Exp 10 Op Amps Lab Notes

8/4/2019 Exp 10 Op Amps Lab Notes

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Experiment 10

OP AMPS

Introduction

The operational amplifier is possibly the most widely used analogue device; the applications inscience, technology, medicine and engineering fields are myriad.

Ideally, the op amp is considered to have1. Infinite gain in the open-loop configuration2. Infinite input impedance which ensures that the signal source is unaffected by the amplifier,and that no current flows into the input terminals3. Zero output impedance which ensures that the amplifier is unaffected by the load and is

capable of delivering whatever current is required. In addition, it enables the amplifier to

operate as a zero-impedance source for a subsequent stage of amplification.

4. Zero response time which ensures that the frequency response is perfectly flat and thebandwidth infinite.

5. Zero voltage and current offset which ensures that when the input signal voltage is zero theoutput signal will also be zero, whatever the source resistance, Likewise, when the voltages

on the two inputs are the same there is no output.

For a number of simple applications, the ideal behaviour of op amps may be assumed. However,in practice the actual parameters are typically (with variations depending on the precision and

cost of the device and the application for which it is intended):

Open loop gain: 105-10

6

Input impedance: Between inputs > 105

Between input and ground > 108

Output impedance: < 100

Frequency response: Decreasing linearly with frequency

Voltage offset: Adjustable and < 5mV without adjustment

Current offset:


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Your device is an operational amplifier type commonly referred to as a 741: this is packaged as

an 8-pin Dual-in-Line (DIL) component.

Pin Description

1 Offset terminal

2 Inverting input

3 Non-inverting input

4 Negative voltage supply (-15V)

5 Offset terminal6 Output

7 Positive voltage supply (15V)

In circuit diagrams, op amps are usually shown as in Fig 1,

Usually the bias and offset connections are omitted as being understood.

Q1.

Study the detailed circuit diagram for the operational amplifier in the manufacturers data sheet

and try to determine qualitatively how the device works. In particular, discuss how the desirable

features of the op amp can be accounted for.

The Inverting Amplifier

An inverting amplifier is typically constructed as shown in Fig 2.

output

inverting input

Non inverting

input

V+

V-

1

2

3

6

6

4

5

Figure 1

_

+

voVi

Zf

ZiFigure 2


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The ideal voltage gain of this configuration is given by

i

f

i

o

Z

Z

v

vA ==

where Zfand Zi are the feedback and input impedances respectively.

Construct the amplifier shown in Fig 2, choosing Zf= 10k and Zi = 220k (the offset

connections may be ignored for the present applications).

Calculate the expectedgain for this configuration. Then apply a 1kHz sine wave input from your

oscillator. Measure the input and output voltages and calculate the actual gain. How does this

compare this to the expected (ideal) value. Comment on any differences.

Increase the amplitude of the input signal and determine the maximum allowable amplitude at

the output for which the amplified signal is not distorted?

Q2 Why does the output waveform distort at high amplitudes?

Q3 Why is the circuit called an inverting amplifier?

Measure the frequency response by taking gain measurements for a range of input frequencies.

Use a 1-2-5 sequence from 10Hz to 106 Hz. Also, for each frequency, measure the phase shift

between the input and output signals

Construct a Bode plot from the results (log10|A(f)| versus log10 (f in Hz)).

Plot the phase shift as a function of log(f).

Q4 For an ideal amplifier, the frequency response should be constant. From your Bode plot,

is the dependence what you would expect?

Q5 For an ideal amplifier, the phase shift between the input and output would be zero for all

frequencies. What causes the phase shift between the input and output signals?

The Non-Inverting Amplifier

Connect up the circuit shown in Figure 3.

vo

vi

Rf = 1M

Ri = 22k Figure 3_

+


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Using the ideal op amp equations, describe how this circuit works and predict its gain. Use the

oscillator to investigate the characteristics of this amplifier. What is the actual gain for a 1kHzinput?

Q6 What happens if you make a non-inverting amplifier with Rf=Ri = 0? What would be the

purpose of such a circuit?

Op Amp Adding Circuit

Consider the circuit shown in Fig 4.

As usual v- v+ 0 and input impedance in terminal is infinite.

Therefore if= ia + ib + ic + id

and

+++=

D

D

C

C

B

B

A

A

f

o

R

v

R

v

R

v

R

v

R

v

If (for example) Rf=RA=RB=RC=RD, then[ ]DCBAo vvvvv +++=

Hence this circuit is an adder to within a change in phase angles. By choosing different ratios forthe feedback and input resistances we can weight each input and, for example, have

[ ]DCBAo vvvvv 432 +++=

Construct an adder circuit as above, using Rf = 1K and appropriate input resistances to sum two

input signals, such that [ ]BAo vvv 52 +=

Apply the same signal to both inputs and verify that the gain is -7.

Rebuild the inverting amplifier and connect it in between the oscillator and one of the adders

inputs (so the output from the oscillator is on input A and the inverted output from the oscillator

on input B.

Measure the gain and compare to the expected gain of +3. Explain any differences.

Figure 4

RfvB

vo

vA

_

+

vC

vD

v+

v-

if

ia

ibic

i

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Exp 10 Op Amps Lab Notes