Simple Class A Amplifier Circuit

The class A amplifier circuit type discussed in this post is suitable for tiny speakers or headphones.

Amplifiers enhance the volume of sounds.

Class A amplifiers use roughly 20 milliamperes of energy making them suitable for devices with lesser power requirements.

This article explains how we can construct one by using Q1 and Q2 transistors that resemble like small switches.

WARNING: Electronics projects are challenging to construct.

This should only be done under adult supervision.

What is Class A Amplifier Circuit:

Both output stages of a Class A amplifier must run constantly at maximum power.

Because of this continuous operation class A power amplifier designs are known to be the least efficient with an average efficiency of about 20% and an estimated maximum of 50%.

Circuit Working:

Parts List:

About 20 milliamperes of quiescent current is drawn by the amplifier.

A key aspect that impacts the transistor biasing and, in turn the amplifiers overall performance is the quiescent current.

The amplifiers quiescent current can easily be controlled by changing the value of resistor R3.

The amplifiers properties can be modified to meet certain needs and the operating point can be improved.

Q1 and Q2 transistors are set up as common emitter amplifiers.

By connecting Q1s output directly to Q2s input a cascaded setup is created that improves overall amplification.

About 80 dB is the total voltage gain Aᵥ of the amplifier circuit which is an important indicator of the circuits amplification capacity.

The function of capacitor C3 is to decouple resistor R3 which is the emitter load for Q2.

This configuration improves stability by ensuring sure that the emitter voltage at Q2 is in close alignment with the average collector voltage at Q1.

Resistor R2 serves to calculate the base bias for Q1 based on the emitter of Q2.

Negative direct current feedback protects the bias in this configuration improving the amplifier’s overall performance and stability.

The volume of the amplifier circuit is easily adjusted using the input potentiometer R4.

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

Formulas:

The collector current in a BJT arrangement is the quiescent current (IQ) that flows through a transistor in a circuit.

Let us calculate this by:

IQ = VCC / R3

• The direct current DC that a BJT collector experiences when no input signal is provided is known as the Current Idle State IQ.
• VCC or the positive voltage provided to the BJTs collector in respect to ground is an example of the circuits collector supply voltage.
• According to this calculation R3 is the value of the collector resistor that joins the BJTs collector to the VCC supply.

Connection among the Variables:

By correctly applying ohms law which states that I = V/R the formula calculates the current IQ that passes through the collector resistor R3 when a voltage VCC is supplied.

Here the collector current IQ is the current flowing through the resistor.

Example:

If VCC is 12V and R3 is 39 ohms then the quiescent current IQ is calculated as:

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

The vital function of quiescent current:

Setting the correct IQ point is crucial for BJT amplifier circuits since it affects components like:

Linear Function:

An proper IQ ensures that a transistor is working in its active area for maximum input signal amplification.

Distortion:

Excessively high IQs lead to saturation distortion while low IQs lead to crossover noise.

Power Consumption:

The quiescent current affects the circuits overall power usage.

Standard Restrictions:

The formula is used here is to provide a rough estimation of the quiescent current.

Other factors like temperature variations and transistor biasing techniques can also have an impact on the actual IQ value in actual circuits.

One BJT configuration is considered in the calculation.

There can be changes in the biasing methods and calculations for more complex BJT amplifier designs.

How to Build:

To Build the high impedance Class A amplifier circuit follow the below mentioned steps for connections:

Discover the Types of Transistors:

• Select suitable transistors for Q1 and Q2 to ensure they satisfy the amplifiers other requirements for impedance.

On the board arrange the transistors:

• Join the Q1 and Q2 transistors onto the PCB or place them onto the breadboard.
• Take note of each transistors pin design to make sure it is positioned correctly.

Include Resistors:

• Connect resistor R1 between Q1s collector and the positive power source VCC.
• From Q1s base to Q2s emitter connect resistor R2.
• Connect resistor R3 to the positive power supply VCC from Q2s collector.

Adjustment of Quiescent Current:

• To modify the amplifiers quiescent current change the value of resistor R3 using the formula IQ = VCC / R3.
• By using this step one can improve the transistors operating point.

Location of Capacitor C3:

• To ensure proper DC feedback connect capacitor C3 to decouple resistor R3.
• By forcing the Q2 emitter voltage to follow the average Q1 collector voltage this capacitor helps in biasing stability.

Negative DC Feedback and Base Bias:

• To get the base bias for Q1 from the emitter of Q2 use resistor R2.
• Negative DC feedback is used in this setup to maintain bias.

Adjust the volume with Potentiometer R4:

• To control the amplifiers volume connect potentiometer R4.
• Users can modify the amplifiers overall volume output and change the input signal with this potentiometer.

Connect the power supply:

• The power supplies positive and negative terminals are connected to the proper spots on the circuit.
• Check that the voltage levels fall within the operational range of the transistor.

Examine the amplifier:

• Turn the circuit ON and then look at the output using an oscilloscope or a speaker and audio source.
• To regulate the volume one should be sure the amplifier is operating as it should by adjusting the potentiometer R4.

Adjust and Enhance:

• Adjust the resistor values and component positions according to the desired performance and test results.
• For more efficiency try out other transistor types and capacitor values.

Conclusion:

This Class A amplifier design provides flexibility with its effective common emitter design, adjustable quiescent current and smart use of improving components like capacitors.

To find ways to satisfy certain audio amplification requirements fans can experiment with changes by understanding formulas and construction details.

References:

Power amplifier classes