Introduction of Power Amplifiers and DC/DC Converters

INTRODUCTION OF POWER AMPLIFIERS AND DC/DC CONVERTERS

The voltage amplifiers operate in the linear portion of the transfer characteristics, closer to Q-point of the amplifier. These amplifiers are called small signal amplifiers because they are mainly used for voltage amplification but not used for providing large power to the loads.

Need for Power Amplifiers

When a large amount of power is required to be delivered to the load, power amplifiers are used.

Example: Loudspeaker, motor, public address system, tape recorders, TV receivers, radio receivers, broadcast transmitters etc.

Power amplifier is also known as large signal amplifier as they are capable of providing large amount of power to the load. These amplifiers convert the input low power signals to high power signals i.e., high voltage, low current signal is converted into high voltage, high current signal. Hence, power amplifier is essentially a current amplifier.

i. They are used as last stage in electronic systems.

ii. A power transistor can be operated over the entire range of its output characteristics i.e from saturation to cutoff. This mode of operation is known as Large Signal Operation and used for digital switching circuits, the output level switches between high and low.

iii. Power amplifier amplifies a weak signal until sufficient power to operate output device such as loudspeaker.

Driver stage - Class A Power amplifier

Output stage - Class B Power amplifier

The input is pre-amplified using voltage amplifier stages. Typical output power rating of a power amplifier will be 1W or higher.

COMPARISON OF SMALL SIGNAL AND LARGE SIGNAL AMPLIFIERS

The voltage amplifiers give an increased voltage output to a high resistance load. The power amplifier produces an increase in output power to a relatively low resistance load.

Features of Power Amplifiers

i. Impedance matching to deliver maximum power to the load.

ii. Need power transistors to withstand large voltages and current.

iii. Power amplifiers are bulk.

iv. Harmonic distortions occur due to non-linear characteristics of transistors.

Performance Parameters

The performance parameters of power amplifiers are

i. Circuit efficiency

ii. Distortion

iii. Power dissipation capacity

i. Circuit Efficiency (or) Amplifier Efficiency

It is defined as the ratio of output ac power to the input dc power. It is also known as Figure of merit. It is also called conversion efficiency.

η depends upon two factors.

1. Way of coupling of load to the transistor

2. Class of operation.

ii. Distortion

The change in shape of output waveform from that of input of an amplifier is known as distortion. It is the difference between output and input of an amplifier.

1. Harmonic or amplitude distortion → occurs due to non-linearity in transistor.

2. Cross over distortion - occurs when transistors are not operating in correct phase with each other.

iii. Power Dissipation Capacity

It is defined as the ability of a power amplifier to dissipate heat.

Heat sink is used with the power transistor to achieve better power dissipation capability. It increases surface areas and allows the heat to escape easily.