, and Power Gain vout, vout/2 RL=Rout, vout2/Rout, V V W
Power Gain
TABLE 18-3 Phase Relations Waveforms Input Output Questions 1. Comment on your experimental data concerning the voltage gain of the emitter follower. 2. How does the input impedance of the common-collector amplifier compare with that of the common emitter? Explain why they differ. 3. How will the size of the bias resistor R1 in Fig. 18-1 affect the input impedance (resistance) of the circuit? 4. How will the size of the emitter resistor R2 in Fig. 18-1 affect the input impedance of the circuit? 5. What is the relationship between the phase of the input and output waveforms in the emitter follower? 6. Explain in detail the method you used to determine phase relations in the experimental circuit. How was your oscilloscope triggered/synchronized for this check? 7. Assume that vout/vin = 1. Write an approximate formula for the power gain of an emitter follower. 8. Explain in detail how you determined the input impedance of the experimental amplifier.
