3.0 Integrated circuit

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Chapter3 Small-signal Audio-frequency Amplifiers

• 3.0 Integrated circuit
• 3.1 Principles of operation (Quiescent
  Operating Point)
• 3.2 Choice of configuration
• 3.3 Determination of gain using a load line
• 3.4 Bias and stabilization
• 3.5 Voltage gain of BJT amplifier
• 3.6 Voltage gain of f.e.t. amplifier
• 3.7 Voltage, current and power amplifiers
• 3.8 Multi-stage amplifiers
• 3.9 Measurements on audio-frequency amplifiers

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n-P-n bipolar transistor
n-P-n bipolar transistor with a buried layer
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Integrated Diode
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Integrated Resistor
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Integrated Capacitor
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simple circuit shown in Fig.2.10a is to be
integrated.
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? Transistors and f.e.t.s may be used as
amplifiers because their output currents can be
controlled by an a.c. signal applied to their
input terminals. ? A f.e.t. has such a high
input impedance that its input current is
negligible it can therefore give only a voltage
gain. ? By suitable choice of collector
current,and hence of input impedance,a transistor
may be considered as either a current-operated
device or a voltage-operated device. ? If the
source impedance is much larger than the input
impedance of the transistor,the transistor is
current operated. If much smaller, it is voltage
operate.
3.1 Principles of operation
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• In the common-emitter connection
• input impedance

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? 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,on the other hand, the power output iS
the important factor. ? ? ?
3.1 Principles of operation
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? The various ways in which a transistor or
f.e.t.may be connected to provide a gain are
shown in Fig.3.1.
3.2 Choice of configuration
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? The short-circuit a.c. current gain hfe of a
transistor connected in the common-emitter
configuration (Fig.3.3) is much greater than the
short-circuit a.c. current gain of thesame
transistor connected with common base, i.e.
hfehfb/(1- hfb ). Resistance-capacitance
coupling of the cascaded stages of an amplifier
is possible and nowadays transformers 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 1000-2000O while the output impedance
is some 10-30 kO.
3.2 Choice of configuration
Fig. 3.3 common-emitter amplifier
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? 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
50O, and an output impedance of about 1 MO.
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.
3.2 Choice of configuration
Fig. 3.2 common-base amplifier
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? 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 amplifying device that can
be conveniently connected between a
high-impedance source and alow-impedance load.
3.2 Choice of configuration
Fig. 3.4 common-collector amplifier
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? In the normal mode of operation (Fig.3.5), the
source is common to the input and output
circuits, the input signal is applied to the
gate, and the output is taken frombetween drain
and earth.This connection provides a large
voltage gain and has a high input impedance.
3.2 Choice of configuration
Fig. 3.5 common-collector amplifier
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? 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.
3.2 Choice of configuration
Fig. 3.5 common-collector amplifier
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(a) giving one of the required points. (b)
giving the second point (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.
3.3 Determination of gain using a load line
3.3.1 The relationship of output voltage and
output current
Fig. 3.6 common-emmitter amplifier
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(a) giving one of the required points.
(b)
giving the second point
3.3 Determination of gain using a load line
3.3.1 The relationship of output voltage and
output current
Fig. 3.7 common-source amplifier
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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) 1000O and (b)
1800O.Use the load lines to determine the steady
(quiescent) collector current and voltage if the
base bias current is 80µA and the collector
supply Ic0 VceVcc9V and is marked A in
Fig.3.3.
Tab. 3.1 data of the common emmitter amplifier
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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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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 characteristicsa
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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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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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 superimposed
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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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
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3.3.4 Current Gain of a Transistor Amplifier
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 2500O load
when an input signal producing a base current
swing of15µA about the bias current is applied
to the circuit. Assume all the capacitors have
zero reactance at signal frequencies.
Fig. 3.12 example 3.2
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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
Fig. 3.13 Potential-divider bias amplifier
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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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? To establish the chosen operating point it is
necessary to apply a bias voltage or current to
a FET or transistor. 1) Why the transistor
amplifier should be biased? __ To amplify the
input signal undistorted. 2) Fixed bias common
emmitter amplifier.3) This circuit does not
provide any d.c. stabilization against changes
in collector current due to change in ICBO or in
hFEand its usefulness is limited.
3.4 Bias and stabilization
3.4.1 Transistor Bias
Fig. 3.15 Fixed bias
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EXAMPLE 3.4The circuit shown in Fig 3.16 is
designed for operation withtransistors having a
nominal hFE of 100. Calculate the collector
current. If the range of possible hFE is from 50
to 160, calculate the collector current flowing
if a transistor having the maximum hFE is used.
Assume ICBO10nA and VBE0.62V.In the above
example the effect of the increased collector
current wouldbe 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.
3.4.1 Transistor Bias
Fig. 3.16 example 3.4