Lab 2: The Emitter Follower

PRELAB

1. What is the approximate value of the voltage gain of an emitter follower? Why may such a circuit be useful?
   
2. What are the values of input and output impedances of the emitter follower you expect to measure? Are they related?

LABORATORY

Equipment needed: ECE 392 parts kit, leads, 2 scope probes.

1.      EMITTER FOLLOWER WITH SINGLE AND DOUBLE POLARITY POWER SUPPLIES.

Wire the circuit shown below. The value of the small protective input base resistor is not critical.

(a) Observe the follower operation.

Drive it with a symmetrical sine wave at the input (check that the signal from the waveform generator has no dc offset voltage). Increase the input voltage from less than 1 V (p-p) to about 2V (do not drive the B-E junction to breakdown on a negative half cycle). Observe input and output waveforms using two channels of an oscilloscope scope and note their amplitudes and phase relation. Note, especially with low signal voltage, the difference in the peak voltage and the time of zero crossing on the two channels. Expand vertical and horizontal scale as needed. Use the scope DC input to see if there is a dc bias on your signal.

Next connect V_CC to 5 V and V_EE to -5 V (see the figure below) and observe the output again. Measure input and output voltages. Calculate voltage gain.
How large output voltage can you get from the circuit (without distortion)?
Explain the difference between this and the previous case.

(b) Measure the input and output impedances, Z_in and Z_out.

Determine Z_in (the impedance looking into the transistor base) by measuring signals on both sides of the 10kΩ resistor connected in series at the input. You may leave in place the small input resistor used in the previous circuit. Recall that the input impedance is the ratio of the input voltage to the input current, which from the Ohm’s law is proportional to the voltage drop across the resistor.

Load the output with a resistor (1kΩ or less) through a 4.7 µF blocking capacitor. Use a sinusoidal signal at the input and observe a change in the output signal when the resistor is connected. Determine the Thevenin equivalent resistance viewing the follower as a voltage source with Z_out in ser0ies. Work at a frequency at which the load circuit does not act as a filter (ω >> 1/RC)

Hint: Note that the load resistor forms a voltage divider with Z_out.

2.      EMITTER FOLLOWER WITH COUPLING CAPACITORS.

Can you use a single polarity power supply for a follower operating with a symmetrical input? Try the circuit shown in the figure below. Measure both ac and dc components of the output voltage with a scope. Measure the emitter and base dc bias with a DVM.
Explain results. How could you eliminate dc component from the output voltage? Try!

Is the frequency response of the last circuit different than those in 1? Determine frequency f_-3dB by decreasing the generator frequency until the output signal drops 3dB

REPORT

Present clearly the result of all measurements. Answer all questions and comment on topics typed in bold print in this manual.
Use Multisim simulation to obtain the frequency responses of the circuit in 2 and compare the measured f-3dB.with the simulation..