Class A Amplifier Providing 24 Watts of Power

Figure 1 

The circuit was designed as Class A amplifier that would provide 24 Watts of power

Amplifier – an electronic device used for intensifying the amplitude or power of a transmitted signal by manipulating the output to counterpart the input signal but with a larger amplitude

Classes of Amplifier:

• Class A – output device conducts through 360 degrees or 100% of the input cycle*
• Class B – output device conducts through 180 degrees or 50% of the input cycle
• Class AB – almost the same as Class B amplifiers but differs in that they have a small idle current flowing from positive supply to negative supply even when there is no input signal
• Class C – output device conducts less than 180 degrees or less than 50% of the input signal
• Class D – uses switching and pulse width modulation of a high frequency to obtain a very high power efficiency
• Class E – refers to an amplifier with pulsed inputs and a tuned circuit output
• Class F – a highly efficient switching power amplifier
• Class G – consists of several power rails at different voltages and switches between rails as the signal output approaches each
• Class H – takes the idea of Class G one step further creating an infinitely variable supply rail

Although Class A cost the most and are least practical, they are still preferred because the sound best, returns very clean signal, it is the most linear and has the lowest distortion. The circuit’s inactive current must be set by the resistor R25 while the maximum resistance must be set when powering the circuit by the trimmer resistor R3, which will be reduced the the inactive current has been achieved. A range of 34 V to 46 V may be used as the supply voltage. The biggest disadvantage of Class A is that it uses a lot of electricity and gets very hot, making it very dependent on a heatsink big enough to maintain its operating temperature so that the amplifier would be safe from thermal runaway.

The circuit operates over the whole of the input cycle such that the output signal is an exact replica of the input with no clipping. The ouput device will conduct all the time which means the power devices must conduct a continuous current which exceeds the maximum peak load current. The output stage of the circuit has bias current greater than the maximum output current which makes all output transistors always conducting current.

To compare the uses of every class of amplifiers:

• Class A – is used as an audio and radio frequency amplifier in radio, radar, and sound systems
• Class B – is used widespread for audio amplifiers that require high power outputs and power amplifier stages of transmitters
• Class C – is used as a radio frequency amplifier in transmitters only and cannot be used for audio
• Class AB – is commonly as push-pull amplifier, designed to function with audio power amplifiers
• Class D – are widely used to control motors and almost limited to small DC motors but are now also used as audio amplifiers using a few additional circuitry to permit analog signals to be converted to a much higher frequency pulse with modulated signal
• Class E – is not used for audio, but commonly used in radio transmitters where the output is at a single narrow band of frequencies
• Class F – is typically used at such high frequencies that the switching time becomes comparable to the duty time
• Class G – is used as rail switched amplifier which has two different power supply voltages
• Class H – is very common in professional audio amplifiers

Source:www.zen22142.zen.co.uk/Circuits/Audio/24wca.htm