YEDITEPE UNIVERSITY ENGINEERING FACULTY
ELECTRONICS LABORATORY
EE 232 - INTRODUCTION TO ELECTRONICS
EXPERIMENT
2 : OPERATIONAL AMPLIFIER
Objective:
Observing the Op-Amp Principal
Grading:
Quiz
Prelab
Simulation
Result Sheet
Performance
:10%
:10%
:25%
:30%
:25%
Equipment:






HP 54603B Oscilloscope
HP 33120A Function Generator
DC Power Supply
LM 741 Op-Amp
Capacitor Series
Resistor Series
General Information:
The operational amplifier is a differential amplifier with an extremely high,
ideally infinite, voltage gain. There are many types available, but perhaps the most
common one is the 741, that is available as an 8 pin dual-in-line (dil) package or as a
14 pin packages containing two devices. You can see from the pin connection
diagram (Fig. 2.1.) how the circuit symbol relates to the practical 8 pin integrated
circuit. Note the notch at one end of the IC. This notch helps us numbering the pins.
Data sheets of LM 741 (A 741) which include comprehensive information
about the device are attached to the end of the experiment sheets, as well. It is better
for you to look through them before coming to the experiment session.
Fig. 2.1. LM 741 Op-Amp Packaging and Pin Connections.
A glance at the circuitry symbol (Fig. 2.2.) for the devices shows that it is an
amplifier that has a single output but two inputs. These inputs are labeled inverting (-)
and non-inverting (+). From amplifier knowledge already gained you should be aware
that a signal applied to the inverting input will produce an anti-phase output, while a
signal applied to the
non-inverting input will
produce an output with
zero phase shift.
Fig. 2.2. Operational Amplifier Circuit Symbol
An ideal operational amplifier has the following characteristics.
Voltage Gain (AV )
Input resistance
Output resistance
Bandwidth
- infinite
- infinite
- zero
- infinite
In practice however, the actual characteristics are different. For 741, these are:
Voltage Gain (AV )
Input resistance
Output resistance
Bandwidth
- 106 dB (20*103)
- 1 M
- 75
- up to 1MHz
Ideally, the input currents drawn by the inverting (+) and non-inverting (-)
terminals are zero. Since the output voltage value of an Op-Amp is forced to be finite
by the power supplies of the Op-Amp (there can be no more voltage value than
supply voltages in a circuit), and under infinite voltage gain assumption the voltage
difference between the inverting and non-inverting inputs can be said to be zero. This
point can be very helpful to analyze circuits that contain Op-Amps.
Fig. 2.3. Gain/frequency characteristic of 741
Although, Op-Amps are used in a wide variety of applications, they initially
used to realize mathematical operations such as differentiation, integration,
logarithmic and anti-logarithmic amplification, weighted summation etc… Therefore
the name “operational amplifier” refers back to these applications.
Op-Amps mostly used in a closed loop circuit topology (feedback).
Here are some examples to the practical operational amplifier circuits:
The Inverting Amplifier:
Since the amplifier is inverting, the output will be 180 out of phase to the input.
The relationship between input and output signals for an inverting amplifier is as
follows:
Rf
V
AV  out  
(2.1)
Vin
Rin
Fig. 2.4. Inverting Amplifier
The Non-Inverting Amplifier:
Since the amplifier is non-inverting, the output will be in same phase to the input.
Rf 


AV  1 
Rin 

(2.2)
Fig. 2.5. Non – Inverting Amplifier
The Difference Amplifier:
The difference amplifier provides an output that is proportional to the difference
between two input signals.
Vo  aV2  bV1  The coefficients a and b can be calculated from the resistor
values in the circuit.
Fig. 2.6. Difference Amplifier
The Inverting Integrator:
If the feedback component used is a capacitor instead of a resistor, as shown in Fig
2.7. , the resultant circuit realizes the mathematical operation of integration.
Fig. 2.7. Inverting Integrator
The relationship between Vin and Vout can be written as
Vout (t )  
1
Vin (t )dt  Vout( t 0 )
Rin C 
Vout(t=0) is the initial value of the output voltage.
(2.3)
Differentiator:
Interchanging the location of the capacitor and the resistor of the integrator circuit
results in the circuit in Fig 2.8. ,which performs the mathematical function of
differentiation.
Fig 2.8. Differentiator
The relationship between Vin and Vout can be written as
Vout (t )   R f C
dVin (t )
dt
(2.4)
In practice, an Op-Amp has some imperfections such as common mode gain
(characterized by CMRR), slew rate etc... that limit the operation of the Op-Amp, in
some applications.
As explained before, output of the op-amp is proportional to the difference
between the two inputs. So, when these two voltages are the same, output must be
zero. But due to the inputs having slightly different gains, output doesn’t become
zero.
 differential mod e gain 
(2.5)
CMRR  20  log 
 dB
 common mod e gain 
The higher the CMRR value the better.
There is a specific maximum rate of change possible at the output of a real OpAmp. This maximum is known as the slew rate (SR) of the Op-Amp and is defined as
SR 
dVo
dt
(2.6)
max
To understand the slew rate limitation of an Op-Amp let’s consider an Op-Amp
connected in the unity-gain voltage follower configuration shown in Fig 2.9. and let
the input signal be the step voltage as shown in Fig 2.10.
Fig 2.9. Unity Gain Follower
vi
n
v
t
vout
v
t
Slope=Slew Rate
Fig 2.10. Slew Rate
Fig 2.10 Input Step Waveform and The Step Response of the Output of the Op-Amp
The output of the Op-Amp will not be able to rise instantaneously to the ideal value
V; rather, the output will be the linear ramp of slope that is equal to SR, shown in Fig
2.10 The amplifier is then said to be slewing, and its output is slew-rate limited. SR is
expressed in volts per microsecond (V/s) and indicates how quickly the amplifier
can respond to a change in input signal. If the input signal applied to an Op-Amp
circuit is such that it demands an output response that is faster than the specified value
of SR, the Op-Amp will not comply. Therefore, the greater the SR, the less the input
frequency limitation by Op-Amp.
.
(DO NOT forget; you have to bring Prelab and Simulation output to the
laboratory.)
Prelab :
(You have to make manuel calculations for his part)
1. Assume that Vin is a sine wave with the amplitude of 400 mVp-p, Rin=1K,
Rf=100K, Vcc=15V in Fig 2.4. Find the output peak to peak amplitude value.
2. For inverting integrator circuit in Fig 2.7. investigate why the feedback resistor
(Rf) is added to the circuit.
Procedure :
(You have to make simulations of all 5 procedure below.)
(Hint: You should use transient (time domain) analysis and be careful to set up
the total run time and maximum step size. You should show the 5 period of the
signals.)
(Draw waveforms of all procedures, smilarly as in Result Sheet.)
1. Build the inverting amplifier circuit in Fig 2.4. with the values of Vcc=15V,
Rin=1K, Rf=10K. Apply 400mVp-p sine wave with the frequency of 1 kHz to the
input. Observe the input and output signals together on the scope and draw them
on a scope sheet.
Change the value of the feedback resistor (Rf) to 100K and observe the output on
the scope, again.
2.
Build the non-inverting amplifier circuit in Fig 2.5. with the values of Vcc=15V,
Rin=1K, Rf=10K. Apply the same procedure as step 1.
3.
Build the inverting integrator circuit in Fig 2.7. with the values of Vcc=15V,
Rin=10K, Rf=1M, C=10nF. Apply 400mVp-p square wave with the frequency of
1 kHz to the input. Observe the input and output signals together on the scope and
draw them on a scope sheet. Does your observation verify the expression (2.3).
Switch the input signal to sinewave, triangle and ramp and in each case, explain
the output waveshapes that occur at the output. (For the simulation of this part,
to take good results and view good waveforms, you should put a tick to the
box of SKIPBP in transient options in PSpice.)
4. Make proper changes to your circuit to build the differentiator circuit in Fig 2.8.
Take Rf=10K, Rs=1.5K, C=1nF. Apply a triangle wave with the amplitude of
10Vp-p and the frequency of 1 kHz to the input. Observe the input and output
signals together on the scope and draw them on a scope sheet. Does your
observation verify the expression (2.4).
Switch the input signal to sinewave, square and ramp and in each case, explain the
output waveshapes that occur at the output.
5. Build the circuit in Fig 2.9. to measure the SR of the Op-Amp with the values of
Vcc=15V, Rin=100K. Apply a square wave with the amplitude of 1Vp-p and with
the frequency of 1 kHz to the input. Measure and record the SR of the Op-Amp.
Is that value same as the one on the data sheets of the Op-Amp?
To see the slew-rate limitation of the circuit, raise the input frequency by
observing the output signal on the scope. How can you explain the distortion in
the output waveshape while the input frequency rises?