EXPERIMENT 8: SINGLE-TRANSISTOR AMPLIFIERS 10/29/20
In this experiment we will measure the characteristics of the standard common emitter amplifier. We will use the 2N3904 npn transistor. If you have time, you can also investigate the characteristics of the common collector amplifier, usually called an “emitter follower”.
I. THE COMMON EMITTER AMPLIFIER
The circuit we will use is shown below. In this circuit resistors R1 and R2 are used to set the base to
the desired DC operating voltage. Usually in circuits of this kind one uses an input capacitor to decouple the DC voltage level of vin from the base.
Before you come to lab, it would be useful to complete the first two homework problems (problems 8.7 and 8.8 in Sprott) that involve designing and calculating the properties of a common emitter amplifier. In any case, be sure to do the prelab, which the analysis of a similar amplifier but with the same component values you will use in the lab. So you will have done most of the calculations called for below.
Equivalent circuit of any voltage amplifier:
1. The circuit we will use is mounted on a pre-wired circuit board. Sketch the circuit in your lab notebook, labeling the values of all components. The value of CC is not apparent from the
markings on the capacitor and will be measured in step 5 below.
2. Use the transistor curve tracer to obtain the collector characteristics for your transistor and place a fully labeled printed copy in your lab notebook.
3. (a) Install the transistor in the circuit board. Connect the +15 V terminal of the small white power supply to VCC and the ground to VEE. For now, the function generator (vin) should
not be connected. Calculate the expected quiescent (Q-point) value of IC, VC, and VCE.
Use your oscilloscope to measure VC and VE and compare with your calculated values.
(b) Draw the load line on your copy of the transistor characteristics. You need two (VCE, IC)
points to define this line. It is usually easiest to find the VCE = 0 and IC = 0 axis
intercepts. You can calculate these from the schematic. Mark the quiescent Q-point on this line using your measured value of VCE. Determine IB at the operating point (from the
plot) and find β. You will probably need to interpolate between two of your base current curves.
(c) Calculate Rin, Rout, AV, and the values of VC at cutoff (IC = 0) and at saturation
(VCE = 0.2 V).
4. (a) Set up the function generator to produce sine waves with frequency f = 10 kHz. Adjust the amplitude of vin to obtain an AC voltage of ~5 V p-p at the collector. Make a sketch or
scope picture with vin and the voltage at the collector on the same plot (remember that the
collector voltage has a DC offset, so be sure to use DC coupling on the scope and show this).
(b) Calculate the measured no-load gain, AV, and compare it to your calculated value.
(c) Draw the general equivalent circuit of an amplifier that defines Rin and Rout in your
notebook and show how you can put external resistances in series with the input and from the output to ground to make voltage dividers that can be used to determine Rin and
Rout. To measure Rin, put a resistor R between the function generator and the base and
adjust R until VB (or vout) drops by a factor of 2. To measure Rout move your resistor R to
go from the output side of CC to ground (VEE) and adjust it until vout drops by a factor of
2. (If you were to connect the resistor on the collector side of CC you would change the
DC operating point of the amplifier.) Compare these with your calculated values.
5. (a) Connect a load resistor RL = 3000 Ω between the output and ground. Measure and tabulate
vin and vout as a function of frequency for 20 Hz ≤ f ≤ 50 kHz (3 points/decade —
1,2,5,10,20,50, etc. — is sufficient except near the corner frequency). Plot the gain as a function of f on logarithmically-spaced axes.
(b) The falloff in the gain at low frequencies is caused by the output capacitor forming a high-pass filter with RL. From your graph determine the corner frequency (i.e. the
frequency at which AV has dropped off by a factor of sqrt(2) compared to the
high-frequency limit). Use this measured corner high-frequency to determine CC.
6. Remove RL, set the function generator to f = 10 kHz, and increase vin to drive vC to both cutoff
and saturation. Make an oscilloscope picture showing a few cycles of the collector and emitter voltages on the same 2 volts/div scales and with the same zero levels (you can set the zeroes at the bottom of the grid). Try to explain what you see. Compare your measured and calculated values of VC(cutoff) and VC(sat).
7. In this step we will observe the behavior of the amplifier in the “grounded-emitter” configuration. Rather than grounding the emitter directly, the idea is to “bypass” RE by
connecting the capacitor CE to ground. This keeps the DC operating point of the amplifier
unchanged, while grounding the emitter as far as AC voltages are concerned. In other words, the emitter voltage will now be ~constant (you can check this on the oscilloscope).
(a) Connect CE to ground and adjust the amplitude of vin to obtain vout = ~5 V peak-to-peak.
Use your oscilloscope to measure the magnitude of vin and vout, and determine the
open-circuit gain. For grounded-emitter operation the gain is given by AV = RC/rtr. Use the
measured value of AV to determine the transresistance and compare your result with the
expected value
rtr (= re) = rohmic + (0.026 ohm-A)/IE.
(b) Switch the function generator to triangle waves and increase vout to 8 V peak-to-peak.
Make a sketch or scope shot of the collector voltage, vC(t), using DC coupling. On the
same graph, indicate the quiescent value of vC (i.e. the value obtained with vin = 0). Can
you explain why vC looks the way it does?
II. THE COMMON COLLECTOR AMPLIFIER, or “EMITTER FOLLOWER” (optional if you have time)
The common collector amplifier has a gain of approximately 1, a moderate input impedance, and a very low output impedance. We will use the same circuit as above, with the output taken from the emitter.
1. Set the function generator frequency to f = 10 kHz. Adjust the amplitude of vin to obtain an
emitter voltage of approximately 0.7 V. Sketch or make a scope picture of vin and vE. Calculate
the gain and compare your result with calculated value (use the measured value of rtr in the
