IL2218 Analog electronics, advanced course
Academic year 2008-2009, period 3
Lab 4 – Operational amplifiers
Name:..............................................................
Personal number:.............................................
Date of approval:..............................................
Assistant:.........................................................
2009‐01‐30 1 Lab 4 –Operational amplifiers
Since its first introduction in the late 1960s, the integrated circuit operational
amplifier has been the most widely used of all linear circuits in production. It
has found application in virtually every analog area such as analog/digital and
digital/analog convertors, voltage reference sources, analog multipliers, waveshaping circuits, oscillators and waveform generators. Today operational
amplifiers such as for example LΜ741 can be obtained at price levels on the
order of $ 0.5 and other newer OPs of high quality even cheaper in large
qualtities. As a result, despite its inherent limitations, namely slew-rate and
gain-bandwidth tradeoff, the versatility of the operational amplifier has made it
an ubiquitous element of analog design, present in most electronic systems and
in many integrated circuits. In this lab, you will use an operational amplifier
together with the output stage you had analyzed previously. Further
understanding on the basic properties of an operational amplifier in open-loop
and closed-loop forms, and the effect of feedback on distortion will be provided.
4.1 Hand calculations
Figure 4.1 The LM741 OP amplifier with an extra output power amplifier.
Voltage supply VCC = 10V; load resistor RL = 8Ω or 100Ω
2 The operational amplifier together with the previously analyzed output circuit
(which actually forms a new operational amplifier) is given in Figure 4.1. For
this lab, you do not need to make any hand calculations before. But analyze the
circuit and try to understand what the aim of the configuration is.
LM741 OP amplifier with an extra output power amplifier for our measurements
will be connected in three different configurations (refer to figure 4.1 ):
1. Open-loop topology: Switch open. LM741 terminal 2 connected to 0V
(GND). Input signal Vi connected to LM741 terminal 3.
2. Closed-loop, Series-shunt negative feedback topology: Switch closed
so that LM741 terminal 2 is connected to switch and feedback resistors.
Input signal Vi connected to LM741 terminal 3.
3. Closed-loop, Unity voltage gain topology (|Av| = |Vo/Vi| = 1) : Switch
open. LM741 terminal 2 connected to output signal voltage Vo of load
resistor RL at output stage. Input signal Vi connected to LM741 terminal 3.
The data sheet for the LM741 op-amp can be downloaded from the home page
of the course or from the world wide web. Make sure you have a look at it and
try to interpret the data given.
Figure 4.2 The previously analyzed output stages (in Lab2) of the new
operational amplifier = LM741 OP + class B/class AB output stage.
3 4.2 Computer simulations (using PSpice)
This section is to be completed before you start the lab measurements:
S1. Find the voltage transfer characteristic using DC sweep simulation Repeat simulations for class B/AB operation and open/closed-loop conditions
(four combinations ) for RL = 100Ω and VCC = 10V. Plot the voltage transfer
characteristic (VTC), Vo = f(Vi) for the voltage interval: −VCC <Vi < VCC
•
Find the small signal gain vo/vi for open/closed-loop conditions.
• Compare the open/closed-loop gain values you find with the theoretical
values (You will find the open-loop gain for the operational amplifier in the
data sheet. Find out what your closed-loop gain is).
•
Observe the effect of feedback basically on gain and distortion from plots.
S1.1 Operational amplifier with class B output stage
S1.1.1 Class B. Open-loop.
Sketch the DC-sweep simulation plot for the voltage transfer function (VTC):
vo
v i
a) Small-signal gain vo/vi from simulation plot:
vo/vi = ……………..
b) Small-signal
vo/vi = ……………..
gain vo/vi from data sheet:
4 S1.1.2 Class B. Closed-loop, unity gain.
Sketch the DC-sweep simulation plot for the voltage transfer function (VTC):
vo
v i
a) Small-signal gain vo/vi from simulation plot:
vo/vi = ……………..
b) Theoretical small-signal gain vo/vi (unity gain) : vo/vi = ……………..
c) The effect of feedback (compare open- and closed-loop configuration)
on gain, distorsion and input range (before output is saturated):
……………………………………………………………………………..
……………………………………………………………………………..
5 S1.2 Operational amplifier with class AB output stage
S1.2.1 Class AB. Open-loop.
Sketch the DC-sweep simulation plot for the voltage transfer function (VTC):
vo
v i
a) Small-signal gain vo/vi from simulation plot:
vo/vi = ……………..
b) Small-signal
vo/vi = ……………..
gain vo/vi from data sheet:
6 S1.2.2 Class AB. Closed-loop, unity gain
Sketch the DC-sweep simulation plot for the voltage transfer function (VTC):
vo
v i
a) Small-signal gain vo/vi from simulation plot:
vo/vi = ……………..
b) Theoretical small-signal gain vo/vi (unity gain) : vo/vi = ……………..
c) The effect of feedback (compare open- and closed-loop configuration)
on gain, distorsion and input range (before output is saturated):
……………………………………………………………………………..
……………………………………………………………………………..
7 S2. Find the Bode plots (magnitude and phase response) of the
voltage transfer function by using the AC sweep simulation
Repeat the following simulations for open/closed-loop conditions of class AB
operation (two combinations in total): Plot the magnitude and phase response
of the vo/vi voltage transfer function.
•
Find the open-loop gain (AC simulation).
•
Determine the poles/(zeroes) of the open-loop gain.
S2.1.1 Magnitude response for class AB. Open-loop condition.
Sketch the magnitude response of the voltage transfer function (VTC):
vo/vi
(dB)
f (Hz) a) Indicate magnitude dB-values on y-axis and frequency (Hz) on x-axis
b) Determine the poles of the magnitude response:
p1= …………………Hz ; p2=…………………………Hz;
8 S2.1.2 Phase response for class AB. Open-loop condition.
Sketch the phase response of the voltage transfer function (VTC):
φ (degrees) 0 ‐60 ‐120 ‐180 f (Hz) For the OP amplifier with a class AB output stage:
•
Find the closed-loop gain (AC simulation).
•
Determine the poles/zeros of the closed-loop gain and
the unity-gain bandwidth.
•
Find the phase margin.
9 S2.2.1 Magnitude response for class AB. Closed-loop unity gain condition.
Sketch the magnitude (AC-) response of the voltage transfer function (VTC):
vo/vi
(dB)
f (Hz) a) Indicate magnitude dB-values on y-axis and frequency (Hz) on x-axis
b) Determine the poles of the magnitude response:
p1= …………………Hz ; p2=…………………………Hz;
c) Unity gain bandwidth (Hz) = …………………………MHz
Note condition: (vo/vi)dB = 0 dB;
10 S2.2.2 Phase response for class AB. Closed-loop condition.
Sketch the phase (AC-) response of the voltage transfer function (VTC):
φ (degrees) 0 ‐60 ‐120 ‐180 f (Hz) Note: See above the plot of the magnitude response (in S2.2.1) to find the
frequency where (vo/vi) = 0 dB.
a) Phase margin: φm = 180 - abs[ φ {(vo/vi)dB = 0 dB} ] =………………
b) Loop-gain βA = .......................................
| A(log f) |
dB
100
βA
80
-20 dB/dekad
60
-40 dB/dekad
40
A/(1 +βA) 1/β
20
log {f /(Hz)}
f0
0
101
102
10 3
11 104
105
10 6
S3. Find output voltage from a time domain (transient) simulation
S3.1 Observe and plot below the output voltage for class AB and the closed-loop
unity gain case by applying a square wave signal with 500mV amplitude and
1kHz frequency to the input.
Input square wave: Vi = 500mV ; frequency f = 1kHz; Closed-loop, unity gain.
Output load voltage VO for class AB operation with load resistor RL = 100Ω .
Output load voltage Vo (V) 0.0
2.0
1.0
t (ms)
S3.2 Repeat the same analysis but now inserting a feedback resistor (you do not
have unity-gain any more) with different values e.g. 10kΩ, 100kΩ and 1MΩ.
Observe and plot in the diagram below the output voltage for all three cases in
the same diagram. Indicate resistor value and gain for each curve !
Output load voltage Vo (V) 0.0
2.0
1.0
t (ms)
• What is the effect of the feedback resistor on the output signal? Do your
observations comply with the theory? ……………………………………
……………………………………………………………………………….
12 S4. Find the distortion of the output voltage by using time
domain (transient) simulation and Fourier analysis.
Repeat the following simulations for class B/AB operation and for open/closedloop conditions (four combinations in total):
S4.1 For the open-loop case (for class B/AB operation); plot the output
voltage by applying a sinusoidal wave signal with a convenient amplitude and
frequency (depending on the results you get in section S2, the amplitude should
be chosen to make sure the output is not saturated, and the frequency should be
in the mid-frequency range of the characteristic) to the input.
Note: See Simulation Settings in PSpice (Analysis type:Time Domain
(Transient)) and fill in values under meny Output File Options.
S4.1.1 Open-loop case for class B operation:
THD (Total Harmonic Distorsion) = ……………………………..
S4.1.2 Open-loop case for class AB operation:
THD (Total Harmonic Distorsion) = ………………………………
S4.2 For the close-loop case (for class B/AB operation); plot the output
voltage by applying to the input a sinusoidal wave signal with 100mV and
500mV amplitudes (for each class of operation) and 1kHz frequency.
• Find the distortion values for each case from the output file. Do the numbers
comply with the theory?
S4.2.1 Closed-loop case for class B operation:
a) vi = 100mV; f=1kHz: THD (Total Harmonic Distorsion) = …………
b) vi = 500mV; f=1kHz: THD (Total Harmonic Distorsion) = …………
S4.2.2 Closed-loop case for class AB operation:
a) vi = 100mV; f=1kHz: THD (Total Harmonic Distorsion) = …………
b) vi = 500mV; f=1kHz: THD (Total Harmonic Distorsion) = …………
13 4.3 Laboration measurements
Provided that you have completed sections 4.1 and 4.2 before you start with this
section, the following measurements will be performed on the operational
amplifier circuit during the lab session:
M1. Sketch the voltage transfer characteristic
Connect the circuit in Figure 4.1. after receiving confirmation from the lab
assistant for your simulation results (See Appendix Printed Circuit Board, PCB).
Repeat the following measurements for class B/AB operation of the closed-loop
case (two combinations in total, since you already have done the open-loop
measurements in Lab 2 Output Stages, section M2).
Measure the voltage transfer characteristic (VTC), Vo = f(Vi) for the voltage
interval of −VCC < Vi < VCC in 1.0V steps (VCC = 10V) . Fill in the table and
plot the VTC in the diagram (on the next page).
• Find the small signal voltage gain vo/vi by using the plots.
M1.1 VTC-measurement: DC-sweep. Class B operation. Closed-loop case.
a) Connect the switch on the PCB for class B operation
b) Measure the voltage transfer characteristic Vo = f(Vi) and fill in the
columns of the table “for class B operation” (Table 4.3.1 next page).
c) Plot VTC-curve in diagram 4.3.1 below (after table Table 4.3.1)
d) Find from plot for class B the small signal gain vo/vi = ……………..
M1.2 VTC-measurement: DC-sweep. Class AB operation. Closed-loop case.
a) Connect the switch on the PCB for class AB operation
b) Measure the voltage transfer characteristic Vo = f(Vi) and fill in the
columns of the table “for class AB operation” (Table 4.3.1 next page).
c) Plot VTC-curve in diagram 4.3.1 below (after Table 4.3.1 next page)
d) Find from plot for class AB the small signal gain vo/vi = ……………
• Compare characteristic and gain with the results you obtained in section S1.
Comments: …………………………………………………………………….
14 Table 4.3.1 Measurements of VTC for class B/class AB operation. Closed-loop
15 Draw in the diagram below for closed-loop case the VTC-curves for both class
B and class AB operation according to the measured values found in the table
(for the measurements M1.1and M1.2)
Voltage transfer characteristic Vo = f(Vi) for the interval −VCC < Vi < VCC
Diagram 4.3.1
vo
vi
16 M2. Sketch the magnitude response of the voltage transfer function
Plot the magnitude response of the vo /vi voltage transfer function for class AB,
closed-loop amplifier.
In order to do this, apply a sinusoidal wave signal with 100mV amplitude and
1kHz frequency to the input. Complete the table and use the diagram with log-xaxis to plot the magnitude response . After sketching the plot:
• Find the closed-loop gain.
• Find the unity-gain bandwidth.
Closed‐loop gain (dB), class AB output stage 1
2
3 4 5 678910
a) Closed-loop gain of the OP amplifier with class AB output stage:
vo/vi = …………………………….
b) Unity gain [vo/vi = 0 dB] bandwidth (Hz) = ……………………..MHz
Compare your results with simulation results!
Comments:………………………………………………………………………..
17 18 M3. Measure the output voltage response for a square wave input signal
M3.1 Observe and plot below the output voltage for class AB and the closedloop unity gain case by applying a square wave signal with 100mV amplitude
and 1kHz frequency to the input.
Input square wave: Vi = 100mV ; frequency f = 1kHz; Closed-loop, unity gain.
Output load voltage VO for class AB operation with load resistor RL = 100Ω .
Output load voltage Vo (V) 0.0
2.0
1.0
t (ms)
M3.2 Repeat the same analysis but now inserting a feedback resistor (you do
not have unity-gain any more) with different values e.g. 10kΩ, 100kΩ and 1MΩ.
Observe and plot in the diagram below the output voltage for all three cases in
the same diagram. Indicate resistor value and gain for each curve !
Output load voltage Vo (V) 0.0
2.0
1.0
t (ms)
• What is the effect of the feedback resistor on the output signal? Do your
observations comply with the theory? ……………………………………
……………………………………………………………………………….
19 Appendix Printed Circuit Board (PCB)
3-Way Switch
class AB
(left position )
NOT connected
class B
(vertical position ) (right position)
+VCC
pinne
b
sva rt
0V
svart
blå
class
AB
ministifthål
vit
vit
V_
blå
vit
V+
röd
OP741
R1
1k
class
B
e
1
grön
D1
gul
1-2 = class AB
2-3 = class B
till R L
grön
grön
1k
1=gul
2= röd
3=blå
b
1
R2
svart
0V
c
pinne
D2
gul
blå
röd
Q1
MJE3055
NPN transistor
c
e
-VCC
Q2
MJE2955
PNP transistor
1) Connect two voltage supplies VCC = 10V in series. Connect the PLUS pole to the PCB PLUS cable
contact, the MINUS pole to the PCB MINUS cable contact and the common connection point of the power
supplies to the ZERO cable contact (0V)
(2) Connect the load RL (either 8Ω or 100Ω ) to the emitter resistors of Q1 and Q2. DON´T forget to
connect the other terminal of the load RL to GROUND (0V) !
(3) NEVER TRY TO CONNECT measurement points directly to the pins of the transistor Q1 and Q2 !
Measure the base voltages at connection point of R1-D1 or R2-D2. Measure the emitter voltage for Q1 and
Q2 at the terminal of the respectively emitter resistor. The collector of Q1 is connected to +VCC.
The collector of Q2 is connected to -VCC.
(4) ERROR: If the SWITCH is in vertical position neither class B or class AB is connceted !
(5) Zero volt (0V) and black mini-holes are connected. White mini-holes and the negative input (V_ ) of
the OP AMP are connected. Red mini-holes and the positive input (V+ ) of the OP AMP are connected. Blue
mini-holes are only connected as an isolated group. Yellow mini-holes are connected to the voltage (Vi )
input terminal of the class B or class AB output stage.
(6) Green mini-holes near Q1 and Q2 are connected to the common point of the two emitter resistors, the
output (Ut). These mini-holes can be used for feedback to the 741 OP Amp in Lab4. The green mini-hole
close the OP Amp 741 is only connected to the output of the 741 to connect the 741 to the yellow-mini
holes of the input (In).
2008‐02‐19 O.Thessén file: Lab4_OPamp_080219_detail_18fin.doc 20 Appendix Schematic Example 1
Class AB Output Stage. Closed-loop condition
21 Appendix Schematic Example 2
Class AB Output Stage. Closed-loop unity gain condition
22 Appendix Schematic Example 3
Class AB Output Stage. Open-loop condition
23