Chapter 1: Low Power Amplifiers
Sub point: 1.1 Classification of Amplifiers, BJT as an
amplifier.
·
Classification
of amplifier:
Amplifier is electronic
device which is used to provide the strength to weak signals. Weak signals are
of different type and need to amplify for different reasons. One example of
weak signal is ECG (Electrocardiogram) which needs to amplify to take proper
decisions by doctor. Another examples of
weak signals like thermocouple current, audio signal and so on.
Amplifiers broadly classified as Small signal
amplifiers/voltage amplifiers and power amplifiers. Following table gives
difference between these amplifiers.
|
Parameter |
Small signal
amplifiers /voltage amplifiers |
Large signal
amplifiers /power
amplifiers |
|
Type of transistor used |
Ordinary transistor |
Power transistor |
|
Type of coupling used |
RC coupling |
Transformer coupling |
|
Beta (β) |
High (greater than 100) |
Low (20-50) |
|
Collector resistor (Rc) |
High (4kΩ to 10kΩ) |
Low (5 to 20Ω) |
|
Output impedance |
High (~12kΩ) |
Low (~200Ω) |
|
Input voltage |
Low (few mV) |
High (2 to 4V) |
|
Collector current (Ic) |
Low (~ 1mA) |
High (greater than 100mA) |
|
Signal distortion |
Low |
High |
|
Cost |
Low |
High |
|
Size |
Small |
Bulky |
|
Applications |
As pre-amplifiers |
Audio amplifiers, public address
systems |
Amplifiers can be
further classified on the basis of various factors as follows
·
Classification based on purpose of
amplification
1 Voltage amplifier
2 Power amplifier
·
Classification based on frequency
range
1 AF amplifier: (20 Hz to 20 KHz)
2 RF amplifier: (typically more than 100 KHz)
·
Classification based on the
position of Q point
1 Class A amplifier
2 Class B amplifier
3 Class C amplifier
4 Class AB amplifier
·
Classification depending on the
type of coupling
1 Direct Coupled amplifier
2 RC coupled amplifier
3 Transformer coupled amplifier
·
Bipolar
Junction Transistor (BJT) as an amplifier:
Basically BJT can be
used switch and as an amplifier. Following table shows state of junctions and
related applications.
|
CB
junction |
EB
junction |
Application |
|
Reverse bias |
Reverse bias |
Open switch |
|
Forward bias |
Forward bias |
Closed switch |
|
Reverse bias |
Forward bias |
Amplifier |
Figure
shows transistor as an amplifier in which transistor is used in common emitter
(CE) configuration and the output is taken across collector Figure shows transistor
as an amplifier in which transistor is used in common emitter (CE)
configuration and the output is taken across collector Figure shows transistor as
an amplifier in which transistor is used in common emitter (CE) configuration
and the output is taken across collector resistor Rc.
If there is a small change in input voltage (Vin), then
we get proportional large change in output voltage.
Due
to small change in input voltage (ΔVin), there will be a small change in base
current IB
ΔIC=βIB
Hence
the corresponding change output voltage is given by
ΔV0=IC*RC
=β*(
ΔVin/RB)*RC
as
RB, RC, β are constants HenceΔV0 is directly proportional to ΔVin.
Hence
for a small change in input voltage (Vin) we get a large change in output
voltage (V0) and voltage amplification takes place. Hence the BJT acts/used as
a voltage amplifier.
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