A power amplifier is a type of amplifier that provides minimum output impedance. It can be used to drive a speaker which requires high power at low impedance. In this project, we are going to build a power amplifier using a push-pull class AB configuration to get a power of 150 watts to drive a load of approx 8 ohms.

Principle Behind Power Amplifier

The different biasing methods of a bipolar junction transistor are the basic principle behind this project. The electric signal we get in the output of a microphone is very low. To make this signal sustainable level, it has to be amplified using the CE configuration of a bipolar junction transistor. In this condition, it is biased in class A mode. So the output signal is an inverted amplified signal. This is a very low-power signal. To amplify it at a power level, two Darlington power transistors are arranged in class-AB configuration. And also a class A mode is used here to drive these transistors.

Must Read Audio Amplifier Using TDA2030

Project

Circuit Diagram

Components Required

• TIP142 NPN Transistor
• TIP147 PNP Transistor
• TIP41 Transistor
• 1N4007 PN Diode (x2)
• BC558 PNP Transistor
• 10uF/25V Electrolyte Capacitor (x2)
• 100uF/25V Electrolyte Capacitor
• 22KΩ (x2), 27KΩ, 3.3KΩ/1W (x2), 1.5KΩ, 220Ω, 33Ω, 1Ω (x2) Resistors
• 45-0-45V Power Supply

Circuit Connection

Power Amplifier Design

To design a class AB amplifier, first, we have to select the right transistors. In this project, our required output power is 150 watts. We know there is a situation of power dissipation in transistors. So our required power is about 200W. We take a (+/-) 45-volt dual power supply. The load is 8Ω. For more efficiency, we used Darlington pair transistors. Those are TIP142 (NPN) and TIP147 (PNP).

In this concept, we have to select the right resistors for biasing. We know the voltage across the bias resistor should be 1.4V. The average collector current is quite large and the biasing current is quite small. So we have to use a large value for the biasing resistor. Thus we take a 3.3KΩ resistor.

From the theory of this project, we see class AB amplifier used two diodes for biasing. In this step, we select those diodes. These diodes provide proper biasing to the power transistors, to eliminate the cross-over distortion. We choose these diodes by their thermal characteristics with the comparison of transistors. Here we use 1N4007 PN diodes. Now we select two swapping resistors to minimize the difference between the characteristics of the matched transistors. This also provides thermal compensation. These resistors have very low value. We take 1Ω.

At the last step of the design and AB amplifier, we have to select bootstrap resistors and capacitors. Bootstrapping is the process of increasing the input impedance of the Darlington transistors. We select a 10uF/63V electrolyte capacitor such its reactance is less at 20Hz. The value of the resistor should be large so it provides high input impedance. We select a 3.3KΩ resistor for this.

Design of Driver

After completing the class AB amplifier, we started to design the driver. Here we used the TIP41 NPN transistor to provide high power and high gain. Then the requirement is emitter resistance. The emitter voltage of the driver transistor = (V_CC/2)-V_be. The V_CC is +45V and Vbe is 0.7V. So the emitter voltage is 21.8V. Now we can see the emitter current is the same as that of the quiescent collector current for the transistor. We get the value of the resistor as around 33Ω. So we select a 33Ω resistor. Then we take the coupling capacitor to provide an AC signal from the output stage of the amplifier to the input of the driver. We select a 10uF/25V electrolyte capacitor for this.

Design of Audio Pre-Amplifier

Now we are going to design the pre-amplifier. Here VCC is around +45V. So we select a transistor with a maximum open-source collector to emitter voltage greater than V_CC. We choose the BC558 transistor. In this step, we choose the load resistor. The quiescent collector current is 20mA for BC558. The resistor is selected when a current of 20mA passes through it. This voltage is across half of V_CC. After calculation, we get the value of the resistor as 26.6KΩ. So we select a 27KΩ resistor for this.

The biasing current is approximately 10 times the base current. As a small gain of the transistor, BC558 is 125. For this, the base current is 200mA peak and so the bias current is 2A. The base voltage is also 10V greater than the emitter voltage. Now V_e is 12% of V_CC.

So V_e = 5.4V

So R₁ = (V_CC-V_b)/I_e = 20KΩ.

We take 22KΩ for this.

And R₂ = V_b/I_e = 3.35KΩ

We take 3.3KΩ for this.

Now we have to select the feedback resistor. In this project, we assume the gain A_v=45. We already calculate the load resistor 22KΩ. So the value of the feedback resistor is 220Ω.

The next step is the emitter resistor. The value of the emitter resistor is V_e/I_e or 3.3KΩ. This resistor is shared with the feedback resistor. So the emitter resistor is 3.3KΩ – 220Ω = 3.08KΩ. We select a 3.3KΩ resistor for this. The final step is to select a coupling capacitor. The value of the emitter capacitor should be less than the reactance of the emitter resistance. Here we take a 10uF/25V electrolyte capacitor for coupling.

Testing the Power Amplifier

After designing the circuit, the input is given by connecting the AC signal voltage to the coupling capacitor of the pre-amplifier. The input is set at 4VCC, 1kHz. We can see the output by connecting a watt meter such that voltage terminals are connected across the load resistor (8Ω). The current terminals are connected between the output terminal and the load resistor. Hence we obtain the output power is nearest 200W.

Working Principle of Power Amplifier

The theory of this circuit is divided into two parts. Those are class AB amplifiers and class A voltage amplifiers. One of the transistors in the class AB configuration produces an amplified output signal for only one-half of the input signal. We know that the class AB amplifier has two transistors. Thus one of them amplified one-half and the other amplified the second half of the input signal. To eliminate cross-over distortion, a class AB amplifier consists of two diodes. This configuration is driven by a transistor arranged in CE mode.

The class-A model of this circuit is used here to produce an inverted output of the input signal. The efficiency of this amplifier is low, and so is the output impedance.

Applications of Power Amplifier

• This circuit can be used in loudspeakers with low impedance.
• It can be used as a driver for a high-power antenna for long-range transmission.

Disadvantages of this Power Amplifier

• This is a theory-based circuit. The distortion may occur in the output.
• The BJT causes more power dissipation. This reduces the efficiency of the system.