Download Biasing the amplifier

Physics Unit 3 2015
10 Voltage Amplifiers
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10. Voltage Amplifiers
• analyse voltage characteristics of amplifiers including linear voltage gain (ΔVout/ΔVin) and clipping;
• identify and apply safe and responsible practices when conducting investigations involving
electrical, electronic and photonic equipment.
Voltage Amplifiers
Input
Small input voltage
Process
amplifier multiplies input
Output
Larger output voltage
An amplifier is a device that converts the input signal into a larger signal at the output. An amplifier
is used to increase the magnitude of either the current or the voltage. An amplifier usually works
over a restricted range and can either be inverting or non-inverting.
An amplifier usually works over a restricted range. The circuit is provided with an external supply of
emf. The fraction of this external emf that appears at the output terminals is controlled by the input
voltage.
Vout (mV)
The shaded region is called the
active region of the graph where
the input voltage has great control Clipping
over the output. The gradient of
the graph in this region is known
as the gain of the amplifier:
ΔVout
Gain =
Vin (μV)
ΔVin
This is an inverting amplifier because the
graph shown is for a device with an output
that is out of phase with the input - when the
voltage is increasing in the forward direction,
the output is increasing in the negative
direction.
Saturation
Vout (mV)
Vin(V)
Vout (mV)
This is a non-inverting amplifier, the gradient
of the output/input is positive. A positive
increase in the input voltage will give a positive
increase in the output voltage
Vin(V)
Clipping
Clipping occurs when the output voltage that is to be obtained is above the voltage supplied. In this
case the output voltage will be equal to the supply voltage. This is when the base current is too
small and the effective resistance of the amplifier (transistor) is very large when compared to the
other resistors. Thus the voltage over the transistor is equal to the supply voltage.
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Biasing the amplifier
The input signal can only be amplified faithfully when in the active region. The best input operating
voltage is halfway along the sloping section of the graph. This is based on the assumption that the
input signals are symmetrical. A symmetrical input will be amplified symmetrically.
Vout
best operating voltage
Vin
Shifting the base voltage of the input signal so that the whole signal now lies within the active
region is called biasing.
Vout
max. output variation
max. input
variation
Vin
Vout
operating point
output
Vin
input
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If the input is too high, the output will exceed the capability of the amplifier, the output will be
clipped.
Vout
operating point
output
Vin
Input.,
If the input is not centred in the active region the output has one or both extremes clipped.
Vout
Output
Vin
Input
Physics Unit 3 2015
10 Voltage Amplifiers
Amplitude distortion due to incorrect biasing
Amplitude distortion due to clipping
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Physics Unit 3 2015
10 Voltage Amplifiers
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The first section of a portable tape player amplifies the signal from the tape head. The figure below
shows the input and output signals of the amplifier as displayed on an oscilloscope.
Example 1
2000 Question 13
(2 marks)
What is the magnitude of the gain of the amplifier?
Example 2
2000 Question 14
(2 marks)
Which one of the diagrams (A – D) below best indicates the input/output characteristics of the
amplifier?
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The input-output voltage characteristics of an amplifier used in a school’s public address system
are shown below.
Example 3 1999 Question 4
(2 marks)
What is the voltage gain of this amplifier?
The figure below shows part of an input voltage Vin to the amplifier.
Example 4 1999 Question 5
(2 marks)
Circle the letter (A – F) of the graph below that best represents the resulting output voltage Vout
of the amplifier.
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Example 5 1998 Question 6
(2 marks)
A voltage amplifier is connected between an AC signal generator and a cathode ray oscilloscope.
The arrangement is shown below.
The output of the AC signal generator is a sinusoidal voltage with a peak-to-peak voltage of
50.0 mV. Determine the gain of the amplifier. Show your working.
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A student uses an oscilloscope to display both the input voltage, Vin , and the output voltage, Vout ,
of an amplifier. The input voltage can be of various shapes, with a magnitude varying from
0 to ±2 Vpeak. When Vin = 0 V, Vout = 0 V.
The results, for two experiments, 1 and 2, are given below, showing the oscilloscope display for
both the input and output voltages. In each experiment the vertical scale is 2V/division, and the
position of zero volts is indicated for both input and output traces.
Example 6 1997 Question 9
(1 mark)
On the axes below, sketch the graph of V out as a function of V in for this amplifier.
Example 7 1997 Question 10
Calculate the gain of the amplifier.
(1 mark)
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The graph following shows the relationship between the output voltage and the input voltage for an
amplifier.
Example 8 1996 Question 4
(1 mark)
Between 0 < Vin < 0.5V, what is the gain of the amplifier?
The graph below shows part of an input voltage for this amplifier,
Example 9 1996 Question 5
(1 mark)
Which of the following best represents the output of this amplifier if the input voltage is as shown
above? All the graphs have the same time scale.
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Four linear amplifiers are available. Their input-output characteristics are illustrated below
Example 10 1995 Question 12
(1 mark)
Which amplifier (A - D) has the largest gain in the linear amplification region?
Example 11 1995 Question 13
(1 mark)
Which amplifier (A - D) will produce an output of -10 V for an input voltage of +2 V?
The voltage signal shown below is to be amplified using amplifier D above.
Example 12 1995 Question 14
(1 mark)
Sketch the output voltage as a function of time. (Use the diagram above)
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A student needs to amplify the output voltage of a microphone. The student chooses and tests a
voltage amplifier, with the characteristics shown below.
Example 13 1994 Question 8
(1 mark)
What is the voltage gain of the amplifier for the input voltage range from -1.5 V to +1.5 V?
The student varies the input between -0 .5 V and+ 0.5V as shown below
Example 14 1994 Question 9
(1 mark)
Which waveform (A - F) shown below best represents the shape of the output voltage of the
amplifier as a function of time?
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As a final test the student connects a signal generator to the input of the amplifier. The signal
varies between -3 V and +3 V as shown below.
Example 15 1994 Question 10
(2 marks)
Sketch the output voltage of the amplifier. (Use the graph above)
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Antonio needs a simple burglar alarm for his car. He designs a circuit which includes a transducer
and a linear amplifier. The circuit senses the displacement of the car accelerator pedal and sounds
an alarm when the pedal moves further than 25 mm from its initial position.
The transducer provides a sinusoidal output voltage with an RMS value which is directly
proportional to the displacement of the pedal, as shown below.
Antonio arranges for the transducer output voltage to be the input voltage to a linear amplifier
Example 16 1993 Question 8
(1 mark)
When the displacement of the accelerator pedal is 25 mm, what is the RMS input voltage to the
linear amplifier?
When the RMS output voltage of the linear amplifier exceeds 15 V, the alarm rings. Antonio
requires an amplifier with the correct gain so that the alarm rings as the accelerator pedal is
displaced 25 mm.
Example 17 1993 Question 9
(1 mark)
What is the correct value of the amplifier gain that Antonio must choose?
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Solutions
Example 1 2000 Question 13
ΔV
gain = out
ΔVin
2-0
gain = 0.04 - 0
2-0
gain = 0.04 - 0
gain = 50
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Example 7 1997 Question 10
For graph 1 the peak to peak value for
Vin is ~ ½ a square i.e. 1 volt
For graph 1 the peak to peak value for
Vout is ~2.5 squares i.e 5 volt
So the gain is 5.
Example 2 2000 Question 14
The amplification is linear and inverting so the
correct answer is A.
Example 3 1999 Question 4
ΔV
gain = out
ΔVin
0 -18
gain =
0.2 - 0
gain = - 90
Example 4 1999 Question 5
The phase of the output must be equal to the
input. The peak value of the input voltage is
0.1 volts, the output will have a peak value 90
times bigger, so it will be -9.0 V. Therefore
the correct answer is B.
For graph 2 the peak to peak value for
Vin is ~1 square i.e. 2 volt
For graph 2 the peak to peak value for
Vout is ~5 squares i.e 10 volts
So the gain is 5.
Example 8 1996 Question 4
The gain is the gradient of the graph in the
specified region.
Between 0 and 0.5 V, the graph is linear and
6
has a gradient of
0.5
 gain = 12
(ANS)
Example 9 1996 Question 5
The amplifier will only amplify the input signal
when it is in the range between 0 and 0.5 V.
When the signal is within this range it will be
amplified by a factor of 12.
The maximum value of the output will be
 0.4 x 12 = 4.8 V
The amplifier only amplifies the positive
values of the input signal.
 C (ANS)
Example 5 1998 Question 6
The gain of an amplifier is the ratio of
Vout
.In
Vin
5
this case that is equal to
= 100.
50×10-3
The value for Vout is read from the CRO
screen. The graph is 5 squares high (peak to
peak) and the scale is one square = 1.0 volt.
Example 6 1997 Question 9
10V
Example 10 1995 Question 12
The gain of an amplifier is the ratio of
40
8
30
In B,
3
40
In C,
8
20
In D,
8
In A,
=5
= 10
=5
= 2.5
B (ANS)
-2
0
+2
Vin
-10V
The graph needs to look like this because the
amplifier is a non-inverter, with a gain of ×5.
Example 11 1995 Question 13
From the graph
C (ANS)
Vout
.
Vin
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Example 12 1995 Question 14
Example 17 1993 Question 9
The input to the amplifier is 25mV, and for the
alarm to sound the output of the amplifier
needs to be 15V.
V
The gain of an amplifier is the ratio of out .
Vin
15V
15000
Therefore,
=
25mV
25
 600
(ANS)
VOUT
5
2.5
-2.5
-5
Example 13 1994 Question 8
The gain of an amplifier is the ratio of
15 of 15
Vout
.
Vin
36
= 12
3
 12 (ANS)
Therefore,
Example 14 1994 Question 9
The amplifier is a non-inverter, with a linear
gain of 12. Therefore the output signal will
remain the same shape and have a range
between -6 to 6 V. The period will remain the
same.
 A (ANS)
Example 15 1994 Question 10
VOUT
18
-18
Example 16 1993 Question 8
From the graph
25mV (ANS)
Note, that there is a lot of reading to get to
this quite simple question. You need to make
sure that you use your reading time
effectively.