IL2218 Analog electronics, advanced course
Academic year 2008-2009, period 3
Lab 3 – Feedback
Name:..............................................................
Personal number:.............................................
Date of approval:..............................................
Assistant:.........................................................
2009-01-30
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Introduction
Many electronic systems incorporate some form of feedback. Feedback has
many good effects; it makes the gain less sensitive to temperature or component
variations, reduces the distortion, and increases the bandwidth. It can also be
used to control the input and output impedances. However, the negative effect is
that it causes reduction in gain.
This lab is designed to provide further understanding of basic properties given in
the lectures by comparing hand calculations based on theory, computer
simulations, and the lab measurements.
3.1 Hand Calculations
Figure 3.1
In this lab, you will design an amplifier using feedback. The amplifier consists
of two stages, a common-emitter amplifier followed by an emitter follower. The
circuit is shown in Figure 3.1.
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C1. Design specifications for the feedback transistor amplifier
Voltage supply: VCC = 15V
Voltage gain: | AV |=|Vo/Vi | = 15 ±10% (from 100Hz to ≥ 100kHz ; AV < 0)
Input resistance: Ri = 10kΩ ±10%
Output resistance: Ro < 50 Ω
Load resistance : RL = 10kΩ
Low cut-off frequency : f1 = 100Hz
Note: The upper cut-off frequency can be greater than 100kHz
Minimum output voltage swing (resistor RL connected): Vo (peak−peak) = 2V
Note:The signal at the output must be a sine wave with low distortion.
Transistors: Q1 and Q2, BC547B
The data sheet for the npn general purpose transistors (BC547B) can be
downloaded from the home page of the course. In case you need transistor
parameters, use the typical values given in the data sheet.
Resistors: 10Ω to 10MΩ (accuracy 10%),
E12-series (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82)
Capacitors (E6-series (10, 15, 22, 33, 47, 68) :
Polyester: 10nF to 0.33µF (100V)
Electrolytic: 10nF to 330µF (25V)
C2. Report of hand calculations for amplifier design synthesis
The report of the amplifier design synthesis should contain a short summary of
calculations for transistor bias points and power consumption (DC-analysis) as
well as a large signal analysis to estimate the maximum output signal peak-topeak swing (for undistorted sinusoidal output signal).
The report of AC small signal analysis should contain calculations for finding
the voltage gain of the transistor feedback amplifier, the input and output
resistance and the low cutoff frequency.
Show the report of calculations to the assistant for confirmation to build the
transistor amplifier on a wood board with discrete components.
Note: Try to keep your report of the amplifier design synthesis very short but
meaningful in the sense that it contains more than only an answere.
It is an advantage if you do/have done the simulations of section 3.2 before you
write the summary of the feedback amplifier design synthesis.
Note: The report is a home work for the laboration and only intended to provide
increased understanding of the basic theory dealt with in the laboration.
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3.2. Computer simulations with PSpice
This section is to be completed before you start the lab measurements.
S1. Bias point simulation
Find the DC voltage and current values of VB1, VC1, VE1, VB2, VC2, VE2, VBE1,
VBE2, VCE1, VCE2 and IC1, IC2. Find the power consumption Ptot from the
simulation output file. Is the circuit functioning properly? Compare the
simulation results with the values you found in the hand calculations.
S2. Gain and distortion using the time domain (transient) simulation
Apply a sinusoidal signal with 10kHz frequency to the input and plot the output
voltage. Repeat this analysis for input signal amplitudes of 1mV and 67mV.
Find the gain Av for each plot and compare them with the value to be satisfied.
Note: An input signal with amplitude 67mV amplified with the specified voltage
gain: | AV |= 15 will result in a output signal with a voltage amplitude approx.
Vˆ ≈ 1000mV = 1V. Thus the output signal for this input signal amplitude
out
should be an undistorted sine wave as the required the swing (Vo,peak-to-peak)
was specified to be Vo,peak-to-peak > 2V.
Observe if the output is capable of a swing of 2V peak-to-peak, and if the
output signal is an undistorted sine wave. Make a Fast Fourier Transform (FFT)
for distortion analysis, check the output file for the Total Harmonic Distortion
(THD).You can also check the harmonic content visually using transient
simulation followed by an FFT plot (and using program Probe to plot a graph).
S3. Frequency response using the AC sweep simulation
Find the -3dB frequency corners (referenced to the gain for frequency 10kHz in
the intermediate frequency pass band) by plotting the output amplitude (use a dB
marker on the output y-axis and a log scale on the x-axis for frequency).
Compare the value you find with the value to be satisfied.
S4. Input and output resistance from the AC sweep simulation
Find the input and output resistance. Use the definitions (Ri = vi/ii and Ro = vo/io)
and make sure the biasing is not affected or altered.
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3.3 Laboration measurements
Provided that you have completed sections 3.1 and 3.2 before you continue with
this section the following measurements will be performed on the feedback
amplifier circuit during the lab session.
M1. Bias point measurement
Build the circuit in Figure 3.1 on a wood board using nails, copper wire and the
components, after having received confirmation from the lab teacher for your
hand calculations and simulation results.
Measure the following DC voltages:
(section 3.1)
(section 3.2-S1)
(section 3.3-M1)
Hand calculations
Computer simulation
Laboration measurement
VB1=
VC1=
VE1=
VB2=
VC2=
VE2=
Calculate values of voltage, current and power consumption Ptot from above:
VBE1= VCE1=
VBE2=
VCE2=
IE2= IC1=
Ptot=
Compare measurement results with calculations and values found in section S1.
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M2. Voltage gain, frequency response, input and output resistances
Connect a signal generator to the input. Check the signal at the output with an
oscilloscope. Adjust the input level so that the output signal is undistorted.
M2.1 Measure the small signal voltage gain in the passband at an intermediate
frequency f = 10 kHz. Also determine the maximum undistorted large signal
output voltage level (Vo,peak-to-peak).
M2.2 To measure the lower and upper cutoff frequencies, start at an
intermediate frequency f =10kHz in the passband . Check the signal at the
circuit output with an oscilloscope. Adjust the input level so that the output
signal is undistorted. From f =10kHz start decrease the frequency “down” to
find the low -3dB frequency corner at the frequency where the voltage gain has
decreased -3dB with reference to its value in the passband . Start again at the
intermediate frequency f =10kHz in the passband and now increase the
frequency “up” from 10kHz to find the upper -3dB frequency corner at the
frequency where the voltage gain has decreased -3dB with reference to the value
in the passband.
M2.3 Measure the input and output resistances (for small signals for linear
conditions) . See in Appendix B how this can be done.
(section 3.1)
(section 3.2-S1)
(section 3.3-M2)
Hand calculations
Computer simulation
Laboration measurement
Gain AV =
VO, Peak-Peak =
f-3dB(low) =
f-3dB(high) =
Rin =
Rout =
Compare your measurement results with the hand calculations and the values
you found in sections S2, S3, and S4.
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Appendix A How to make the metal thread wiring and where to “hammer”
down the nails on the wood board of the prototype transistor amplifier before
soldering the discrete electronics components.
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Appendix B Measurements to determine the small signal input and output
resistance of the transistor feedback amplifier
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Appendix C Cut out the lower figure below. Nail positions are indicated
with small circles (o), lines between “circles” show how metal wiring can be
done. Solder wires before you place components. Be sure you place the
bipolar transistors in a correct position! See datasheet for BC547B
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