Audio amplifier is useful in electronics.
It makes weak sound signals strong to run loudspeakers.
Class AB Stereo Amplifier Circuit are special as they mix Class B which is for good power use and Class A which is for good sound features.
This article shows a simple Class AB stereo amplifier which explains how it works, formulas, how to make it and what we can learn from it.
Circuit Working:
Parts List:
| Component | Specification | Quantity |
|---|---|---|
| Resistors | 1k, 3.9k, 470k, 10Ω 1/4 watts | 1 each |
| 680Ω, 0.47Ω 1/4 watts | 2 each | |
| Potentiometer 4.7k | 1 | |
| Preset 1k | 1 | |
| Capacitors | Ceramic 10pF, 47nF | 1 each |
| Electrolytic 10μF 25V, 22μF 25V, 1000μF 25V | 1 each | |
| Semiconductors | Transistor 2N3904 | 2 |
| Transistor TIP31A | 3 | |
| Transistor TIP32A | 1 | |
| Loudspeaker 8Ω | 1 |
This Class AB stereo amplifier uses smart design with complementary transistors.
It gives good power and clean sound.
It can run 8 ohm speaker with less distortion.
TIP31A and TIP32A transistors help boost output.
Signal comes in through VR1 potentiometer to adjust input strength.
Capacitor C1 blocks DC and lets only AC audio to go in.
Q1 and Q2 2N3904 are differential pair and they give first signal boost and keep circuit stable.
R3 sets Q2 base and C3 helps stability at high frequencies.
Q3 TIP31A and Q4 TIP32A are drivers and they make signal stronger for final stage.
R4 and R5 set bias current for drivers.
Q5 and Q6 both TIP31A work in push-pull.
This reduces crossover distortion and keeps amplifier efficient.
R6 senses current and gives feedback for more stability.
C4 and C5 are decoupling capacitors as they clean power and reduce noise.
We can adjust the idle (quiescent) current in output stage.
Formulas:
Below are simple formulas to design Class AB stereo amplifier:
1. Bias Current (I_Q):
Sets idle current in output stage.
Formula:
I_Q = V_BE / R_6
where,
- V_BE is the base-emitter voltage of Q5 & Q6 transistors which is 0.7V
- R_6 is the resistor used for bias current
2. Voltage Gain (A_v):
Tells how much signal is amplified.
Formula:
A_v = R_C / R_E
where,
- R_C is the collector resistor R4
- R_E is the emitter resistor
3. Output Power (P_out):
How much power goes to speaker.
Formula:
P_out = V_peak² / (2 × R_load)
where,
- V_peak is the peak output voltage
- R_load is the speaker impedance for 8Ω
These formulas help design and understand how the amplifier works.
How to Build:
To build a Class AB Stereo Amplifier Circuit following steps need to be followed:
- Put all parts as shown in the circuit diagram
Q1 Connections:
- Collector connects to third leg of VR2
- Base connects to center leg of VR2
- Emitter connections to second leg of VR2
Q2 Connections:
- Collector goes to emitter of Q1
- Base goes to C3 and R3
- Emitter goes to GND
VR1 Connections:
- Center leg connects to negative side of C1
- Upper leg connects to audio input
- Third leg goes to GND
Q3 Connections:
- Collector connects to +30V
- Base connects to collector of Q1
- Emitter connects to emitter of Q4 through R4
Q4 Connections:
- Collector connects to GND through R5
- Base connects to collector of Q2
Q5 Connections:
- Collector connects to +30V
- Base connects to emitter of Q3
- Emitter connects to collector of Q6 through R6
Q6 Connections:
- Base connects to collector of Q4
- Emitter connects to GND through R6
- R1 and R2 connect from collector of Q1 to +30V
- C2 connects between R1 and R2 and also to R3 and base of Q2
- C4 connects from collector of Q6 and emitter of Q5 and one side of 8Ω speaker
- Other side of speaker goes to GND
- C4 negative side goes to R8 and C5 in series of GND
Conclusion:
This simple and easy to make Class AB Stereo Amplifier Circuit is smart and helpful and it boosts sound well without wasting power.
It shows how push-pull works and gives both good power and clean audio.
If we follow the above steps and learn how it works we can build and change this amp for many uses.
It helps us grow in electronics and in audio skills.