Simple Class A Amplifier Circuit

This post jumps into a type of amplifier circuit called “class A” that’s great for small speakers or headphones.

Amplifiers make sounds louder.

Class A amplifiers are good for lower power devices because they use less energy about 20 milliamps.

This post shows you how to build one using transistors like tiny switches called Q1 and Q2.

WARNING: Building electronics projects can be tricky.

It is best to do this with adult supervision.

What is Class A Amplifier Circuit:

A Class-A amplifier is characterized by having both output stages operating continuously at full power.

Due to this constant operation class A is recognized as the least efficient among power amplifier designs, typically achieving an average efficiency of around 20%, reaching a theoretical maximum of 50%.

Circuit Working:

Parts List:

The amplifier draws a quiescent current of around 20 milliamperes.

The quiescent current is a crucial parameter that influences the biasing of the transistors and, consequently the overall performance of the amplifier.

By adjusting the value of resistor R3 it is possible to modulate the quiescent current of the amplifier.

This provides a means to optimize the operating point and tailor the amplifiers characteristics to specific requirements.

Transistors Q1 and Q2 are configured as common emitter amplifiers.

The output of Q1 is directly connected to the input of Q2 establishing a cascaded configuration that enhances overall amplification.

The total voltage gain Aᵥ of the amplifier circuit is approximately 80 dB, a critical parameter indicating the amplification capability of the circuit.

Capacitor C3 is strategically placed to decouple resistor R3, which serves as Q2s emitter load.

This configuration ensures that the Q2 emitter voltage closely tracks the average collector voltage of Q1 enhancing stability.

The base bias for Q1 is obtained from the emitter of Q2 using resistor R2.

Negative DC feedback stabilizes the bias in this setup, contributing to the amplifiers overall reliability and performance.

The loudness of the amplifier circuit is conveniently controlled by the input potentiometer R4.

This potentiometer allows users to adjust the input signal and modulate the overall volume output of the amplifier.

Formulas:

Lets calculates the quiescent current IQ flowing through a transistor in a circuit, specifically the collector current in a BJT configuration.

I_Q = V_CC / R₃

• Current Idle State I_Q: This is the direct current DC that a BJTs collector experiences in the absence of an applied input signal.
• It depicts the current that passes through the transistor when it is biased to operate in a certain linear region while it is in the active state.
• V_CC: This is an illustration of the circuits collector supply voltage.
• It is the positive voltage that is applied to the BJTs collector in relation to ground.
• R3: The value of the collector resistor that connects the BJTs collector to the VCC supply is shown by this.
• This resistor experiences a voltage drop across it as a result of the current passing through the transistor and across it.

Connection among the Variables: The formula determines the current I_Q that flows through the collector resistor R3 when a voltage V_CC is applied by essentially using ohms Law : I = V/R.

In this instance, the current passing through the resistor is the collector current I_Q.

Example:

If VCC is 12V and R3 is 39 ohms, then the quiescent current I_Q can be calculated as:

I_Q = VCC / R3 = 12 V / 39 Ω = 0.308 A = 308 mA (milliamps)

Critical Role of Quiescent Current

For BJT amplifier circuits, it is essential to set the right I_Q point, it influences elements such as:

Linear Operation: For the best possible amplification of input signals, a transistor operating in its active region is ensured by a suitable I_Q.

Distortion: Saturation distortion results from an I_Q that is too high, while crossover distortion is caused by a low I_Q.

Power Consumption: The circuits total power consumption is influenced by the quiescent current.

Constraints on the Formula:

A basic estimate of the quiescent current can be obtained using this formula.

The real I_Q value in practical circuits can be affected by additional variables like as temperature changes and transistor biasing methods.

The calculation takes into account a single BJT setup.

For more intricate BJT amplifier circuits, there may be differences in the biasing techniques and computations.

How to Build:

Building the high impedance Class A amplifier follow the below steps for connections:

Identify Transistor Types:

• Choose suitable transistors for Q1 and Q2 ensuring they meet the impedance requirements and other specifications of the amplifier.

Place Transistors on the Board:

• Insert Q1 and Q2 transistors into the breadboard or solder them onto the PCB.
• Note the pin configuration of each transistor, ensuring proper placement.

Add Resistors:

• Connect resistor R1 between the positive power supply VCC and the collector of Q1.
• Connect resistor R2 from the base of Q1 to the emitter of Q2.
• Connect resistor R3 from the collector of Q2 to the positive power supply VCC.

Quiescent Current Adjustment:

• Adjust the value of resistor R3 to modulate the quiescent current of the amplifier, using the formula I_Q = V_CC / R₃
• This step allows you to optimize the operating point of the transistors.

Capacitor C3 Placement:

• Connect capacitor C3 to decouple resistor R3 and ensure proper DC feedback.
• This capacitor helps stabilize the biasing by making the Q2 emitter voltage follow the average collector voltage of Q1.

Base Biasing with Negative DC Feedback:

• Use resistor R2 to obtain the base bias for Q1 from the emitter of Q2.
• This configuration employs negative DC feedback for bias stabilization.

Control Loudness with Potentiometer R4:

• Connect potentiometer R4 to control the loudness of the amplifier.
• This potentiometer allows users to adjust the input signal and modulate the overall volume output of the amplifier.

Connect Power Supply:

• Connect the positive and negative terminals of the power supply to appropriate points on the circuit.
• Ensure that the voltage levels are within the operating range of the chosen transistors.

Test the Amplifier:

• Power up the circuit and use an oscilloscope or an audio source and a speaker to test the output.
• Adjust potentiometer R4 to control the loudness and verify that the amplifier is functioning as expected.

Fine-Tune and Optimize:

• Fine tune the resistor values and component placements based on testing results and desired performance.
• Experiment with different transistor types and capacitor values for further optimization.

Conclusion:

This Class A amplifier design offers a versatile solution with adjustable quiescent current, efficient common emitter configuration, and strategic use of components like capacitors for stability.

By understanding the formulas and construction details, enthusiasts can experiment with variations to meet specific audio amplification needs.

References

Power amplifier classes