Objectives:

1. To understand the operational characteristics of a common collector (CC) amplifier - also known as an emitter follower (EF).

2. To examine the gain characteristics of a, CC, amplifier.

3. To determine the input impedance of the amplifier.

4.  To determine the output impedance of the amplifier.

Parts and Equipment:

Computer with MicroCap software or some other electronic simulation program.

Textbook Reference:

Hambley, Allan R., "Electronics," 2^nd Ed. Prentice Hall, 2000, pp.258-266

Serda and Smith, "Microelectronic Circuits," 4th Ed. Oxford University Press, New York, NY, 1998, pp. 290-295 or Savant, Roden, and Carpenter, "Electronic Design," The Benjamin/Cummings Publishing Company, Inc., Redwood City, CA, 1991, pp. 106-109, 224-226.

Procedure 1: Biasing Characteristics

Enter the following circuit using a 2N3904 transistor for Q1 and the values shown in the diagram:

Common Collector or Emitter Follower amplifier

1.  Run an AC analysis of the circuit over the frequency range 100 kHz to 10 Hz.  Print graphs of the gain(dB), gain(V/V), phase, and input impedance for this frequency range.  Use the cursor mode on the analysis results window to read the values at 2 kHz

2.  Next determine the lower 3 dB cutoff frequency by leaving the right cursor at the high frequency end of the graph and moving the left cursor until the delta for the gain is as close to 3 dB as possible and then reading the frequency.

3.  Return to the circuit window and turn on DC node voltages and record the values.

4.  Then turn on and record the branch currents.

5.  Turn on and record the power dissipated for each element in the circuit.

Repeat the previous steps using values of R_L = 10 k Ohms, 100 Ohms, and no load.

CALCULATIONS / GRAPHS:

5. Plot a graph of V_out vs. I_L (use I_L = V_out / R_L) using the gain(V/V) data obtained at 2 kHz with the Four different values of R_L.  Use the cursor mode, set the cursor at 2 kHz and read the gain value given for that frequency.  Assume a 1 V input and calculate V_out and I_L = V_out / R_L for each case.  This graph gives a straight line V_out = V_OC - I_LR_out. The slope of this line is -R_out. The value of V_OC can be found by extending the line to I_L = 0 or by using regression analysis (available in most spreadsheets) to find both the SLOPE and INTERCEPT.

6. Determine the Thévenin equivalent circuit of the amplifier output. Using a spreadsheet program will be very helpful as you can use it to find the best straight line through your four data points. You can then plot that straight line on the same graph with your data points.

The Thévenin equivalent of the output can be found by solving for V_OC and R_out.  V_OC = A_VOC*V_IN. 

R_out = -DV_out/DI_L.

Small Signal Mid-Band Model of the Amplifier

7.  Using your calculated values of,  R_in, and voltage gain for each load resistance calculate the current gain for each case.

Questions:

1. Investigate the main differences in performance between the CE amplifier and the CC amplifier .

2. Investigate the difference between voltage gain and current gain in the two amplifier configurations and give a possible use for each.

3. Does the CC amplifier provide current gain or voltage gain?

4. What is the maximum voltage gain achieved from this amplifier configuration?  When did it occur?

5. What is the maximum current gain achieved from this amplifier configuration?  When did it occur?

CONCLUSIONS:
 

This page last modified 06 February 2006.