Experiment 4: Op-Amp Circuits
Objective:
In this experiment, you will learn about operational amplifiers (or op-amps). Op amps can be used
in many different ways. Two of the most common uses are as
1) Comparators
2) Amplifiers (inverting and non-inverting amplifiers)
EQUIPMENT AND PARTS LIST
Digital Multimeter (DMM)
Adjustable Dual DC power supply
Circuit breadboard
Op-Amp: uA741 or LM741
Potentiometer: 10 kΩ
Resistors: 10 kΩ and 33 kΩ
BACKGROUND & THEORY
Operational amplifiers can be basically viewed as voltage amplifiers with very high open loop gain, very
high input resistance, and very low output resistance. They are commonly available in an integrated
circuit (IC) package. An IC can include many components such as transistors, diodes, resistors, and
capacitors integrated onto a tiny silicon chip. There are literally hundreds of different uses for op amps,
but we'll only experiment with three of very basic applications.
The op amp you'll be using is the type 741. Fig. 1 shows the pin configuration of the 741 in a DIP (dual inline package).
Figure 1: Pin-diagram of the 741 op amp
The pins denoted by V- and V+ are often referred to as -VCC and +VCC , respectively, and must be
connected to regulated dc power supplies. Typically, they are at -15 V and +15 V although other values
are also quite common. Since the power supply must always be applied in order for the op amp to work,
its connection to the V+ and V- pins are customarily omitted in a schematic diagram in order to avoid
over-crowding the diagram.
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Pins 2 and 3 are the inverting and non-inverting inputs of the op amp, respectively. The op amp
produces amplified output of the voltage difference between the two inputs; it is therefore sometimes
also referred to as a differential amplifier.
The output, Vo, of the op amp is related to the inputs by Vo=A (V+in-V-in), where A is the open-loop
voltage gain of the op amp. Since the open loop gain of an op amp is typically larger than 105, its gain
must be controlled through a feedback network. Otherwise the op amp would be easily saturated even
for a very small input voltage difference. The negative feedback drastically reduces the overall gain and
keeps op amp from going into saturation.
The balance (or null offset) pins (1 and 5) provide a way to eliminate any offset in the output
voltage of the amplifier. The offset voltage (usually denoted by Vos) is an artifact of the integrated
circuit. The offset voltage is additive with VO (pin 6 in this case). It can be either positive or negative
and is normally less than 10mV. Because the offset voltage is so small, in most cases we can ignore
the contribution VOS makes to VO and we leave the null offset pins open. Pin 8, labeled "NC", has no
connection to the internal circuitry of the 741, and is not used.
1. COMPARATOR
In the circuit below, the non-inverting input is connected to “ground”, so V+in=0.
Figure 2: Comparator
Vary the voltage applied to the inverting input (using the potentiometer) to show that:
If V-in > 0 then Vo = ____________
and if V-in < 0 then Vo = ____________
Repeat the experiment with the INVERTING input connected to ground and the non-inverting input
connected to the variable potential divider. Now we find:
if V+in > 0 then Vo = ____________
and if V+in < 0 then Vo = ____________
Find two applications for this circuit and have a short description for each application.
2. INVERTING AMPLIFIER
The negative feedback drastically reduces the overall gain and keeps op amp from going into
saturation. Two of the most basic op amp circuits are shown in Fig. 3 and Fig. 4. Both circuits show
that the closed-loop gain, 𝐴𝑣 , is independent of the op amp hence permitting easy substitution of
devices.
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Figure 3: Inverting amplifier
In general, the definition of the voltage gain 𝐴𝑣 of an amplifier is defined as follows:
𝑣0
𝐴𝑣 =
𝑣𝑖
In the particular case of an op amp with negative feedback (as shown in Fig. 3), you should prove that
the gain can be calculated using
𝑅𝑓
𝐴𝑣 = −
𝑅1
Using 𝑅1 = 10 kΩ and 𝑅𝑓 = 33 kΩ, take at least three readings of 𝑣𝑖 and 𝑣0 in order to verify this
relation.
3. Non-Inverting Amplifier
Try a similar experiment but using the circuit shown below:
Figure 4: Non-inverting Amplifier
In this case, you should be able to prove that the voltage gain of this amplifier is
𝑅𝑓
𝐴𝑣 = 1 +
𝑅1
Again, using 𝑅1 = 10 kΩ and 𝑅𝑓 = 33 kΩ, take at least three readings of 𝑣𝑖 and 𝑣0 in order to verify this
relation.
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