ANALOG & TELECOMMUNICATION ELECTRONICS
LABORATORY EXERCISE 1
Lab 1:
TRANSISTOR AMPLIFIER:
SMALL SIGNAL BEHAVIOR
Goal
Design a single-transistor amplifier for audio frequencies, and verify the performances in
linearity (small signal).
Specifications
Design a single transistor amplifier according to the following specifications:
 voltage gain (value) = 13;
 3 dB bandwidth from 200 Hz to 20 kHz;
 output dynamic range (for 2 kHz signal) 3 V (peak to peak), with a load resitance of 10kΩ.
Use a 2N2222A BJT;
Available supply voltage: 12 V.
The gain is specified as nominal value (small differences up to 10% are allowed in both
directions), while the specs on bandwidth and dynamic output range are the minimum required
(the amplifier must provide at least the values of the specs, taking into account normalization of
R and C values, and tolerances).
The circuit must comply with these specs with the nominal load, and power supply at the
nominal value +-3%, at ambient temperature (25° C).
The numeric values of above parameters may be different in the various AY; for actual design
use the values provided in the lessons.
WARNING !!
The 2N2222 “silicon” is packaged in metal case (TO-18) or plastic case. Plastic case is used for
two versions of the same device: PN2222 and P2N2222. The internal silicon is the same, but
   Pinout is different   .
The various packages are mixed in the LED drawers. Verify the label on the device you are
actually using, and find the proper data sheet.
Most common problems and mistakes
-
Not considering the scaling factor of scope probes in the measurements.
Using the scope for direct differential measurements (probe ground not connected to circuit
ground).
Get very hig precision values from the measurements carried out with oscilloscope (or other
low-precision instrument).
Missing analysis of measurement errors.
ATLCELabTranLin12h.doc – DDC - 21/03/2012 11.26.00
1
Design
This section describes an example of design procedures; other procedures can also bring to
correct final results. The actual design sequence must be described in the lab report.
The issues marked by “►” must be specifically addressed in the report.
► Define the bias point of the transistor. Select one of the following sequences (the first
paramenter is selected by the designer), or propose another one:
 select steady state VE (VER); from this value compute VO (no load) and RC ;
 select VU (no load); from this value compute RC and VER;
 select RC ; from this value compute VU (no load) and VER.
From RC and VER compute IC in such a way as to position the steady-state collector voltage
about midway between VER and VCC. In this way the Vu variations (at the Collector node) are
symmetric (select an adequate Vcesat margin to keep the transistor away from saturation).
Compute RE to get the required IC .
Select the Base voltage divider resistors, in such a way as to avoid wide variations of the
working point caused by β spreading.
Find the transistor small signal parameters for the actual bias point.
The RE resistance consist of the two devices RE1 and RE2, this last is shunted by Ce. Compute
RE1 to get the desired gain (taking into account the load).
► Select the capacitors to get the (minimum) bandwidth indicated in the specs.
► Choose passive devices with standard values (see the general lab guide for more details),
and compute the changes in amplifier specs caused by the shift from computer values to
standard values.
► Compute the changes in amplifiers specs caused by tolerances of active and passive
devices. If the bandwidth does not comply specifications, change the value of capators as
required.
► Draw the overall mask for the transfer function of the actual circuit, taking into account
standard values and device tolerances.
► Review the selection of R and C devices and modify (if required), in order to fulfill all the
design specs also taking into account device values standardization and tolerances. This
analysis may point out the need for more precise devices in some part of the circuit.
Simulations
Using PSPICE (or any other electrical level simulation program), verify the bias point, the
bandwidth (bode diagram), and the output voltage dynamic (no load and full load).
Put only a summary of simulation results in the report.
ATLCELabTranLin12h.doc – DDC - 21/03/2012 11.26.00
2
Measurements
► Verify the bias point (Ic, Vce).
All current mesurements should be carried out with indirect procedure: measure the voltage
drop across a resistor where the current flows, measure the exact value of the resistor
(removed from the breadboard !), and compute the current. In this way the circuit is not modified
for the measurement (as would happen with direct measurement using series-inserted current
meters).
Measure the base current Ib and evaluate the current gain (hFE) of the BJT. Warning: this
measurement (when done with indirect procedure) require the evaluation of a small difference
among two currents. The original measurements must be carried out with enough precision to
avoid large error in the difference. Is it enough to use nominal values of R1 and R2, or should
the actual components be measured? Only for this current, compare results from direct and
indirect masurements.
► In-band gain: with very low level input signal (to get good approximation with the transistor
linear model), verify the gain at center (in logarithmic scale) of designed passband, with the
specified load.
► Plot the frequency response (bode diagram), and compare with the design specification and
possible result band (computed from component tolerances).
► Verify the output voltage dynamic range, without load and with load. Verify compliance with
corresponding specifications, and find the maximum output level for no visibile distortion.
► At center frequency, measure the input and output impedance (real part only). Compare
results with computed values.
Analysis of results
► Compare the specs with simulation results and with measurements on the actual circuit.
Suggestion: put results in a table with colums specs, design, design with tolerances, simulation,
measurement. Discuss any discrepancy.
If the actual bias point is widely different from the designed one, find the reason, and indicate
which changes should be made to get the correct transistor bias. The new circuit will be used in
lab experiment 2.
ATLCELabTranLin12h.doc – DDC - 21/03/2012 11.26.00
3
Demo experiment
A spectrum analyzer or a FFT can be used to verify the harmonic content of the output signal;
this operation is usually carried out by the assistent on the demo bench. Some of the digital
scopes available in the lab can directly compute the FFT of the input signal; in such cases the
experiment can be carried out directly on the students’ benches.
This lab exercise addresses the linear behavior of the amplifier. The input level should be very
low, and no (or very low) harmonic should appear at the output.
To recognize more easily the harmonic position, set the spectrum analyzer with linear frequency
scale, and a span covering till 4-5th harmonic. In these measurements set the input signal
frequency at bandpass center.
Before any measurement, verify the spectral content of input signal. Some signal generators
have significant harmonics in the output signal. The actual distorsion of the amplifier is the
difference between input and output spectra.
Verify (qualitatively) the changes in the output spectrum with changes in the input level.
Lab report
Start with a (short) summary of the design procedures.
Describe how to carry out the project starting from the specifications, not the details of device
values computation.
Provide a part list with all information required for component acquisition: type, value, other
parameters, …(Bill Of Material: BOM). At least for some of them give the reference (part
number) for a commercial provider (e.g. Digi-Key, RS, …).
Draw the complete schematic diagram, with component labels pointing to the table above.
Describe the measurement carried out (follow the sequence proposed in these instructions),
and the result of each measurement.
Analyze and discuss any difference between expected and actual results.
The report must also include an evaluation and discussion of measurement errors.
For the other sections of the report follow the instructions of the general lab guide, available at:
http://areeweb.polito.it/didattica/corsiddc/01NVD/MatdidNew/LabGuide.pdf
ATLCELabTranLin12h.doc – DDC - 21/03/2012 11.26.00
4