Download Op-Amp Voltage Amplifiers Word Document

Topic 2.4 – Analogue Communications.
2.4.2 Op-Amp Voltage Amplifiers.
Learning Objectives:
At the end of this topic you will be able to;
 draw the circuit diagram for a non-inverting amplifier;
 select and use the formula G  1 
RF
to calculate the gain of a non
R1
inverting amplifier;
 draw the circuit diagram for an inverting amplifier;
 select and use the formula G  
RF
to calculate the gain of an
R IN
inverting amplifier;
 in each case, use the gain formulae to select resistors to produce
the given gain;
 interpret and produce graphs showing input and output signals.
1
GCSE Electronics.
Unit E2 : Applications of Electronics
2.4.2
Op-Amp Voltage Amplifiers
All of the amplifier circuits we are going to consider in this topic are built
around the operational amplifier or Op-Amp for short.
The circuit symbol for an operational amplifier (or Op-amp) is shown below:
+V
Inverting input
-
Non-inverting input
+
VOUT
-V
There are several things to be aware of when dealing with Op-amps;
i.
ii.
iii.
iv.
v.
The ‘+’ & ‘-‘ signs do not refer to power supply connections.
There are two inputs; the non-inverting input ‘+’ and the inverting
input ‘-‘.
There is one output, labelled VOUT.
There are two power supply connections labelled +V and –V, since an
op-amp requires a dual rail power supply. This provides both a positive
and a negative voltage (e.g. ±12V) to allow the AC signal to swing
above and below 0V.
The saturation voltage of an op-amp is usually 1 or 2 volts less than
the power supply voltage. Typically for a supply of ±9V, the saturation
voltage would be ±8V
Two commonly used op-amps are the LM741 and the much better TLO81. Both
have the same pinout as the industry standard op-amp, the 741. The op-amp is
usually packaged in an 8-pin DIL package as shown below
2
Topic 2.4 – Analogue Communications.
2.4.2 Op-Amp Voltage Amplifiers.
We need to consider two types of voltage amplifier in this topic, the noninverting amplifier and the inverting amplifier.
Non-inverting Amplifier
The input impedance of a non-inverting op-amp is very high which makes it
ideal to be used as a pre-amplifier. The circuit diagram for non-inverting
amplifier is shown below:
RF
+V
Note : The power supply
connections are often left off
the diagram of the amplifier to
make the circuit easier to follow.
+
-V
VIN
VOUT
R1
0V
The voltage gain of this amplifier is given by the following formulae.
Voltage Gain 
VOUT
R
 1 F
VIN
R1
Important things to remember for this amplifier are:
i.
ii.
iii.
Voltage gain can be determined if either VOUT and VIN are known or if
RF and R1 are known.
The gain is positive so at any instant, if the input voltage is positive,
the output will be also be positive, and vice versa.
If you are designing an amplifier of this type then all resistors
chosen must be greater than 1kΩ, to reduce power dissipation.
We will now look at some examples:
3
GCSE Electronics.
Unit E2 : Applications of Electronics
Example 1:
A non-inverting amplifier is required to act as a preamplifier for a
microphone. The amplifier requires a voltage gain of +100.
(a)
Draw the circuit diagram for a non-inverting amplifier.
RF
+V
+
-V
VIN
VOUT
R1
0V
(b)
Determine a suitable resistor for RF if R1 = 1kΩ
In the question we are told that the gain needs to be +100, so we
now apply the gain formula as shown below;
Gain  1 
RF
R1
100  1 
RF
R1
100  1 
RF
R1
R F  99  R1
R F  99  1k  99k
OR if you find it difficult to rearrange the formula try this
method:
For a non-inverting op-amp with a gain of 100, RF is 99 times
bigger than RIN so:
R F  99 1k  99k
4
Topic 2.4 – Analogue Communications.
2.4.2 Op-Amp Voltage Amplifiers.
Example 2:
The following circuit shows a non-inverting amplifier connected to a ±10V
power supply. The saturation voltage is ±8V.
RF = 47kΩ
+10V
+
-10V
VIN
R1= 1kΩ
VOUT
0V
(a)
(b)
What is the voltage gain of this amplifier?
Gain  1 
RF
R1
Gain  1 
47
 48
1
If the peak value of VIN = 100mV, determine the peak value of
VOUT.
Gain 
VOUT
VIN
VOUT
100
 48  100  4800mV  4.8V
 48 
VOUT
5
GCSE Electronics.
Unit E2 : Applications of Electronics
(c)
The graph below shows the 100mV AC signal applied to VIN. On the
axes below sketch the graph of VOUT.
VIN/mV
100
75
50
25
0
-25
-50
-75
-100
VOUT/V
Output graph not inverted - i.e.
positive i/p gives positive o/p etc.
Peak output voltage = peak input voltage x gain
4.8
3.6
2.4
1.2
0
-1.2
-2.4
-3.6
-4.8
Frequency remains the same
6
Topic 2.4 – Analogue Communications.
2.4.2 Op-Amp Voltage Amplifiers.
(d)
The AC signal applied to VIN is now increased to 200mV. On the
axes below sketch the new graph of VOUT.
VIN/mV
200
155
100
50
0
-50
-100
-150
-200
Output graph not inverted - i.e.
positive i/p gives positive o/p etc.
VOUT/V
Peak output voltage = 200mV x 48 = 9.6V
Saturation occurs at 8V, therefore output voltage
is clipped at saturation values.
9.6
7.2
4.8
2.4
0
-2.4
-4.8
-7.2
-9.6
Frequency remains the same
HORRIBLE SCALES – I ENCOURAGE MINE TO DRAW IN THE FULL SINE
WAVE, USING A DOTTED LINE, WITH THE CALCULATED AMPLITUDE,
AND THEN CLIP IT AT THE SATURATION VOLTAGES.
Here are a couple of examples for you to do!
7
GCSE Electronics.
Unit E2 : Applications of Electronics
Exercise 1:
1.
The following circuit shows a non-inverting amplifier connected to a ±10V
power supply.
RF = 78kΩ
+V
+
-V
VIN
R1= 2kΩ
VOUT
0V
(a)
What is the voltage gain of this amplifier?
.......................................................................................................................................
.......................................................................................................................................
(b)
If the peak value (amplitude) of VIN = 75mV, determine the peak
value of VOUT.
.......................................................................................................................................
.......................................................................................................................................
.......................................................................................................................................
8
Topic 2.4 – Analogue Communications.
2.4.2 Op-Amp Voltage Amplifiers.
(c)
The graph below shows the AC signal applied to VIN. On the axes
below sketch the graph of VOUT. Add a suitable scale to the VOUT
axis.
VIN/mV
100
75
50
25
0
-25
-50
-75
-100
Vout/mV
0
9
GCSE Electronics.
Unit E2 : Applications of Electronics
2.
A non-inverting amplifier is required to act as a preamplifier for a
microphone in a public address system. The amplifier requires a gain of
35.
(a)
Draw the circuit diagram for a non-inverting amplifier.
(b)
Determine a suitable resistor value for R1 if RF = 68kΩ.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................
10
Topic 2.4 – Analogue Communications.
2.4.2 Op-Amp Voltage Amplifiers.
Inverting Amplifier
One use of the inverting amplifier is as part of a mixer circuit which we will
look at later. First we consider the inverting amplifier on its own. The circuit
diagram is shown below:
RF
+V
RIN
Note : The power supply
connections are often left off
the diagram of the amplifier to
make the circuit easier to follow.
VIN
+
-V
VOUT
0V
The voltage gain of this amplifier is given by the following formulae.
Voltage Gain 
VOUT
R
 F
VIN
R IN
Important things to remember for this amplifier are:
i.
ii.
iii.
Voltage gain can be determined if either VOUT and VIN are known or if
RF and RIN are known.
The ‘-‘ sign in the formula indicates the inverting action of this
amplifier, so at any moment if the input voltage is positive, the
output will be negative, and vice versa.
If you are designing an amplifier of this type then all resistors
chosen must be greater than 1kΩ, to reduce power dissipation.
We will now look at some examples:
11
GCSE Electronics.
Unit E2 : Applications of Electronics
Example 3: An inverting amplifier is required with a voltage gain of -20.
(a)
Draw the circuit diagram for an inverting amplifier.
RF
RIN
+
VIN
VOUT
0V
(b)
Determine a suitable resistor for RF if RIN has a value of 10kΩ.
We apply the gain formula as shown below;
Gain  
RF
R IN
RF
10
 R F  20  10
 20  
R F  200k
OR if you find it difficult to rearrange the formula try this
method:
For an inverting op-amp with a voltage gain of -20, RF is 20 times
bigger than RIN so:
R F  20 10k  200k
12
Topic 2.4 – Analogue Communications.
2.4.2 Op-Amp Voltage Amplifiers.
Example 4:
The graph below shows the input and output waveforms of an inverting
amplifier
VIN/mV
10
7.5
5
2.5
0
-2.5
-5
-7.5
-10
VOUT/mV
800
600
400
200
0
-200
-400
-600
-800
13
GCSE Electronics.
Unit E2 : Applications of Electronics
(a)
What is the voltage gain of this amplifier
From the graph:
The amplitude of the input signal is 10mV
The amplitude of the output signal is 600mV
Gain 
VOUT 600mV

 60
VIN
10mV
but, the output signal is inverted, so the voltage gain is -60
(b)
Either:
Here is the circuit diagram for the inverting amplifier.
Calculate the value of RF
Gain  
RF
R IN
RF
2 k
R
60  F
2 k
R F  60  2k  120k
Or:
For an inverting op-amp with a
gain of -60, RF is 60 times
bigger than RIN so:
 60  
14
R F  R IN  60
 2k  60
 120k
Topic 2.4 – Analogue Communications.
2.4.2 Op-Amp Voltage Amplifiers.
Exercise 2:
1.
The following circuit shows an inverting amplifier connected to a ±12V
power supply. The output saturates at ±10V.
390kΩ
13kΩ
VIN
+
VOUT
0V
(a)
What is the voltage gain of this amplifier?
.......................................................................................................................................
.......................................................................................................................................
15
GCSE Electronics.
Unit E2 : Applications of Electronics
16
(b)
Graph 1 below shows the signal applied to VIN. On graph 2 axes
sketch the graph of VOUT. Label the graph with suitable values.
(c)
VIN is increased to 0.5V. On graph 3 axes sketch the graph of the
new signal at VOUT. Label the graph with suitable values.
Topic 2.4 – Analogue Communications.
2.4.2 Op-Amp Voltage Amplifiers.
2.
An inverting amplifier is required with a voltage gain of -50.
(a)
Draw the circuit diagram for an inverting amplifier.
(b)
Determine a suitable resistor for RIN if RF =75k.
.......................................................................................................................................
.......................................................................................................................................
.......................................................................................................................................
.......................................................................................................................................
.......................................................................................................................................
.......................................................................................................................................
17
GCSE Electronics.
Unit E2 : Applications of Electronics
Solutions to Student Exercises
Exercise 1:
1.
(a)
Gain  1 
RF
R1
Gain  1 
78
 40
2
(b)
Gain 
VOUT
75
 40  75  3000mV  3V
 40 
Vout
(c)
VOUT/V
4
3
2
1
0
-1
-2
-3
-4
18
VOUT
VIN
Topic 2.4 – Analogue Communications.
2.4.2 Op-Amp Voltage Amplifiers.
2.
(a)
RF
+V
+
-V
VIN
VOUT
R1
0V
(b)
In the question we are told that the voltage gain needs to be 35,
so applying the gain formula;
Gain  1 
RF
R1
35  1 
68
R1
35  1 
68
R1
68  34  R 1
R1 
68
 2 k
34
OR if you find it difficult to rearrange the formula using the
other method:
For a non-inverting op-amp with a gain of 35, RF is 34 times bigger
than R1 so:
R1 
68
 2k
34
19
GCSE Electronics.
Unit E2 : Applications of Electronics
Exercise 2:
1.
(a)
(b)
VOUT/V
600
400
200
0
-200
-400
-600
-800
20
RF
R IN
Gain  
390
 30
13
For a voltage gain of-30 the peak value of VOUT will be 30 times
larger than VIN and will also be inverted:
Graph 2
800
Gain  
Topic 2.4 – Analogue Communications.
2.4.2 Op-Amp Voltage Amplifiers.
(c)
When VIN is increased to 0.5V, the output will try to reach an
amplitude of 30 x 0.5V = 15V
The op-amp saturates at ±10V, so the output will be clipped at 10V,
so Graph 3 should look like that shown below.
Vout/V
Graph 3
12
9
6
3
0
-3
-6
-9
-12
2.
RF
(a)
RIN
VIN
+
VOUT
0V
(b) Either:
(b) Or:
Gain  
RF
R IN
 50  
75k
R IN
R IN
75k

 1.5k
50
For an inverting op-amp with a gain of 50, RF is 50 times bigger than RIN so:
R F  R IN  50
75k  R IN  50
R IN 
75k
 1.5k
50
Now for some Examination Style Questions
21
GCSE Electronics.
Unit E2 : Applications of Electronics
Examination Style Questions
1.
A voltage amplifier is needed which will produce the output waveform shown below, when the
input signal shown is applied to it.
(a)
Which of the following describes the performance of this amplifier?
A.
B.
C.
D.
E.
F.
(b)
It is an inverting amplifier with a voltage gain of 39.
It is a non-inverting amplifier with a voltage gain of 39
It is an inverting amplifier with a voltage gain of 40.
It is a non-inverting amplifier with a voltage gain of 40.
It is an inverting amplifier with a voltage gain of 390.
It is a non-inverting amplifier with a voltage gain of 390.
Answer:
……
[1]
Complete the following circuit diagram for the amplifier you chose in part (a) based on an
op-amp.
[2]
(c)
The input voltage is increased to 1V.When testing the amplifier, it is found that the output
voltage never goes above +12V, or below –12V. Give an explanation for this finding.
………………………………………………………………………………………………
……………………....………………………………………………………………………
[2]
22
Topic 2.4 – Analogue Communications.
2.4.2 Op-Amp Voltage Amplifiers.
2.
The following diagram shows an inverting amplifier.
The power supply is ±15V and the output saturates at ±12V
(a)
A voltage gain of 100 is required. Choose a suitable values for RF if R1 = 2.2k.
..................................................................................................................................................
..................................................................................................................................................
(b)
................................................................................................................................................
[2]
An AC voltage of peak value 2V is applied to the input.
Complete the graph showing the corresponding output voltage.
[3]
23
GCSE Electronics.
Unit E2 : Applications of Electronics
3.
The graph shows the input signal applied to four different voltage amplifiers, A, B, C and D.
These are the graphs of outputs of the four amplifiers.
24
Topic 2.4 – Analogue Communications.
2.4.2 Op-Amp Voltage Amplifiers.
Use the graphs to answer the following questions.
(a)
What is the amplitude of the output of amplifier A?
The answer is ....................................
[1]
(b)
What is the voltage gain of amplifier A?
The answer is ....................................
[1]
(c)
Which amplifier, A, B, C, or D, is clipping the output?
The answer is ....................................
[1]
(d)
What is the saturation voltage of amplifier D?
The answer is ....................................
[1]
25
GCSE Electronics.
Unit E2 : Applications of Electronics
4.
The following diagram shows a non-inverting amplifier.
(a)
On the circuit diagram, clearly label:

The output of the amplifier with the letter O;

The feedback resistor with the letter F;

The inverting input of the op-amp, with the letter I.
[2]
(b)
Use the values for RF and R1 to calculate the gain of this amplifier.
................................................................................................................................................
(c)
................................................................................................................................................
[2]
The gain of the amplifier is changed to 100. Use the following frequency response graph to
determine the bandwidth of the amplifier.
Show clearly on the graph how you obtain your result.
Bandwidth = .............................
26
[2]
Topic 2.4 – Analogue Communications.
2.4.2 Op-Amp Voltage Amplifiers.
5.
(a)
Draw a diagram showing how an op-amp and 2 resistors can be used to produce a noninverting amplifier
[3]
(b)
One undesirable effect which can occur in this amplifier is clipping of the output
waveform. Describe one cause of clipping.
………………………………………………………………………………………………..
(c)
………………………………………………………………………………………………..
[1]
The amplifier has a voltage gain of 100
The same voltage gain can be obtained by using two amplifiers each with a gain of 10.
Explain how this might be better than a single amplifier with a gain of 100.
...............................................................................................................................................
...............................................................................................................................................
[2]
27
GCSE Electronics.
Unit E2 : Applications of Electronics
6.
The diagram shows an incomplete non-inverting amplifier circuit.
(a)
(i)
Which input, C or D, on the diagram is the inverting input?
....................
[1]
(ii)
Complete the diagram for the non-inverting amplifier.
[2]
(iii)
Which resistor, A or B, is the feedback resistor?
....................
[1]
28
Topic 2.4 – Analogue Communications.
2.4.2 Op-Amp Voltage Amplifiers.
(b)
The resistors A and B are chosen to give a gain of 40.
The following signal is the input voltage V1.
(i)
Calculate the peak value (amplitude) of the output voltage V2 in mV.
................................................................................................................................................
................................................................................................................................................
................................................................................................................................................
[2]
(ii)
Draw a graph of this output voltage V2.
[3]
(c)
The Gain-Bandwidth Product of the amplifier is 1.6MHz. Calculate the bandwidth of the
amplifier.
……………………………………………………………………………………
[1]
29
GCSE Electronics.
Unit E2 : Applications of Electronics
7.
The following diagram shows a non-inverting amplifier.
(a)
Use the values for RF and R1 to calculate the gain of this amplifier.
................................................................................................................................................
................................................................................................................................................
[2]
(b)
The resistors are now changed to give a gain of 10.
The following signal is then applied to VIN.
(i)
Calculate the maximum value of the output voltage VOUT in mV.
................................................................................................................................................
................................................................................................................................................
[2]
30
Topic 2.4 – Analogue Communications.
2.4.2 Op-Amp Voltage Amplifiers.
(ii)
Draw a graph of this output voltage VOUT on the following grid.
[3]
31
GCSE Electronics.
Unit E2 : Applications of Electronics
8.
A non-inverting amplifier is used as a pre-amplifier.
(a)
(i)
Which input, C or D, on the diagram is the non-inverting input?
................................................................................................................................................
[1]
(ii)
Complete the diagram for the non-inverting amplifier.
[2]
(iii)
Choose suitable values for resistors A and B to give a gain of 50.
................................................................................................................................................
................................................................................................................................................
................................................................................................................................................
[2]
32
Topic 2.4 – Analogue Communications.
2.4.2 Op-Amp Voltage Amplifiers.
(b)
The following signal is the input voltage V1.
(i)
Calculate the peak value (amplitude) of the output voltage V2 in mV.
................................................................................................................................................
................................................................................................................................................
[2]
(ii)
Draw a graph of this output voltage V2.
[3]
33
GCSE Electronics.
Unit E2 : Applications of Electronics
9.
(a)
Complete the following diagram to show an inverting amplifier by adding two resistors R IN
and RF and any necessary connections.
[3]
(b)
RIN = 11k and RF = 330kWhat is the gain of the amplifier?
.................................................................................................................................................
.................................................................................................................................................
.................................................................................................................................................
[3]
(c)
34
RF is changed to give a gain of -80
The following signal is then applied to the input.
The amplifier output saturates at ±9V.
Topic 2.4 – Analogue Communications.
2.4.2 Op-Amp Voltage Amplifiers.
(i)
Use the graph to find the maximum value of the input voltage VIN in mV.
.................................................................................................................................................
[1]
(ii)
Calculate the maximum value of the corresponding output voltage VOUT in mV.
.................................................................................................................................................
[1]
(iii)
Draw a graph of this output voltage VOUT on the following grid.
[3]
35
GCSE Electronics.
Unit E2 : Applications of Electronics
(d)
The power supply changes, and the amplifier is found to saturate at ±7V.
Draw a graph of the new output voltage on the following grid.
[2]
36
Topic 2.4 – Analogue Communications.
2.4.2 Op-Amp Voltage Amplifiers.
10.
An audio pre-amplifier is shown below.
The amplifer has a voltage gain of 250.
The op-amp is powered from a ±15V supply and saturation occurs at ±12V.
(a)
A test signal of amplitude 8mV is applied to the amplifier input.
(i)
Calculate the amplitude of the output voltage.
[1]
.................................................................................................................................................
.................................................................................................................................................
37
GCSE Electronics.
Unit E2 : Applications of Electronics
(ii)
The test signal is shown on the following graph.
Draw the graph of the amplifier output on the axes below.
Mark on the graph any significant voltage.
[2]
38
Topic 2.4 – Analogue Communications.
2.4.2 Op-Amp Voltage Amplifiers.
(b)
The amplitude of the test signal is increased to 80mV. Use the axes provided to sketch the
resulting output signal. [The time axis has the same scale.]
[2]
(c)
The amplifier (voltage gain 250) has a bandwidth of 25 kHz. Use the axes provided to
sketch the frequency response of this amplifier.
[2]
39
GCSE Electronics.
Unit E2 : Applications of Electronics
11.
A non-inverting voltage amplifier has a gain of 25
The amplifier uses a ±15V power supply.
The diagrams below show two input signals applied to this amplifier.
In each case, draw the corresponding output signal using the axes provided.
In the part (a) the voltage scale for the output signal is provided.
In the part (b), you must label the output signal with appropriate voltages.
[5]
(a)
40
Topic 2.4 – Analogue Communications.
2.4.2 Op-Amp Voltage Amplifiers.
(b)
41
GCSE Electronics.
Unit E2 : Applications of Electronics
Self Evaluation Review
My personal review of these objectives:
Learning Objectives



draw the circuit diagram for a noninverting amplifier;
select and use the formula G  1 
RF
R1
to calculate the gain of a non
inverting amplifier;
draw the circuit diagram for an
inverting amplifier;
select and use the formula
G
RF
to calculate the gain of an
R IN
inverting amplifier;
in each case, use the gain
formulae to select resistors to
produce the given gain;
interpret and produce graphs
showing input and output signals.
draw the circuit diagram for a noninverting amplifier;
Targets:
1.
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
2.
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
42