Analog Electronics II (EMT212) Laboratory Module
UNIVERSITI MALAYSIA PERLIS
ANALOG ELECTRONICS II
EMT 212
EXPERIMENT # 1
OP-AMP (INVERTING & NON-INVERTING)
MARKST1 / G1 / T2 / G2 / T3 / G3 / Q / C / Total Marks / 100%
12 / 5 / 12 / 5 / 3 / 1 / 6 / 6 / 50
EXPERIMENT 1
Op-Amp (Inverting and Non-Inverting)
1. OBJECTIVE:
1.1 To demonstrate an inverting operational amplifier circuit
1.2 To demonstrate a non-inverting operational amplifier circuit
1.3 To investigate the operational amplifier voltage follower
2. INTRODUCTION:
The name operational amplifier was originally intended to describe an amplifier capable of performing various mathematical operations in an analogue computer. Such a device had to be capable of handling a wide range of input signal frequencies right down to DC. The first operational amplifiers were constructed from discrete components: transistors, resistor etc. However, these were large and cumbersome and the development of integrated circuit technology meant that very high performance operational amplifiers could be produced in small sized packages.
Analogue computing is dead, replaced by digital computing, though operational amplifiers still find widespread used in electronics. Their main application is in the construction of high performance AC and DC amplifiers. They have the advantage that the gain and frequency response of the amplifier is completely determined by a number of external components according to simple formulae; there is no need to worry about the variation of the gain of the transistors with temperature and voltage supply. Furthermore, using operational amplifier (OP AMPS) it is straightforward to build a simple amplifier for use as part of an experiment, for example, a DC amplifier with a gain of 1000 to boost the output of a thermocouple to levels suitable for driving a digital meter.
The OP AMP is a differential amplifier that is the output is proportional to the difference between two inputs, i.e.
(2.1)
where V+ is the voltage at the so-called non-inverting input, V- is the voltage at the so-called inverting input and A is the gain which is normally very high, typically 100,000 or more. Such an amplifier would appear to be useless because the output would saturate at the smallest value at the input. In fact operational amplifiers are used with large amounts of negative feedback to produce practical amplifiers. We will consider the effect of feedback later, for now we will consider the properties of an ideal and a typical operational amplifier.
The operational amplifier used in this experiment is a type 741, which is a very widely used general purpose amplifier. It requires balanced ± power supplies around zero volts. The zero volt level is referred to as a ground (GND). The input and output connections are made with respect to the power supply ground. The circuit symbol of an operational amplifier and the pin connections of the type 741 are shown in Figure 2.1.
Figure 2.1 741 OP AMP pin out and schematic symbol
2.1 Ideal Op Amp:
In this lab, we will generally assume that our op amps are truly perfect; i.e. Ideal op amp. Ideal op amp can be designed following two simple rules.
i. No current flows into or out of the positive or negative input terminals.
ii. The voltage across the op amp input terminals remains at zero.
2.2 Inverting and Non-inverting Amplifier:
There are two basic configurations for operational amplifier circuits: the inverting amplifier, and the non-inverting amplifier. Operational amplifiers ideally have infinite open-loop gain and infinite open-loop input resistance. Open-loop characteristics refer to those of an amplifier having no feedback resistance between output and input. Closed-loop characteristics are those of an amplifier having an external feedback resistor. The resistor provides negative feedback, whereby a portion of the output voltage is subtracted from the input. Both the inverting and non-inverting amplifier use the principle of negative feedback to control the overall (closed-loop) voltage gain.
2.2.1 Inverting amplifier:
Figure 2.2 Inverting Amplifier
Figure 2.2 shows a typical inverting amplifier configuration. The input signal (Vin) is connected to the inverting input and so an increase in the input voltage will result in a proportionate decrease in the output voltage. So an input of 1V DC with a gain of 10 would result in an output voltage of -10V. The voltage gain of an inverting amplifier is calculated as follows.
Owing to the very large open loop gain (A) of the op amp we can say that, under normal operating conditions (linear amplifier), . This is because the smallest difference between and would cause the output to saturate. Since , the current flowing into the inverting input terminal is virtually zero. So, applying Kirchhoff’s current law at the node between R1 and RF gives;
(2.2)
which leads to
(2.3)
Remember the – (minus) sign is an expression of the phase inversion.
2.2.2 Non-inverting amplifier:
Figure 2.3 shows a typical non-inverting amplifier configuration.
Figure 2.3 Non-inverting Amplifier
In this circuit the output is connected back to the inverting input via a potential divider network consisting of R1 and RF to provide negative feedback. This means that the amplifier will try to oppose any change in the voltage between its input terminals. We can easily calculate the so-called closed loop gain under such conditions, starting from the basic equation describing an operational amplifier (remember that in this case Vin is connected at + terminal).
(2.4)
Now, is derived from via the potential divider and is given by ,
(2.5)
Therefore, combining (2.4) and (2.5) gives
(2.6)
For an ideal op amp A = ¥, this reduces to
(2.7)
The non-inverting amplifier is so called because the input signal is fed to the non-inverting input pin and so an increase in the input voltage will give rise to an increase in the output voltage (i.e. the input and output signals are in phase).
2.3 Unity gain amplifier:
Figure 2.4 Voltage Follower
The circuit sketched in Figure 2.4 is called a voltage follower or unity gain buffer. The feedback line with no load gives
Moreover, because of the condition we will have; which implies
(2.8)
The output follows the input voltage with unitary gain. Considering that the high impedance input and the low impedance output values of Op-Amps are close to the state of the art in the electronic design, the voltage follower can be used as an isolation stage (buffer) between two circuits.
3. COMPONENT AND EQUIPMENT:
3.1 Resistors:
3.1.1 1MW
3.1.2 100 kW
3.1.3 4.7 kW
3.1.4 1 kW (2)
3.2 LM 741 OP-AMP
3.3 DC Power Supply
3.4 Function Generator
3.5 Oscilloscope
3.6 Breadboard
4. PROCEDURE:
4.1 Prepare the power supplies. The op-amp requires two 15 volt supplies as shown in Figure 4.1.
Figure 4.1 Power supply configuration
4.2 To investigate an op-amp used as an inverting amplifier, connect the
circuit in Figure 4.2. The small numbers in the diagram correspond to the integrated circuit’s (chip’s) pin numbers.
Figure 4.2 Inverting Amplifier circuit
4.2.1 Connect a dual-trace oscilloscope to observe both the input Vin and the output Vout. With VS = 0.2Vpp sine wave at 1 KHz, measure and record in TABLE 1 the output voltage Vout for each value of RF listed in TABLE 1. Also, note the phase angle of the output Vout with respect to the input Vin.
4.2.2 Now replace RF with a 1 MW resistor and sketch the resulting waveform Vout as well as the input waveform in GRAPH 1.
4.3 To investigate an operational amplifier used as a non-inverting amplifier, connect the following circuit as in Figure 4.3.
Figure 4.3 Non-Inverting Amplifier Circuit
4.3.1 Connect a dual-trace oscilloscope to observe both the input and the output. Repeat procedure step 4.2.1 for the non-inverting amplifier using the values of RF in TABLE 2.
4.3.2 Replace RF with 1 MW resistor and sketch the resulting output waveform as well as the input waveform in GRAPH 2.
4.4 To investigate the operational-amplifier voltage follower, connect the circuit in Figure 4.4.
Figure 4. 4 Unity Gain Amplifier
4.4.1 With VS = 2 Vpp sine wave at I kHz, measure the output voltage Vout. Record the result in TABLE 3. Note the phase angle of the output with respect to the input. Plot the waveform in GRAPH 3.
Name :______Date : ______
Matrix No. : ______
5. RESULTS:
TABLE 1
RF (ohms) / Vout (volts) / (pre-calculate) / Phase shift,θ
1 kW
4.7 kW
100 kW
(12marks)
(4 marks)
GRAPH 1 (1 marks)
Name :______Date : ______
Matrix No. : ______
RF (ohms) / Vout (volts) / (pre-calculate) / Phase shift,θ
1 kW
4.7 kW
100 kW
TABLE 2
(12 marks)
(4 marks)
GRAPH 2
(1 marks)
Name :______Date : ______
Matrix No. : ______
Vout (volts) / Phase shift, θTABLE 3
(3 marks)
GRAPH 3
(1 marks)
Name : ______Date : ______
Matrix No. : ______
6. QUESTIONS:
markQ1
A1 / What does inverting mean if a sine wave is connected to the op-amp (-) terminal? / (2)
Q2
A2 / What does non-inverting mean if a sine wave is connected to the op-amp (+) terminal? / (2)
Q3
A3 / What discrete circuit is the unity gain follower the op-amp equivalent? / (2)
7. CONCLUSION: (6 marks)
Based on your experiment, make an overall conclusion by observation to the input and output of each circuit.
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