09-02-2013, 02:08 PM
Power Amplifier (Class A)
Power Amplifier.ppt (Size: 741 KB / Downloads: 141)
Introduction
Power amplifiers are used to deliver a relatively high amount of power, usually to a low resistance load.
Typical load values range from 300W (for transmission antennas) to 8W (for audio speaker).
Although these load values do not cover every possibility, they do illustrate the fact that power amplifiers usually drive low-resistance loads.
Typical output power rating of a power amplifier will be 1W or higher.
Ideal power amplifier will deliver 100% of the power it draws from the supply to load. In practice, this can never occur.
The reason for this is the fact that the components in the amplifier will all dissipate some of the power that is being drawn form the supply.
Amplifier Efficiency
A figure of merit for the power amplifier is its efficiency, h .
Efficiency ( h ) of an amplifier is defined as the ratio of ac output power (power delivered to load) to dc input power .
By formula :
As we will see, certain amplifier configurations have much higher efficiency ratings than others.
This is primary consideration when deciding which type of power amplifier to use for a specific application.
Typical Characteristic
Previous figure shows an example of a sinusoidal input and the resulting collector current at the output.
The current, ICQ , is usually set to be in the center of the ac load line. Why?
(DC and AC analyses discussed in previous sessions)
Transformer-Coupled Class-A Amplifier
An important characteristic of the transformer is the ability to produce a counter emf, or kick emf.
When an inductor experiences a rapid change in supply voltage, it will produce a voltage with a polarity that is opposite to the original voltage polarity.
The counter emf is caused by the electromagnetic field that surrounds the inductor.
DC Operating Characteristics
The dc load line is very close to being a vertical line indicating that VCEQ will be approximately equal to VCC for all the values of IC.
The nearly vertical load line of the transformer-coupled amplifier is caused by the extremely low dc resistance of the transformer primary.
VCEQ = VCC – ICQ(RC + RE)
The value of RL is ignored in the dc analysis of the transformer-coupled class-A amplifier. The reason for this is the fact that transformer provides dc isolation between the primary and secondary. Since the load resistance is in the secondary of the transformer it dose not affect the dc analysis of the primary circuitry.