Power Amplifiers: Types, Functions, and Key Features
What is the Function of a Power Amplifier?
The function of a power amplifier is to amplify an input signal applied to it, typically a few hundred millivolts from the preamplifier, to a signal of a considerable level, a few tens of watts, to vibrate the speaker membrane.
Define the Volume Control
The volume control adjusts the output power or volume developed by the speaker.
What is the Excitatory Stage?
The excitatory stage is a second stage of voltage preamplifiers, and its principal role is to raise the output to appropriate levels of voltage and current, capable of exciting the power stage.
What is the Feedback Loop?
The feedback circuit is responsible for collecting a portion of the output signal and reinjecting it into the preamplifier or previous stage. This achieves a major effect of minimizing distortion and improving amplifier linearity.
Circuit Protection Function of Final Output Transistors
The protection circuit of the final output transistors limits short-circuit current to a value lower than the maximum current permitted by the final output transistors, thereby preventing their destruction. It also protects the speakers in adverse situations.
Main Features of Class A Amplifiers
Class A amplifiers receive constant polarization, which makes them work in the most linear part of their load line.
Disadvantages of Class A Amplifiers
The disadvantages of amplifiers in class A are:
- They have a constant consumption stream from the power supply to maintain the polarization of the transistors.
- The bias voltages make this kind of amplifier inefficient.
Class B Amplifiers Explained
Class B amplifiers are characterized because no current flows through the output transistors when there is no audio signal applied to their input. The input signal itself polarizes the transistor to turn it on and thus be able to excite the speakers.
Disadvantages of Class B Amplifiers
These amplifiers have problems, especially when operating with weak input signals. In these circumstances, the transistors work in the non-linear curved area near the elbow of the characteristic curve. The signal obtained at its output is not a true reflection of the input signal and produces distortion in the output signal.
Class AB Amplifiers Explained
In Class AB amplifiers, the output current flows for more than half the period of the input signal and less than a full period. These amplifiers are situated on the transistor operating point above the elbow of the non-linear characteristic curves. The transistor base is polarized with a relatively low constant current of appropriate value, so it can amplify not only half the period of the input signal but also part of the other. Class AB is most used by manufacturers in the output stages and settings used in contracts, as the AB class has no crossover distortion.
Factors Involved in the Power Stage
- Yield: Measures the conversion efficiency of a power amplifier.
- The power dissipated in the load: Given by the expression Pac = PL(Veff)2 / RL, where Veff is the effective stress and load present in the RL is the load value in ohms.
- The maximum power dissipation in the load: Achieved when the signal applied to the load is maximized and there is no distortion.
Why Performance Should Be High in the Power Stage
This parameter must be high in power amplifiers since poor performance in these stages involves a high expenditure due to the large amounts of power that may exist in the power stage. A large portion of the power is extracted from the power supply and not dissipated in the load; therefore, it does so in the active elements and resistors in the circuit.
Typical Configurations Used by the Output Power Stages
The most typical configurations used in power amplifiers are the Darlington amplifier and the push-pull or complementary symmetry booster.
Darlington Amplifier Explained
The Darlington amplifier consists of two transistors in an emitter follower configuration or direct-coupled common-collector. It is characterized by high current gain, high input impedance, low output impedance, and voltage gain close to unity.
Push-Pull or Complementary Symmetry Amplifier Explained
The amplifier in push-pull or complementary symmetry often works in class AB (to prevent crossover distortion), amplifying a single half period of the input signal. This involves placing the operating point in the cutoff region so that it passes into the active region by presenting only the right half period of the input signal to the transistor. When the half period is the opposite, the transistor remains in court, as in the absence of a signal. To obtain an output signal equal to the input, it will have two additional transistors to amplify each half period of the input signal.