Transcript BTEC-Electronics

Chapter3: Small-signal Audio-frequency Amplifiers
Chapter3: Small-signal Audio-frequency Amplifiers
3.0
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
Integrated circuit
Principles of operation (Quiescent Operating Point)
Choice of configuration
Determination of gain using a load line
Bias and stabilization
Voltage gain of BJT amplifier
Voltage gain of f.e.t. amplifier
Voltage, current and power amplifiers
Multi-stage amplifiers
Measurements on audio-frequency amplifiers
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Chapter3: Small-signal Audio-frequency Amplifiers
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Chapter3: Small-signal Audio-frequency Amplifiers
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Chapter3: Small-signal Audio-frequency Amplifiers
n-P-n bipolar transistor
n-P-n bipolar transistor with a buried layer
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Chapter3: Small-signal Audio-frequency Amplifiers
Integrated Diode
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Chapter3: Small-signal Audio-frequency Amplifiers
Integrated Resistor
R
l
Wd

l
ld


d
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Chapter3: Small-signal Audio-frequency Amplifiers
Integrated Capacitor
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Chapter3: Small-signal Audio-frequency Amplifiers
simple circuit shown in Fig.2.10a is to be integrated.
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Chapter3: Small-signal Audio-frequency Amplifiers
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Chapter3: Small-signal Audio-frequency Amplifiers
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Chapter3: Small-signal Audio-frequency Amplifiers
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Chapter3: Small-signal Audio-frequency Amplifiers
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Chapter3: Small-signal Audio-frequency Amplifiers
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Chapter3: Small-signal Audio-frequency Amplifiers
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Chapter3: Small-signal Audio-frequency Amplifiers
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Chapter3: Small-signal Audio-frequency Amplifiers
3.1 Principles of operation
◆ Transistors and f.e.t.s may be used as amplifiers because th
eir output currents can be controlled by an a.c. signal applied t
o their input terminals.
◆ By suitable choice of collector current,and hence of input im
pedance,a transistor may be considered as either a current-ope
rated device or a voltage-operated device.
◆ If the source impedance is much larger than the input impe
dance of the transistor,the transistor is current operated. If mu
ch smaller, it is voltage operate.
◆ A f.e.t. has such a high input impedance that its input curre
nt is negligible ；it can therefore give only a voltage gain.
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Chapter3: Small-signal Audio-frequency Amplifiers
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Chapter3: Small-signal Audio-frequency Amplifiers
In the common-emitter connection:
 Ic
h fe 
 IB
I C  h feI B
 I C  VBE  VBE



gm  I B  IC
 IB
h fe
input impedance:
 Ic
gm 
 VBE
h fe  g m ri
VBE
ri 
I B
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Chapter3: Small-signal Audio-frequency Amplifiers
3.1 Principles of operation
◆ The mutual characteristics of a f.e.t or a transistor always exhibit some
non-linearity. If a suitable operating point is chosen and the amplitude of
the input signal is limited, the operation of the circuit may be taken as
linear without the introduction of undue error.
◆ The function of a small-signal amplifier is to supply a current or voltage
to a load, the power output being unimportant. In a large-signal amplifier,o
n the other hand, the power output iS the important factor.
◆
change in output current
current gain Ai =
change in input current
◆
◆
change in output voltage Ai RL
voltage gain A v =

change in input voltage
RIN
change in output power Ai 2 RL
power gain A p =

 Ai Av
change in input power
RIN
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Chapter3: Small-signal Audio-frequency Amplifiers
3.2 Choice of configuration
◆ The various ways in which a transistor or f.e.t.may be connected to provide a
gain are shown in Fig.3.1.
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Chapter3: Small-signal Audio-frequency Amplifiers
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Chapter3: Small-signal Audio-frequency Amplifiers
3.2 Choice of configuration
◆ The short-circuit a.c. current gain hfe of a transistor connected in the common-emitte
r configuration (Fig.3.3) is much greater than the short-circuit a.c. current gain of the
same transistor connected with common base, i.e. hfe=hfb／(1- hfb ). Resistance-capacita
nce coupling of the cascaded stages of an amplifier is possible and nowadays transforme
rs are rarely used. Generally, common-emitter stages are biased, so that the transistor
is current operated. Then the input impedance is in the region of 1000-2000Ω while the
output impedance is some 10-30 kΩ.
Fig. 3.3 common-emitter amplifier
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Chapter3: Small-signal Audio-frequency Amplifiers
3.2 Choice of configuration
◆ A transistor connected as a common-base amplifier (Fig. 3.2) has a short
circuit a.c. current gain hfb less than unity (typically about 0.992), a low input
impedance of the order of 50Ω, and an output impedance of about 1 MΩ.
Because the current gain is less than unity, common-base stages cannot be
cascaded using resistance-capacitance coupling but transformer coupling
can be used. Transformers, however, have the disadvantages of being relatively
costly, bulky and heavy and having a limited frequency response, particularly
the miniature types used in transistor circuits.
Fig. 3.2 common-base amplifier
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Chapter3: Small-signal Audio-frequency Amplifiers
3.2 Choice of configuration
◆ The common-collector circuit,or emitter follower as it is usually called, is shown in
Fig.3.4. This connection hasa high input impedance, a low output impedance, and a
voltage gain less than unity. The main use of an emitter follower is as a power amplifyin
g device that can be conveniently connected between a high-impedance source and a
low-impedance load.
Fig. 3.4 common-collector amplifier
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Chapter3: Small-signal Audio-frequency Amplifiers
3.2 Choice of configuration
◆ In the normal mode of operation (Fig.3.5), the source is common to the input and out
put circuits, the input signal is applied to the gate, and the output is taken from
between drain and earth.This connection provides a large voltage gain and has a high in
put impedance.
Fig. 3.5 common-collector amplifier
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Chapter3: Small-signal Audio-frequency Amplifiers
3.2 Choice of configuration
◆ Fig.3.6 shows the f.e.t. equivalent of the emitter follower, this is known as the source
follower circuit. The follower circuit will be treated in greater detail in Chapter 4.
Fig. 3.5 common-collector amplifier
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Chapter3: Small-signal Audio-frequency Amplifiers
3.3 Determination of gain using a load line
3.3.1 The relationship of output voltage and output current
VCC  VCE  IC RL
(a) giving one of the required points.
Let I c  I D  0 ;then VCC  VCE
(b) giving the second point
Let VCC  VCE  0 ;then VCC  IC RL
Fig. 3.6
common-emmitter amplifier
(c) If these two points are located
on the characteristics and joined by
a straight line,the load line for the
particular load resistance and
supply voltage is obtained.
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Chapter3: Small-signal Audio-frequency Amplifiers
3.3 Determination of gain using a load line
3.3.1 The relationship of output voltage and output current
VDD  VDS  I D RL
(a) giving one of the required points.
Let I c  I D  0 ; VDD  VDS
(b) giving the second point
VDD  I D RL
Fig. 3.7
common-source amplifier
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Chapter3: Small-signal Audio-frequency Amplifiers
3.3 Determination of gain using a load line
3.3.1 The relationship of output voltage and output current
example 3.1
A transistor connected in the common-emitter configuration has the data
given in Table 3.1. Plot the output characteristics of the transistor and draw
the load lines for collector load resistances of (a) 1000Ω and (b) 1800Ω.Use
the load lines to determine the steady (quiescent) collector current and
voltage if the base bias current is 80μA and the collector supply Ic=0
Vce=Vcc=9V and is marked A in Fig.3.3.
Tab. 3.1 data of the common emmitter amplifier
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Chapter3: Small-signal Audio-frequency Amplifiers
3.3 Determination of gain using a load line
3.3.2 Choice of Operating Point
◆In practice, some non-
linearity always exists, and to
minimize signal distortion care
must be taken to restrict
operation to the most nearly
linear part of the characteristic.
◆ For this a suitable operating
point must be selected and the
signal amplitude must be
restricted.
Fig. 3.8
Choice of Operating Point
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Chapter3: Small-signal Audio-frequency Amplifiers
3.3 Determination of gain using a load line
3.3.3 A.C.Load Lines
◆ Very often the load into which
the transistor or fet works is not
the same for both ac and dc
conditions.
◆ When this is the case two load
lines must be drawn on the out
characteristics：a dc load line to
determine the operating point, and
an ac load line to determine the
current or voltage gain of the
circuit.
◆ The ac load line must pass
through the operating point.
Fig. 3.9 Potential-divider bias amplifier
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Chapter3: Small-signal Audio-frequency Amplifiers
3.3 Determination of gain using a load line
3.3.3 A.C.Load Lines
Fig. 3.10
A.C.Load Lines
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Chapter3: Small-signal Audio-frequency Amplifiers
3.3 Determination of gain using a load line
3.3.4
Current Gain of a Transistor Amplifier
Fig. 3.9 Potential-divider bias amplifier
Fig. 3.11 Current Gain of a Transistor Amplifier
◆ When an input signal is applied to a transistor amplifier, the signal
current iS super imposed upon the bias current.
◆ suppose that the base bias current is IB2 and that an input signal current
swings the base current between the values IB1 and IB3.
◆ The resulting values of collector current are found by projecting onto the
collector-current axis from the in tersection of the a.c.load line and the curves
for IB1 and IB3.
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Chapter3: Small-signal Audio-frequency Amplifiers
3.3 Determination of gain using a load line
3.3.4
Current Gain of a Transistor Amplifier
Fig. 3.9 Potential-divider bias amplifier
Fig. 3.11 Current Gain of a Transistor Amplifier
peak-to-peak collector current
Ai 
peak-to-peak base current

I C ( MAX )  I C ( MIN )
I B 3  I B1
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Chapter3: Small-signal Audio-frequency Amplifiers
3.3.4
Current Gain of a Transistor Amplifier
Fig. 3.12 example 3.2
example 3.2
The transistor used in the circuit has the
data given in Table. Plot the output
characteristics of the transistor. Draw the dc
load line and select a suitable operating
point. Draw the ac load line and use it to
find the alternating current that flows in the
2500Ω load when an input signal producing
a base current swing of±15μA about the
bias current is applied to the circuit. Assume
all the capacitors have zero reactance at
signal frequencies.
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Chapter3: Small-signal Audio-frequency Amplifiers
3.3 Determination of gain using a load line
3.3.5 Voltage Gain of a FET Amplifier
◆ The voltage gain of a fet can also be
found with the aid of a load line. For
example, Fig. 3.13 shows an ac load line
drawn on the drain characteristics of a fet
and the dotted projections from the load
line show how the drain voltage swing,
resulting from the application of an
input signal voltage, can be found.The
voltage gain Av of the fet amplifier stage is
Av 
Fig. 3.13 Potential-divider bias amplifier
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peak-to-peak drain voltage
peak-to-peak gate-source voltage

VDS ( MAX )  VDS ( MIN )
VGS 3  VGS 1
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Chapter3: Small-signal Audio-frequency Amplifiers
3.3.5 Voltage Gain of a FET Amplifier
example 3.3
Draw the d.c.load line and select a
suitable
operatin point. Draw the a.c.load line and
use it to find the voltage gain when a
sinusoidal input signal of 0.3 V peak is
applied.
Fig. 3.14 example 3.3
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Chapter3: Small-signal Audio-frequency Amplifiers
3.4 Bias and stabilization
◆
To establish the chosen operating point it is necessary to apply a bias
voltage or current to a FET or transistor.
3.4.1 Transistor Bias
1) Why the transistor amplifier should be biased?
__ To amplify the input signal undistorted.
2) Fixed bias common emmitter amplifier.
VCC  I B R1  VBE
VCC  VBE
 R1
IB
3) This circuit does not provide any d.c.
stabilization against changes in collector
current due to change in ICBO or in hFE
and its usefulness is limited.
I C  I B  I CEO I CEO  (1   ) I CBO
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Fig. 3.15 Fixed bias
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Chapter3: Small-signal Audio-frequency Amplifiers
3.4.1 Transistor Bias
EXAMPLE 3.4
The circuit shown in Fig 3.16 is designed for operation with
transistors having a nominal hFE of 100. Calculate the collector
current. If the range of possible hFE is from 50 to 160, calculate the collector c
urrent flowing if a transistor having the maximum
hFE is used. Assume ICBO=10nA and
VBE=0.62V.
In the above example the effect of
the increased collector current would
be to move the operating point along
the d.c.load line,and this would lead
to signal distortion unless the
input signal level were reduced.
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Fig. 3.16 example 3.4
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