Common Emitter (CE) amplifiers are designed to produce a large output voltage swing from a relatively small input signal voltage of only a few millivolt’s and are used mainly as “small signal amplifiers” as we saw in the previous tutorials.
However, sometimes an amplifier is required to drive large resistive loads such as a loudspeaker or to drive a motor in a robot and for these types of applications where high switching currents are needed Power Amplifiers are required.
The main function of the power amplifier, which are also known as a “large signal amplifier” is to deliver power, which is the product of voltage and current to the load. Basically a power amplifier is also a voltage amplifier the difference being that the load resistance connected to the output is relatively low, for example a loudspeaker of 4Ω or 8Ω resulting in high currents flowing through the collector of the transistor.
Because of these high load currents the output transistor(s) used for power amplifier output stages such as the 2N3055 need to have higher voltage and power ratings than the general ones used for small signal amplifiers such as the BC107.
Since we are interested in delivering maximum AC power to the load, while consuming the minimum DC power possible from the supply we are mostly concerned with the “conversion efficiency” of the amplifier.
However, one of the main disadvantage of power amplifiers and especially the Class A amplifier is that their overall conversion efficiency is very low as large currents mean that a considerable amount of power is lost in the form of heat. Percentage efficiency in amplifiers is defined as the r.m.s. output power dissipated in the load divided by the total DC power taken from the supply source as shown below.
Power Amplifier Efficiency
- η% – is the efficiency of the amplifier.
- Pout – is the amplifiers output power delivered to the load.
- Pdc – is the DC power taken from the supply.
For a power amplifier it is very important that the amplifiers power supply is well designed to provide the maximum available continuous power to the output signal.
Class A Amplifier
The most commonly used type of power amplifier configuration is the Class A Amplifier. The Class A amplifier is the simplest form of power amplifier that uses a single switching transistor in the standard common emitter circuit configuration as seen previously to produce an inverted output. The transistor is always biased “ON” so that it conducts during one complete cycle of the input signal waveform producing minimum distortion and maximum amplitude of the output signal.
This means then that the Class A Amplifier configuration is the ideal operating mode, because there can be no crossover or switch-off distortion to the output waveform even during the negative half of the cycle. Class A power amplifier output stages may use a single power transistor or pairs of transistors connected together to share the high load current. Consider the Class A amplifier circuit below.
Single Stage Amplifier Circuit
This is the simplest type of Class A power amplifier circuit. It uses a single-ended transistor for its output stage with the resistive load connected directly to the Collector terminal. When the transistor switches “ON” it sinks the output current through the Collector resulting in an inevitable voltage drop across the Emitter resistance thereby limiting the negative output capability.
The efficiency of this type of circuit is very low (less than 30%) and delivers small power outputs for a large drain on the DC power supply. A Class A amplifier stage passes the same load current even when no input signal is applied so large heatsinks are needed for the output transistors.
However, another simple way to increase the current handling capacity of the circuit while at the same time obtain a greater power gain is to replace the single output transistor with a Darlington Transistor. These types of devices are basically two transistors within a single package, one small “pilot” transistor and another larger “switching” transistor. The big advantage of these devices are that the input impedance is suitably large while the output impedance is relatively low, thereby reducing the power loss and therefore the heat within the switching device.
Darlington Transistor Configurations
The overall current gain Beta (β) or hfe value of a Darlington device is the product of the two individual gains of the transistors multiplied together and very high β values along with high Collector currents are possible compared to a single transistor circuit.
To improve the full power efficiency of the Class A amplifier it is possible to design the circuit with a transformer connected directly in the Collector circuit to form a circuit called a Transformer Coupled Amplifier. The transformer improves the efficiency of the amplifier by matching the impedance of the load with that of the amplifiers output using the turns ratio ( n ) of the transformer and an example of this is given below.
Transformer-coupled Amplifier Circuit
As the Collector current, Ic is reduced to below the quiescent Q-point set up by the base bias voltage, due to variations in the base current, the magnetic flux in the transformer core collapses causing an induced emf in the transformer primary windings. This causes an instantaneous collector voltage to rise to a value of twice the supply voltage 2Vcc giving a maximum collector current of twice Ic when the Collector voltage is at its minimum. Then the efficiency of this type of Class A amplifier configuration can be calculated as follows.
The r.m.s. Collector voltage is given as:
The r.m.s. Collector current is given as:
The r.m.s. Power delivered to the load (Pac) is therefore given as:
The average power drawn from the supply (Pdc) is given by:
and therefore the efficiency of a Transformer-coupled Class A amplifier is given as:
An output transformer improves the efficiency of the amplifier by matching the impedance of the load with that of the amplifiers output impedance. By using an output or signal transformer with a suitable turns ratio, class-A amplifier efficiencies reaching 40% are possible with most commercially available Class-A type power amplifiers being of this type of configuration.
However, the transformer is an inductive device due to its windings and core so the use of inductive components in amplifier switching circuits is best avoided as any back emf’s generated may damage the transistor without adequate protection.
Also another big disadvantage of this type of transformer coupled class A amplifier circuit is the additional cost and size of the audio transformer required.
The type of “Class” or classification that an amplifier is given really depends upon the conduction angle, the portion of the 360o of the input waveform cycle, in which the transistor is conducting. In the Class A amplifier the conduction angle is a full 360o or 100% of the input signal while in other amplifier classes the transistor conducts during a lesser conduction angle.
It is possible to obtain greater power output and efficiency than that of the Class A amplifier by using two complementary transistors in the output stage with one transistor being an NPN or N-channel type while the other transistor is a PNP or P-channel (the complement) type connected in what is called a “push-pull” configuration.
This type of power amplifier configuration is generally called a Class B Amplifier and is another type of audio amplifier circuit that we will look at in the next tutorial.