Class A Amplifier Circuit

This article talk about Class A amplifier.

It is good for small speaker or headphone.

Amplifier make sound more loud.

This Class A Amplifier Circuit uses near 20 milliamp power so it good for low power thing.

We show in this post how to make it using Q1 and Q2 transistor and this transistor work like tiny switch.

WARNING: Making electronic project is not easy.

Only do it with adult help.

What is Class A Amplifier Circuit:

Both output parts of Class A amplifier always work with full power all the time.

Because it is always ON, Class A amplifier is not so efficient.

It usually give only 20% efficiency but sometimes maybe it goes up to 50% max.

Circuit Working:

Parts List:

Amplifier uses about 20 milliamp quiescent current.

This current is important because it sets transistor bias and how amplifier works.

We can change quiescent current by changing resistor R3.

Amplifier can be changed a little to fit different needs and make working point better.

Q1 and Q2 transistors work as common emitter amplifiers.

Q1 output goes to Q2 input by making cascade to amplify more.

Amplifier gives about 80 dB voltage gain which means it makes signal strong.

Capacitor C3 blocks resistor R3, so R3 works as emitter load for Q2.

This makes amplifier stable by keeping Q2 emitter voltage near Q1 collector voltage.

Resistor R2 gives base bias to Q1 using voltage from Q2 emitter.

Negative DC feedback keeps bias safe and amplifier stable.

Volume can change by input potentiometer R4.

R4 lets user control sound volume and input signal level.

Formulas:

Collector current in BJT is called quiescent current IQ.

It is the current that flow in transistor when no signal is given.

We can find IQ like this:

IQ = VCC / R3

where,

• DC current in collector with no input is called Idle Current or IQ.
• VCC is positive voltage for BJT collector, measured from ground.
• R3 is resistor that connects BJT collector to VCC.

How to Build:

To Build a Class A Amplifier Circuit follow the below mentioned steps for connections:

• Choose good transistors for Q1 and Q2.
• They must match the amplifiers need like impedance.
• Place Q1 and Q2 on PCB or breadboard.
• Check the pin layout and be sure we put them correct way.
• Connect resistor R1 between Q1 collector and positive power of VCC.
• Connect resistor R2 from Q1 base to Q2 emitter.
• Connect resistor R3 from Q2 collector to VCC.
• To change amplifier idle current change value of R3 this help fix transistor working point.
• Connect capacitor C3 with resistor R3 which will help block DC and keep good bias.
• C3 help make Q2 emitter voltage follow Q1 collector average voltage and this will give better stability.
• Use resistor R2 to give base bias to Q1 from Q2 emitter.
• Negative DC feedback help keep bias in control.
• Connect potentiometer R4 to control volume.
• Join power supply wires like positive and negative to correct place on circuit.
• Turn ON the circuit and use speaker or oscilloscope with audio signal to check output.
• Adjust potentiometer R4 to change volume and check amplifier work proper.

Conclusion:

This Class A Amplifier Circuit is easy and flexible project.

It uses common emitter setup, adjustable quiescent current and smart parts like capacitors.

People can test and improve it.

By learning how it works they can make it fit for different audio needs.

References:

Power amplifier classes