An electronic amplifier is a device that is used to amplify the signal by increasing its power. It functions by taking energy from a power supply then processing the output signal to have larger amplitude than the input signal. This process does not affect the shape of the signal or any of its other characteristics. Amplifiers have a multiplication factor relating the magnitude of the output signal to the input signal. The gain may be specified as "output/input" of any combination of current, voltage and power. There are many types of amplifiers; one of these types is power amplifiers.
Power amplifiers are used to deliver a relatively high amount of power, usually to a low resistance load. The power gain of an amplifier depends on the source and load impedances used as well as its voltage gain. The typical load values range from 300W (for transmission antennas) to 8W (for audio speaker). 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 dissipate some of the power that is being drawn from the supply.
Power amplifiers are classified based on their conduction angles, T. The conduction angle is the portion of the input signal cycle in which the amplifier conducts. So, if the amplifier is always "ON", then T=360. Based on this, there are various classes of power amplifiers, such as: A, B, AB, C, D and E. These classes, according to their circuit configurations and methods of operation, range from entirely linear with low efficiency to entirely non-linear with high efficiency. This paper will look at one of these classes, Class A, and examine its internal structure, applications, and advantages and disadvantages.
Class A amplifiers use transistors with a standard Common Emitter circuit configuration. In this configuration, the transistor conducts during one complete cycle of the input signal waveform (T=360). In other words, it is always biased "ON". A single transistor or couples of transistors may be used in Class A amplifier output stages to share the high load current. They are often used in output stages of op-amps; sometimes as medium-power, low-efficiency, and high-cost audio amplifiers. Class A amplifiers have many configurations which differ in efficiency, complexity and size. The various configurations are introduced below:
Single-ended Amplifier Circuit
This configuration (Fig. 1) is considered as the simplest type of Class A amplifiers. This type works using a single ended transistor for the output stage, while the resistive load is connected directly to the Collector terminal. Once the transistor switches "ON", the output current is pushed to the Collector terminal which then results in a voltage drop across the Emitter resistance. This limits the negative output capability.
This type of Class A configuration has a very low efficiency (about 20% only) where it delivers small portions of power for large consumptions of DC power supply. Moreover, the load current always passes even in the absence of input signal. This produces a lot of heat and large heat sinks will be needed for the output transistor.
Darlington Transistor Configurations
In order to obtain a greater power gain and increase the current handling capacity, this configuration (Fig. 2) replaces the single output transistor with a Darlington Transistor. A Darlington transistor is basically two transistors connected in such a way that the base current from the first transistor enters the base of the second one. In this configuration, the current that is amplified by the first transistor is amplified also by the second one which results in much higher current gain. The overall current gain of the Darlington pair is the product of the two individual transistors:
So, for example, if two transistors have low current gains of 60, then the overall current gain would be 60 x 60 = 3600.
The advantage of using this configuration is that it has large input impedance and at the same time relatively low output impendence. In this way, the power loss is reduced and so is the heat within the switching device.
Transformer-coupled Amplifier Circuit
Another configuration of Class A power amplifiers is to connect a transformer to the circuit, specifically in the Collector circuit. This type of circuits is called Transformer Coupled Amplifier (Fig. 3). The basic operation of this configuration is that the magnetic flux in the transformer falls down when the variations in the Base current causes the Collector current to reduce and go below the quiescent Q-point set up by the Base bias voltage. This causes an induced emf in the transformer primary windings which in consequence, causes the Collector voltage to be twice the supply voltage 2Vcc. This gives a maximum Collector current of twice Ic when the Collector voltage is at its minimum.
Matching the impedance of the load with that of the amplifier improves the efficiency of the amplifier:
An efficiency of 40% can be reached by using this type of configuration for Class A power amplifiers. Another advantage is that transformer-coupled amplifier can be easily converted into the tuned amplifier which is a type of amplifier that is used extensively in communications. However, this configuration increases the cost and the size of the circuit due to the addition of the transformer.