Digital and Interfacing Systems
CENG 215
Lab 7 An Introduction To The Operational Amplifier
Name _________________
Objective:
To gain a basic understanding of Operational Amplifiers.
Equipment:
1. LM 741 Op Amp, Oscilloscope, Function
generator, DVM, Parts kit
Introduction:
The Op Amp is the most versatile and widely
used analog integrated circuit.
It replaces
discrete transistors in virtually all small signal
applications at frequencies up to 1 MHz. The
most popular Op Amp is the LM 741 whose
representative circuit diagram is shown at right.
Note: When using an Op Amp circuit always
connect the supply voltages and ground first
and measure them with a DVM to verify they
are correct.
Part 1: Open Loop Operation
1. Open loop operation does not use negative
feedback and cannot be used in a practical
amplifier configuration because of the very
large gain involved.
2. Set up the circuit shown at right.
3. Connect
a 100 Hz sine wave of
approximately 50 mV P-P to Vin. Be sure
that the signal has no DC component or
offset. Set the scope to DC coupling.
4. Connect a scope to Vin and Vout and draw
sketches of Vin and Vout showing peak
voltages with respect to ground, and phase
relationships.
Measure the positive and
negative saturation voltages - they are
usually not the same values.
+VSAT = ……………. –VSAT =………………
Part 2: Closed Loop Inverting Amplifier
A closed loop amplifier uses negative feedback. This involves taking a sample
of the output voltage and ‘feeding it back’ so that it subtracts from the input
voltage which reduces and stabilizes the voltage gain and results in many other
benefits.
1. Set up the circuit shown at right, using a ±
10V supply.
2. Connect a 500 Hz sine wave of approx 200 –
300 mV with no DC offset to VIN. Use DC
Coupling. Connect a scope at VIN and VOUT.
Draw sketches of VIN and VOUT showing P-P
voltages with respect to ground and phase
relationships. Adjust the VIN signal so that VOUT
is not saturated (not clipping at the
top/bottom) before making your sketches.
3. Press the Measure button on the scope and record the values of VIN & VOUT
VIN = .........………..... VOUT = ...........………......
Determine the closed loop voltage gain of the
amplifier as:
AV,CL = VOUT / VIN = .......................
Note: There are no units for GAIN, it is a simple
Ratio of the output and input voltages.
4. Calculate the voltage gain of the amplifier as:
AV,CL = - RF / R1 = ..............
Note: The minus sign in the formula indicates the output
of the amplifier is 180° out of phase with input. It does
NOT indicate negative gain or attenuation.
5. Compare the Gain values found in steps 3 and 4. Do they agree? ………………
6. Observe VOUT on the scope. Adjust the amplitude of VIN so that VOUT is, for
example, exactly 6V P-P.
7. Increase the frequency until the amplitude of Vout has dropped to 0.707 of
what it was in step 6. (0.707 * 6 V P-P = 4.2 V P-P). This frequency is the
corner frequency or Bandwidth (BW) of the amplifier.
fcorner = Bandwidth = ..............................
8. Replace RF with a 100 kΩ resistor and repeat steps 3, 4, 5, 6, and 7.
VIN = ................
VOUT = ...............
AV.CL = - RF / R1 = ..................
AV,CL = VOUT / VIN = ..…….........
fcorner = ........................
Did AV,CL increase or decrease when RF was increased to 100 kΩ?
Did fcorner increase or decrease when RF was increased to 100 kΩ?
Write a statement about Gain Vs Bandwidth:
Procedure Part 3: Closed Loop Non-Inverting Amplifier
1. Set up the circuit shown below right.
2. Apply a 1 kHz sine wave of 200 – 300 mV (with
no DC offset) to Vin. Adjust the Vin amplitude so
the O/P does not saturate. Connect a scope at
Vin and Vout. Draw sketches of Vin and Vout
showing peak voltages with respect to ground
and phase relationships. Be sure that Vout is
not saturated before making your sketches.
3. Measure the closed loop voltage gain of the
amplifier:
VIN = ..…….… VOUT = .………..…
AV,CL = VOUT / VIN = .........…
4. Calculate the voltage gain of the amplifier as
AV.CL = 1+ (RF / R1 ) = ..........
5. Do the values found in steps 3 and 4 agree?
……....
Voltage Follower
A special case of the non inverting amplifier is
the voltage follower. It uses 100% negative
feedback. It is often used as a buffer between
two circuits or an impedance matching device
when a low O/P impedance is desirable.
6. Set up the circuit at right and replace RF with a piece of wire and remove
R1. Make a calculation for the measured value using the equivalent values
for RF and R1 .
AV,CL = 1+ (RF / R1 ) Calculated AV,CL = ..........
7. Measure the voltage gain.
VIN = .……........... VOUT = .….……........
Measured AV.CL = VOUT / VIN = ....….............
Do the calculated and measured values agree? ………..
Questions
1. In all op-amp circuits, the voltage difference between the inverting input
and non-inverting inputs (between pins 2 & 3) is considered to
be……………………………Volts
2. In your own words, state why most op-amps are not used as amplifiers in
the open loop mode.
3. Explain briefly, in your own words, what effect negative feedback has on the
voltage gain of a closed loop amplifier.
4. Decreasing voltage gain increases/decreases the band width (corner or cut
off frequency fcorner)
Signed …………………………………………
Date……………………….