Chapter
2: High Power Amplifiers
|
Sub point 2.1 |
Power Amplifier:
Comparison between small signal
amplifier and power amplifier, performance
parameter of power amplifier
like : bandwidth,
gain, frequency band,
efficiency |
- Comparison between small signal
amplifier and power 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 milliVolts) |
High (2 to 4Volts) |
|
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 |
- Power amplifier:
Power
amplifiers are the amplifiers in which power transistors are used that are
capable of handling high power and provides large amount of power to the loads
like loudspeaker, motor etc. Power amplifiers are generally used at last stage
of practical amplifier. For example consider the block diagram below,
Figure shows practical amplifier in which number of voltage amplifiers
are used in previous stage and in final stage power amplifiers are used. Let’s
see brief working of the circuit.
1] Voltage amplifiers: Are used to increase the strength of weak signals
2] Driver stage: Supplies necessary power to power amplifier.
Generally consists of class – A power amplifier with transformer coupling
3] Power amplifier: Generally class – B push-pull amplifier with
transformer coupling is used which is last stage of practical amplifier and
feeds the power directly to the load, in our case, speaker.
- Performance parameters of power amplifiers
1] Gain: Gain is defined as it is the ratio of output and input.
AV = Vout / Vin (Voltage gain)
Ai = Ic / Ib (Current
gain)
Ap = Pout / Pin (Power
gain)
2] Bandwidth: It is the range over which the output of circuit is satisfactory.
3]
Overall Efficiency: It is the ratio of AC power delivered to
the load to the total power supplied by DC source.
Overall
Efficiency = AC power delivered to the load / total power supplied by DC source
4]
Collector Efficiency: It is the ratio of AC power
delivered to the load to the total power supplied by DC source to transistor.
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