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ETEC 421 Digital Controls 1
Lab # 7 – Signal Conditioning with Op Amps
Purpose:
The purpose of this lab is study signal conditioning with various op amp configurations.
Equipment Required:
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Protoboard
Various components as specified in each part of the experiment.
Digital Voltmeter
OpAmpApplicationsGuide.pdf
Discussion:
Procedure:
1.
Inverting Op Amp:
The figure at the left shows the circuit for an inverting op
amp. Its important characteristics are that it can have gains
both greater and less than 1 and it has a negative output
voltage for a positive input voltage. One of the
disadvantages is that in most cases a dual positive and
negative supply are required. The output voltage is given by
the equation:
R 
Vout    2 Vin
 R1 
and that
R3  R1 || R2
Assemble the circuit as shown and perform the following operations on that circuit:
 Measure Vout with Vin = 1.0, 3.0, and 6.0 volts.
 Swap R1 and R2 and measure Vout again with Vin = 1.0, 3.0, and 6.0 volts.
 Record actual values of all resistances and voltages.
 Calculate the % difference between measured and calculated.
2.
Non-Inverting Op Amp:
The figure at the left shows the circuit for a non-inverting
op amp. An important characteristic of this configuration
is that it can only have a gain greater than 1. However, it
does give a positive output voltage for a positive input
voltage.
 R1  R 2 
Vout  Vin 

 R1 
Assemble the circuit as shown and perform the following operations on that circuit:
 Measure Vout with Vin = 1.0, 2.0, and 3.0volts.
 Swap R1 and R2 and measure Vout again with Vin = 1.0, 2.0, and 3.0 volts.
 Record actual values of all resistances and voltages.
 Calculate the % difference between measured and calculated.
3.
Voltage Follower:
The figure at the left shows the circuit for a voltage follower
circuit. This is the simplest of all of the op amp
configurations. It is used to provide impedance translation
and buffering. It has a gain of one which cannot be changed.
Therefore:
Vout  Vin
Assemble the circuit as shown and perform the following operations on that circuit:
 Measure Vout with Vin = 1.0, 5.0, 7.0, and 9.0 volts.
 Determine the value for Vin beyond which Vout does not change.
 Calculate the % error.
4.
Differential Amplifier
The figure at the left shows the circuit for a differential
amplifier circuit. This circuit allows two voltages to be
independently amplified and then subtracted from each
other. In its most general form, the relationship between
the input and output voltages is:
 R R  R 
R 
Vout   1 2  4 V2   2 V1
 R1 
 R3 R4  R1 
which simplifies to:
R 
Vout  V2  V1  2 
 R1 
if R1 = R3 and R2 = R4.
Assemble the circuit as shown and perform the following operations on that circuit:
 Measure Vout with V1 = 2.0 and V2 = 2.0 volts.
 Measure Vout with V1 = 3.0 and V2 = 2.0 volts.
 Calculate the % error.
5.
Summing Amplifier
The figure at the left shows the circuit for a summing
amplifier circuit used to independently scale individual
voltages and then sum them. The circuit is not limited to
the number of inputs shown in the diagram but can be
expanded as needed. The relationship between the input
and output voltages is:

 R 
 R  
 R  
Vout   V1  5    V2  5    V3  5 
 R1  
 R2  
 R3 

Assemble the circuit as shown and perform the following operations on that circuit:
 Measure Vout with V1 = 2.0, V2 = 1.0 and V3 = 1.0 volts.
 Measure Vout with V1 = 3.0, V2 = 0.5, and V3 = 2.0 volts.
 Calculate the % error.
6.
Voltage Controlled Current Source
The figure at the left shows the circuit for a voltage
controlled current source. Similar in organization to a
non-inverting op amp, this circuit provides a current
proportional to the input voltage independent of the
value of Rload. This circuit is particularly good in
eliminating the effects of wiring resistance for loads
remotely located from the circuit. The relationship is:
I load 
 Vin
R1
Assemble the circuit as shown and perform the following operations on that circuit:
 Measure VRload with V1 = 1.0 volts and Rload = 100 ohm.
 Measure VRload with V1 = 1.0 volts and Rload = 200 ohm.
 Calculate the % error.
7.
Schmitt Trigger
The Schmitt Trigger adds hysteresis to the response of the op amp to
an input boltage. This helps with control circuits by creating a deadband through which the voltage must travel before the circuit
responds. (See the Op Amp reference above for a complete
discussion.) The circuit values define an Upper Trip Point and a
Lower Trip Point. The distance between these trip points defines
the dead band of the circuit. The relationship is:
 R2 
 R2 
Vsat  and LTP  
 Vsat 
UTP  
 R1  R2 
 R1 R2 
Where Vsat = saturation voltage of the op amp.
Assemble the circuit as shown and perform the following operations on that circuit:
 Measure both the Upper Trip Point and the Lower Trip Point.
 Calculate the % error.