Power amplifiers are amplifiers used to increase the power level of a signal to sufficient level to drive a given load. There are many classes of power amplifier such as class A power amplifier, class B power amplifier or class AB, class C, class D, class E etc. The power amplifiers are classified according to conduction cycle of the input signal. There are advantages, disadvantages and application of each of the power amplifier classes. Class AB is one class of power amplifier in which the signal input conduction cycle is below 180 degree. It's power efficiency is higher than class A and lower than class C. The class ab power amplifier efficiency can reach 70%. Because of this, class AB are used in battery powered electronics systems, in wireless transceivers such as FM transmitter, AM transmitter and others. Here it is shown how one can easily build a DIY class AB power amplifier. This class AB power amplifier is driven by a common emitter(CE) driver which works as a swamped amplifier. The driver is used to increase the signal amplitude at the output of the power amplifier. There are class AB power amplifiers which do not use such drivers. Using driver in front of the AB power amplifier is better than not using them. It increases gain which provides higher output power. The class ab power amplifier can be build using mosfet or BJT. Here we will use BJT transistors 2N3904 and 2N3906. The CE driver for the class AB amplifier is also build using 2N3904 general purpose transistor.
The power amplifier is easy to build on breadboard and can be tested using audio from PC speaker. The following picture shows the class AB power amplifier build on breadboard.
The following video demonstrates how this DIY class AB power amplifier works.
The class ab power amplifier circuit schematic is shown below.
The transistor Q1 forms the CE driver to the push pull amplifier. The gain of the push pull amplifier alone is approximately unity. Due to the transistor Q1and resistor R2 the voltage gain is increased and so there will be power amplification by the whole circuit. The resistor R2 is negative feedback resistor. It biases the driver 2n3904 transistor Q1 so that is turned on. The input signal via the potentiometer and coupling capacitor C1 enters the driver transistor Q1. Q1 amplifies the input signal and the output of the Q1 transistor at the collector is fed to both the complementary transistors Q2 and Q3. The 10kohm potentiometer is used to control the magnitude of the input signal into the circuit. The coupling capacitor C1 couples the input signal into the class AB power amplifier circuit.
The operation of class ab power amplifier is based basically on turning on one transistor while the other is off and vice versa. These transistors are the 2n3904 NPN transistor Q2 and 2n3906 PNP transistor Q3. The circuit they make is called the push pull circuit. The input signal to these transistor is such that when the transistor Q2 is on the transistor Q3 is off and vice. During the positive half cycle of the input signal the NPN transistor Q2 conducts while Q3 is off and during the negative half cycle of the input signal the PNP transistor Q3 conducts while Q2 is off. Thus full cycle of the input signal gets through and the input signal is recreated at the junction between these two transistor. For this to happen however, both the transistor must be barely on. That is these transistor must be biased such that the base voltage is just above their base-emitter voltage which is usually between 0.6V to 0.7V. When they are biased in this way they are near their cutoff region. The resistor R1, the two diodes and the transistor Q1 together provides this bias. Here 1N4007 diodes were used. The diode characteristics should match the Q2 and Q3 transistor characteristics. These diodes are used in the circuit also to compensate any changes in the environment temperature that would otherwise destabilize the bias condition for the push pull circuit.
The output signal from the class AB amplifier appears at the junction of the transistors Q2 and Q3. This output signal is then coupled to the load via the output coupling capacitor C2. This signal then goes into the load.
The following video animation explains how the push pull class AB amplifier works with CE driver. It shows how the signal flows in the circuit. Also it is shown how input signal waveform gets amplified by the circuit.
In this diy class AB power amplifier the audio signal input was from audio jack which can be connected to PC or mobile phones. One can also easily make electret microphone amplifier using LM358 or just make audio pre-amplifier circuit using BJT or FET transistors and connect to the input of the potentiometer. The tutorial audio amplifier design with uA741 op-amp explains how simple old operational amplifiers an be used instead of this discrete push-pull amplifier. Actually there are dedicated audio amplifier IC such as LM386 audio amplifier which is very popular.
