![]() 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. The average power drawn from the supply (Pdc) is given by:Īnd therefore the efficiency of a Transformer-coupled Class A amplifier is given as:Īn output transformer improves the efficiency of the amplifier by matching the impedance of the load with that of the amplifiers output impedance. Power delivered to the load (Pac) is therefore given as: Then the efficiency of this type of Class A amplifier configuration can be calculated as follows. 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. Darlington Transistor ConfigurationsĪs 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. 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. These types of devices are basically two transistors within a single package, one small “pilot” transistor and another larger “switching” transistor. 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. 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. 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. 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. It uses a single-ended transistor for its output stage with the resistive load connected directly to the Collector terminal. This is the simplest type of Class A power amplifier circuit. Consider the Class A amplifier circuit below. Class A power amplifier output stages may use a single power transistor or pairs of transistors connected together to share the high load current. 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. 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. 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 most commonly used type of power amplifier configuration is the Class A Amplifier. Pdc – is the DC power taken from the supply.įor 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.Pout – is the amplifiers output power delivered to the load.η% – is the efficiency of the amplifier.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.īecause 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. 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. 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 common emitter class-A amplifier is 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.
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