Power Amplifier Efficiency
Where: η% – is the efficiency of the riser. Pout – the amplifier output power transmitted to the load. PDC – DC power from feed. For a power amplifier, it is very important that the amplifier's power supply is well designed to provide maximum usable continuous power to the output signal.
Class A Amplifiers
The most commonly used 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 previously seen, to produce an inverted output. The transistor is always routed to "ON", so that the input signal transmits during a full cycle of the waveform, producing minimal distortion of the output signal and maximum amplitude. This means that the Class A Amplifier configuration is the ideal mode of operation, since even in the negative half of the cycle there can be no crossover or shutdown distortion in the output waveform. The Class A power amplifier output stages can use a single power transistor or pairs of interconnected transistors to share 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 the output stage, where the resistant load is connected directly to the Collector terminal. When the transistor switches to the "ON" position, it reduces the output current through the Collector, which causes an inevitable voltage drop throughout the Transmitter resistance, thereby limiting the negative output capacity. The efficiency of this type of circuit is very low (less than 30%) and provides small power outputs for a large discharge in the DC power supply. A Class A amplifier stage passes the same load current even when no input signal is applied, so large coolers are needed for output transistors. However, another simple way to increase the current carrying capacity of the circuit while also achieving a greater power gain is to replace the single output transistor with a Darlington Transistor. Such devices are basically two transistors in a single package, one a small "pilot" transistor and the other a larger "switching" transistor. The biggest advantage of these devices is that the input impedance is properly large when the output impedance is relatively low, thereby reducing power loss and therefore the heat inside the switching device.
Darlington Transistor Circuit
The total current gain of a Darlington device Beta (β) or hfe value is the product of multiplying two separate gains of transistors, and very high β values are possible with high collector currents compared to a single transistor circuit. To increase the full power efficiency of the Class A amplifier, it is possible to design a circuit with a transformer directly connected to the Collector circuit to create a circuit called transformer connected amplifier. The transformer increases the efficiency of the amplifier by pairing the impedance of the load with the impedance of the amplifier output using the transformer's rotation rate (n), and an example of this is given below.
Transformer Connected Amplifier Circuit
As collector current, The Ic is lowered below the sedentary Q point established by the base pre-voltage due to changes in the base current, the magnetic flux in the transformer core collapses, causing an induced emk in the primary windings of the transformer. This causes the instant collector voltage to increase to twice the 2Vcc supply voltage and two Ic maximum collector currents, while the collector voltage is minimal. Then the efficiency of this type of Class A amplifier configuration can be calculated as follows. r.m.s. The collector voltage is given as follows: r.m.s. Collector current is given as follows: r.m.s. The power transmitted to the load (Pac) is therefore given as follows: The average power (Pdc) drawn from the source is given as follows: and therefore, the efficiency of a Transformer-connected Class A amplifier is given as follows: An output transformer increases the efficiency of the amplifier by matching the impedance of the load to that of the amplifier's output impedance. By using an output or signal transformer with an appropriate rotational rate, class A amplifier efficiency of up to 40% is possible with most Class A power amplifiers on the market in 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 should be avoided, as any manufactured back imp can damage the transistor without adequate protection. In addition, another major drawback of this type of transformer coupling class A amplifier circuit is the additional cost and size of the required sound transformer. The type of "Class" or classification given to an amplifier really depends on the transmission angle, which is 360o part of the input waveform cycle transmitted by the transistor. In the Class A amplifier, the transmission angle is exactly 360o or 100% of the input signal, while in other amplifier classes the transistor transmits during a smaller transmission angle. It is possible to achieve more power output and efficiency than the Class A amplifier by using two complementary transistors during the output phase, where one transistor is of NPN or N-channel type and the other transistor is PNP or P-channel. complementary) type, called "push-pull" configuration. This type of power amplifier configuration is often called a Class B Amplifier and is another type of audio amplifier circuit that we will examine in the next tutorial.