Operational amplifier
Introduction
An operational amplifier (op-amp) is a differentional high-gain electronic voltage amplifier
which is widely used in numerous electronic circuits. In other words, an operational amplifier
produces on its output voltage that is typically 106
-times larger than the difference between
voltages on its inputs. An ideal op-amp has infinite gain, infinite input impedance (i.e.
zero currents flowing to the inputs) and zero output impedance. Real op-amps usually have
voltage gain over 10 000, input resistivity at least 50 k and output impedance approx.
50 .
Usually, op-amps have two inputs (inverting and non-inverting) and one output. When
the potential of the non-inverting input is higher than the potential of the inverting input,
the output voltage is positive and vice-versa.
Tasks
1. Operational amplifier connected in the inverting amplifier configuration
The inverting amplifier configuration (see fig. 1) well demonstrates the behaviour of opamps.
Since the output voltage cannot reach infinite, the voltage between the inputs must
be approx. zero. The non-inverting input is grounded in this configuration. Therefore, the
potential of the inverting input (point A) must be zero as well. Further, the current flowing
through the resistor R1 must be identical with current flowing through the resistor R2 since
the input impedance of the operational amplifier is high. Consequently, U1/R1 = -UO/R2
and the configuration amplifies the input voltage according to the relation
UO = -
R2
R1
U1 (1)
In this task, you will
* connect the operational amplifier in the inverting configuration. Choose such resistors
that the configuration amplifies the input signal by ca 2 and verify whether the
circuitry works well. Several pairs of the input and output DC voltages should be
measured and the slope of the measured dependence should agree with the calculated
gain.
* determine the bandwidth of the particular operational amplifier. Bandwidth is the
frequency range where the op-amp amplifies the signal, conventionally range where the
gain does not drop below Au,max/

2. Au,max is the maximum gain for low frequencies
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Figure 1: Operational amplifier connected in the inverting amplifier configuration.
given by eq. (1). Connect AC signal to the circuitry and measure the dependence of
the gain on the frequency by means of an oscilloscope. From a graph showing this
dependence define the op-amp bandwidth.
2. Low-pass filter
The configuration in the fig. 2 is similar to the inverting amplifier configuration (fig. 1).
However, the capacitor decreases the impedance of the feedback branch for high frequencies
leading to the gain
Au = -
RF
RA
1
1 + iCF RF
(2)
In this task, measure the dependence of the gain of the low-pass filter depicted in the fig. 2
on frequency and define its bandwidth.
Figure 2: Low-pass filter configuration.
3. Non-inverting amplifier configuration
Take a look at the configuration in the fig. 3. The input voltage is connected to the noninverting
input whereas the inverting input is connected to ground through the resistor
2
R1. The feedback is connected to the inverting input through the resistor R2. By means
of a procedure analogical to the procedure used in the task 1 you should calculate what
makes the configuration in the fig. 3. During the practicum, you will verify whether your
calculation was right.
Figure 3: Operational amplifier connected in the non-inverting amplifier configuration.
4. Differential amplifier
Verify whether the configuration in the fig. 4 amplifies the difference between two input
voltages in accordance with the equation
U0 = U2
R4(R1 + R2)
R1(R3 + R4)
- U1
R2
R1
(3)
For example if your choose R1 = R3 = 10 k and R2 = R4 = 22 k, you get:
UO = 2,2 (U2 - U1) (4)
Figure 4: Operational amplifier connected in the differential amplifier configuration.
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5. Voltage comparator
Since operational amplifier multiplies the difference between the input potential by a very
high number, it is very sensitive tool to compare two signals. Fig. 5 shows an example of
the comparator configuration. Explore behaviour of this comparator.
Figure 5: Comparator configuration.
6. Differentiator
There are four various configurations with an op-amp in the fig. 6. One of them is a
differentiator, it means that its output voltage is directly proportional to the derivative
of the input voltage. Verify whether this configuration works according to the mentioned
statement. During this task you will have to solder one component to the network.
Who likes brain-teasers, can try to guess, what are the other configurations in the fig. 6.
In addition, you can try to think out how to make a network with an op-amp which behaves
as a summing amplifier of as a negative resistivity. But these tasks are outside of the scope
of an usual practicum.
4
+
-
+
-
-
+
-
+
Figure 6: Several examples of op-amp configurations.
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