ECE3424 Electronic Circuits Laboratory
Experiment #3
TITLE:
SINGLE-TRANSISTOR AMPLIFIERS
OBJECTIVE: Measure characteristics of single-transistor (BJT) transfer circuits (amplifiers) and
ascertain their dependence on the bias network of resistances.
Commentary: Transistors, as the name implies, are “trans-resistors”. An input signal modulates a small
current or voltage across the input terminals of the transistor. This control signal then modulates an
output path at a much larger current level. The larger current level (and signal thereto) can be used to pull a
large signal voltage across a (resistive) load. The ratio of signal in and signal out, = signal gain, which is a
transfer gain. And the signal gain is why these circuits are called “amplifiers”.
Amplifiers, however, have operational constraints. Just because we develop a larger signal voltages the
output does not imply that our amplifier is capable of lighting a 60W bulb. The amplifier’s ability to
transfer power is constrained by its internal resistance at the output. We call this resistance the “output
resistance” (Rout), and it is one of the measurables.
And since the transistor amplifier is a circuit, also has a finite input resistance, which we will call Rin.
So the basic transfer characteristics of an amplifier that were wish to measure are:
as we represent by the two-port model of an amplifier shown by figure 3.1. The output signal is labelled as
vL because it is applied across the load RL.
These signals, and the amplifier transfer characteristics, are all based on small increments of voltage at the
input and output, for which VI = vI, VL = vL. Therefore input and output resistances are not just
equivalent resistances, but are slopes. For example: Rin = dVI/dII. So measurement cannot be done by a
DMM. We will need to use other techniques.
So, the purpose of this experiment is to measure amplifier characteristics and identify the factors that define
them. And in the process learn some of the special measurement techniques needed.
PROCEDURE:
A-1: The amplifier topology that we will consider is the one shown by figure 3A-1. This
topology is called a common-emitter (CE) configuration, and is a general-purpose amplifier topology.
Notice that the amplifier is a set of components (resistances and transistor) strung in between two voltage
rails, just like those associated with the prototyping motherboard. In this case the upper rail is provided by
the 20V (external) power supply, and the lower rail is at ground.
This is a more detailed circuit than what you have had in previous experiments. So be sure that everything
is connected up properly before turning on the power supply. Notice that if you lay out the circuit on the
motherboard with roughly the same topology as the circuit diagram, there is less aptitude for connection
errors.
A-3: Be sure to leave resistances R4 and R5 readily accessible, for exchange with other resistance
values. Set the resistance box at 999.999k until further notice.
B-1: Power up the circuit with V+ = 20V. As a check you should use the DMM (with the probe
leads) to probe the circuit and check the voltages at the base (VB) and the emitter (VE) of the transistor. VB
should be approximately 6V and VE should be approximately 5.3V. If they are not reasonably close to
these values, you might best turn off your power supply and run a quick snake check on your circuit.
B-2: Put a shorting wire (jumper wire: banana-banana) across the resistance box. Apply an
input signal (4kHz) of peak-peak amplitude 100mV. Connect CH1 of the O-scope to the input point vS of
the amplifier and CH2 of the O-scope to the output Vo of the amplifier. Output peak-peak amplitude should
be considerably larger than that of the input amplitude (on the order of 1.5V). Using the O-scope scales
and cursors, measure the ratio Vp-p(out) vs Vp-p(in). This ratio is the gain AV of the amplifier. Record this
value in your data table.
B-3: Now increase the input amplitude and monitor the output amplitude at Vo until distortion
becomes evident. Identify the upper and lower limits of the output levels. These upper and lower limits are
called the Compliance limits of the circuit.
C-1: Now remove the shorting wire from the resistance box. You should note that Vp-p(in) drops
in amplitude. Set amplitude of vS to 200mV. Reduce the Rbox (resistance Rx) until the amplitude at vI
becomes 0.5 vS. Hint: You may find it convenient to concurrently monitor vS and vI with CH2 to vS, and
setting CH1 and CH2 to the same scale. The value of Rx for which vI = 0.5vS is Rin. The principle is
illustrated by figure 3C-1. Record this value of Rx in your data table.
C-2: Replace the R-box by a shorting wire (on the motherboard). Move the connections of the
Rbox so that they are emplaced across RL. Disconnect the wire from RL to the lower rail, (which effectively
disconnects RL). This procedure effectively replaces RL by the R-box. Move the wire connected to CH1
so that it monitors vL.
C-3: With input signal at 100mV, monitor vL as a function of the setting of Rx. Start with Rx =
999,999andreduce it in value until the amplitude drops by a factor of 2. This value is the value of Rout
and is another entry in your data table. The principle is illustrated by figure 3C-2.
D-1: Restore the circuit configuration of part A-1. Replace R4 and R5, respectively by R4 = 680
and R5 = 4.3k. Repeat the measurement process as used in parts B and C to determine (1) voltage gain AV
and compliance, and (2) Rin and Rout for these new values of R4 and R5.
D-2: Restore the circuit configuration of part A-1. Replace R4 and R5, respectively by R4 =
1.0k and R5 = 3.9k. Repeat the measurement process as used in parts B and C to determine (1) voltage
gain AV and compliance, and (2) Rin and Rout for these new values of R4 and R5.
D-3: Restore the circuit configuration of part A-1. Replace R4 and R5, respectively by R4 =
2.0k and R5 = 3.0k. Repeat the measurement process as used in parts B and C to determine (1) voltage
gain AV and compliance, and (2) Rin and Rout for these new values of R4 and R5.
ANALYSIS:
1. The following relationships should be of help in your analysis of the data.
2. (Logbook) Complete your data table with three more columns that show AV, Rin, and Rout as evaluated by
the mathematical relationships. Comment on the comparisons.
3. (Logbook) Use the results of your measurements to determine the best fit value of the forward current
gain F for the 2n3904 transistor. This parameter is one of the internal parameters for the transistor, and it
should be on the order of 100 - 150.
4. (Logbook) Make a plot of Rin vs R4, comparing measurements against theory.
5. (Logbook) Make a plot of AV vs R4, comparing measurements against theory.
6. (Logbook) What are your conclusions about the effect of emitter feedback resistance R4 on AV , Rin, Rout
and the compliances?
7. (Formal Report) For the formal report run pSPICE on the circuit of figure 3A-1 with a large-amplitude
(700mV) input signal, for each of the four different cases of parts B-3, D-1, D-2, D-3 of the Procedure,
overlaying the four traces onto a single plot output. From this SPICE output confirm and compare to the
compliances measured in parts B-3 and D-1 thru D-3.
In order to overlay the outputs of four different circuits, create a schematic the circuit of figure 3A-1. Then
make a copy of this circuit and paste it three more times, (= total of four circuits). For each of these
circuits, change the values of R4 and R5, respectively, to those used in steps B-3, D-1, D-2, D-3. You now
have four circuits just like the ones that you tested.
At the input of each circuit include a special part, = the ideal VVT (Part = E from your parts list). It will
default to unity transfer ratio, which is what you will want. The inputs to all of the VVT will be the source
VS, which should be in the form of a VSIN part, with f = 4 kHz and Vamp = 200mV. The respective
outputs of the VVTs go to each of the four circuits that you have created.
The sole purpose of the VVT is to provide a “buffer” between the source and each of the four circuits,
isolate each circuit, one from the other.
Now label the outputs of each of these circuits as VL1, VL2, VL3, and VL4, respectively. These labelled
outputs will then be the reference labels that you can touch when you pull up the traces and overlay these
four outputs.