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:
| Category | Description | Quantity |
|---|---|---|
| Resistors | 5.6k CFR 1/4 W | 1 |
| 47k CFR 1/4 W | 1 | |
| 39Ω CFR 1/4 W | 1 | |
| Potentiometer 100k | 1 | |
| Capacitors | Electrolytic 1µF 25V | 2 |
| Electrolytic 100µF 25V | 1 | |
| Electrolytic 220µF 25V | 1 | |
| Semiconductors | Transistor 2N2222 | 1 |
| Transistor TIP31 | 1 | |
| Speaker 65Ω | 1 |
Amplifier take around 20 milliamp quiescent current.
This current is important as it affect how transistor get bias and how amplifier work.
We can change quiescent current easily by changing resistor R3.
Amplifier can be changed little to fit different needs and also we can make working point better.
Q1 and Q2 transistor are used like common emitter amplifier.
Q1 output goes straight to Q2 input and this make cascade type and help amplify more.
Amplifier gives around 80 dB voltage gain (Aᵥ) and this show how strong amplifier makes signal.
Capacitor C3 is used to block resistor R3 and thus R3 is emitter load for Q2.
This setup help make amplifier more stable which keeps Q2 emitter voltage close to Q1 collector voltage.
Resistor R2 gives base bias to Q1 which uses voltage from Q2 emitter.
There is also negative DC feedback and this help keep bias safe and make amplifier more stable and better.
Volume can be change by input potentiometer R4.
R4 lets user control sound volume and change 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,
- This DC current in collector when no input is called Current Idle State or IQ.
- VCC is the positive voltage given to collector of BJT and which is measured from ground.
- R3 is the resistor that connect collector of BJT 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.