How to build Fixed-Bias Class A Power Amplifier

Class A power amplifiers are widely used in audio systems due to their linear output characteristics, high efficiency, and low distortion. In this guide, we will walk you through the process of building a fixed-bias Class A power amplifier. 

Class A amplifiers are a type of linear power amplifier where the output device (transistor, tube, etc.) conducts for the full cycle of the input signal. This results in a linear output waveform with minimal distortion, but also results in a low power efficiency (typically 25%). Class A amplifiers are typically used in applications where low distortion is of paramount importance, such as in audio systems. They are considered to have a "warm" and "smooth" sound, compared to other types of amplifiers.

A power amplifier, also called large signal amplifier, are amplifier which experiences large excursion in current and voltage producing high output power at the load. A power amplifier uses power transistor. Here we will illustrate power amplifier design with power bipolar junction transistor. Some of the BJT power transistor are as follows.

  • 2N3055: a popular NPN power transistor widely used in medium power applications such as audio amplifiers, DC-DC converters, etc.

  • TIP41C: a widely used PNP power transistor commonly used in audio amplifiers, linear regulators, and switch mode power supplies.

  • MJ2955: a widely used NPN power transistor that provides high gain and low saturation voltage. It is commonly used in power amplifier circuits, DC-DC converters, etc.

  • BD135: a popular NPN BJT widely commonly used in power amplifier circuits.

  • TIP31C:a bipolar junction transistor (BJT) designed for use in switching applications and in power amplifiers.

Materials needed

  • BD135 Power Transistor (NPN)
  • Resistor
  • Capacitor
  • Heat sink
  • Power supply
  • Breadboard or PCB

Step 1: Circuit Design

The circuit for a fixed-bias Class A power amplifier consists of a transistor, a resistor, a capacitor, and a heat sink. The first step is to choose a suitable transistor for your amplifier. The transistor should be able to handle the desired power output and voltage swing.

Here BD135 will be used as a power amplifier transistor in the design of fiexed biased class A power amplifier. The BD135 is an NPN transistor that is commonly used in power amplifier circuits. It has a maximum collector current of 1.5A and a maximum collector-emitter voltage of 80V, making it suitable for use in low to medium power amplifier applications. When choosing a transistor for a power amplifier, it is important to consider the specifications of the transistor such as its maximum power rating, voltage rating, gain, and frequency response, in addition to the specific requirements of the application.


We will consider the design of fixed bias BD135 power amplifier with Vcc=9V, Ic=50mA, load resistor of 8Ohm and input signal frequency of 20KHz.

The following is the completed circuit diagram. 

Fixed bias BJT Power Amplifier

Step 2: Component Selection

Once you have chosen the transistor, you need to select the resistor, capacitor, and heat sink. The value of the resistor will determine the bias current of the transistor, which in turn affects the efficiency and distortion of the amplifier. The capacitor will provide stability to the circuit and prevent unwanted oscillations. The heat sink is used to dissipate heat generated by the transistor during operation.

For a fixed bias amplifier also called base bias amplifier, the base resistor (Rb) and collector resistor (Rc) determine the bias point for the transistor. The input and output coupling capacitors (Ci and Co) are used to couple the signal to and from the amplifier, removing any DC component from the signal.

To determine the values of RB, RC, C1, and C2, we need to consider the specifications of the BD135 transistor, the supply voltage (Vcc), the collector current (Ic), the load resistor (RL), and the frequency of the signal (f) assumed in the above specification.

  • Rb: To calculate the value of Rb, we need to determine the base current (Ib). This can be done using the equation:

Ib = Ic / beta

Where beta is the current gain of the transistor. For a BD135, the typical current gain is between 100 and 300. We'll use a value of 200.

Ib = 50mA / 200 = 0.25mA

Next, we can calculate the value of Rb using Ohm's law:

Rb = (Vcc - Vbe) / Ib

Where Vbe is the base-emitter voltage, which is typically around 0.7V for a bipolar junction transistor.

Rb = (9V - 0.7V) / 0.25mA = 33.2kΩ

We'll round this up to the nearest standard value, which is 33kΩ.

  • Rc: To calculate the value of Rc, we'll use the load resistor (RL) and the collector current (Vc) of 4.5V:

Rc = (Vcc-Vc)/Ic

Rc = (9V-4.5V) / 50mA = 90Ω

We'll round this down to the nearest standard value, which is 100Ω.

  • Ci: The value of the input coupling capacitor (Ci) depends on the frequency of the signal and the impedance of the input source. At 20kHz, a value of 1µF is typically used.

  • Co: The value of the output coupling capacitor (Co) depends on the frequency of the signal and the load impedance. At 20kHz, a value of 10µF is typically used.

So, the values for Rb, Rc, Ci, and Co are:

Rb = 33kΩ Rc = 100Ω Ci = 1µF Co = 10µF

An easy way to determine the biasing resistor values and the coupling capacitor values is to use the online fixed bias BJT amplifier design calculator

Fixed bias BJT Power Amplifier calculator

 To calculate the power at the load and the efficiency of the amplifier, we need to know the supply voltage (Vcc), the collector current (Ic), and the load resistor (RL).
  • Power at the Load: The power at the load can be calculated using Ohm's law:

P = V^2 / R

Where V is the voltage across the load and R is the load resistor.

V = I * R = 50mA * 8Ω = 0.4V P = 0.4V^2 / 8Ω = 0.1W

  • Efficiency: The efficiency of the amplifier can be calculated by dividing the power at the load by the power consumed by the amplifier:

Efficiency = Pout / Pin

Where Pout is the power at the load and Pin is the power consumed by the amplifier.

Pin = Vcc * Ic = 9V * 50mA = 0.45W Efficiency = 0.1W / 0.45W = 22.2%

So, the power at the load is 0.1W and the efficiency of the amplifier is 22.2%.

Step 3: Breadboard Assembly

Next, assemble the components on a breadboard or PCB. The transistor should be mounted on the heat sink and connected to the power supply. Connect the resistor between the transistor's collector and base, and the capacitor between the collector and ground. Connect a load resistor or a speaker of 8Ohm as load as is done here.

The following shows completed power amplifier assembled on breadboard.

Breadboard Assembly of Fixed bias BJT Power Amplifier

Connect the power supply to the circuit. Make sure that the power supply voltage is within the recommended range for the transistor you have chosen.

Step 4: Testing

Finally, test the amplifier by connecting an input signal and measuring the output. For this you can use an oscilloscope to measure and see the input and output signal waveform. Following shows a picture of output signal on oscilloscope of this audio amplifier circuit on breadboard.

Testing of Fixed bias BJT Power Amplifier with oscilloscope

Check for any signs of distortion or instability. Adjust the value of the resistor if necessary to achieve the desired bias current and distortion performance.

If you don't have oscilloscope you can use a PC soundcard oscilloscope to fed signal into the amplifier and then see the output waveform. The base biased BJT amplifier on breadboard and test with PC soundcard based oscilloscope tutorial shows how you can use free oscilloscope + function generator + spectrum analyzer software to test actual circuit without the need of high end oscilloscope. Another way is to use electronics design software like Proteus or Matlab to test actual electronics circuit on breadboard. For this see Instrumentation Amplifier actual circuit testing with Proteus and  Matlab Simulink as Oscilloscope to test real electronics circuit

Video demonstration of Class A Power Amplifier

The video below demonstrates how this fixed biased class A power amplifier works and it performs.



In conclusion, building a fixed-bias Class A power amplifier is a simple process that can be done with a few basic components and some knowledge of electronics. This guide should give you a good starting point, but it is important to always be careful and make safety a priority when working with electricity. It is important to note that this is a power amplifier and so a pre-amplifier(small signal amplifier) maybe required in front of the amplifier. There are many ways to build small signal pre-amplifier and some of them are follows:

- 2n3904 audio amplifier

- Audio amplifier using op amp

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