A power amplifier circuit is designed to have the lowest possible output impedance and is intended for driving high-power loads, such as speakers, that operate at low impedance levels. In this project, we have crafted a power amplifier circuit using a push-pull class AB configuration to generate a power output of 150 watts, suitable for driving an 8-ohm load, such as a speaker.
- Principle Behind Power Amplifier Circuit:
- Theory Behind Power Amplifier Circuit:
- Circuit Diagram of 150W Power Amplifier Circuit:
- Power Amplifier Circuit Design:
- Design of Class AB Amplifier Stage:
- Design of Driver Stage:
- Design of Audio Preamplifier Stage:
- Testing the Power Amplifier Circuit:
- Applications of Power Amplifier Circuit:
- Power Amplifier Circuit Design:
Principle Behind Power Amplifier Circuit:
The core concept of this circuit revolves around various biasing methods for a bipolar junction transistor (BJT). The electrical signal generated by a microphone tends to be quite weak. To address this, we employ a BJT in a Common Emitter (CE) configuration biased in class A mode. This configuration amplifies the low-voltage signal to a sustainable level, albeit with an inverted amplification. However, the signal remains at a very low power level. To boost the power level of this signal, we employ two Darlington power transistors in a class AB arrangement. These transistors are driven by another transistor operating in class A mode.
Theory Behind Power Amplifier Circuit:
This circuit incorporates two essential elements: class AB amplifiers and class A voltage amplifiers. In the class AB mode, a transistor amplifies the output signal to half of the input signal. A class AB amplifier comprises two matched transistors, with each one conducting for half of the input signal duration. To mitigate crossover distortion, practical class AB amplifiers employ diodes for biasing these two transistors. The amplifier is driven by a common emitter transistor configuration.
Meanwhile, a class A bias mode is used with another transistor to generate an inverted version of the input signal.
Circuit Diagram of 150W Power Amplifier Circuit:
Power Amplifier Circuit Design:
Design of Class AB Amplifier Stage:
- Transistors should be used: The required output power is 150W. We estimate the required power to be around 200W, taking into account transistor power dissipation. We’ve chosen a +/-50V dual supply, with Vcc = 50V and an 8Ohm load. We use Darlington pair transistors – TIP142(NPN) and TIP147 – to improve the circuit’s efficiency (PNP).
- Bias Resistor Selection: The bias resistor should have a voltage of around 1.4V less than Vcc. The biassing current is also small because the average collector current is rather huge. As a result, bigger resistors are used. A 3K resistor is used in this example.
- Diode Selection: The two diodes are needed to provide adequate biassing to the power transistors, hence preventing cross over distortion. The diodes should be chosen so that they have similar thermal characteristics to the transistors. We’re utilising 1N4007 diodes here.
- Output Resistors: To minimise any differences in the characteristics of the two matched transistors and provide thermal compensation, two swamping resistors are utilised. These resistors should be small, therefore we went with 0.33 Ohms.
- Selection of Bootstrap Resistor and Capacitors: Bootstrapping is used to boost the Darlington transistors’ input impedance. We choose a 10uF electrolyte capacitor that has a lower reactance at a frequency of at least 20Hz. To achieve a high input impedance, the resistor value should be substantial. We’ll use a 3K resistor in this case.
Design of Driver Stage:
- Transistor Selection: We’ve chosen a TIP41 power transistor to produce a high-power, high-gain output.
- Emitter Resistance: The difference between half of Vcc and Vbe is the emitter voltage of the driver transistor. Because Vcc is 50 volts and Vbe is 0.7 volts, the emitter voltage is 24.3 volts. The value of resistor Re is around 50 Ohms since the emitter current is the same as the quiescent collector current for the transistor. However, we’ll use a 40-Ohm resistor in this case.
- Coupling Capacitor Selection: The coupling capacitor is utilised to provide AC signal from the previous amplifier’s output stage to the driver stage’s input. We’ll use a 10uF electrolyte capacitor here.
Audio Preamplifier Stage:
- Because Vcc is roughly 50V in this case, we choose a transistor with a maximum open source collector to emitter voltage larger than Vcc. The NPN transistor BC546 is ideal for this application.
- R3: Load Resistor Selection According to the BC546 datasheet, the quiescent collector current is roughly 2mA. The value of the load resistor is set so that the voltage across it is half of Vcc when a current of 2mA runs through it. This yields a 12.5K load resistor. We’ll use a 10K resistor here.
- R1 and R2 are used to select biassing resistors. The biassing current is supposed to be 10 times the base current. The base current is roughly 0.016mA, while the bias current is 0.16mA, because to the BC546’s weak signal gain of around 125. In addition, the base voltage is 0.7V higher than the emitter voltage. Assume that the emitter voltage, Ve, is 12% of Vcc, or 6V.
- This gives,
- R1 = (Vcc-Vb)/Ie = 24.5K. Here we select a 25K resistor
- R2 = Vb/Ie = 3.35K. Here we select a 3K resistor.
Selection of feedback resistor,R5:
Here we assume the required gain, Av = 50. Since load resistor is about 10K, value of feedback resistor is calculated to be around 200Ohms.
Selection of emitter resistor, R4:
The total emitter resistor value is given by Ve/Ie, i.e. 3K. However since this resistor is shared with the feedback resistor, the emitter resistor is around 3K-200 = 2.8K. Here we select a 2K resistor.
Selection of emitter capacitor:
The value of this capacitor should be such that the reactance is less than the total emitter resistance. Here we select a 0.01uF electrolyte capacitor.
Selection of coupling capacitor:
The coupling capacitor is an electrolyte capacitor of 10uF.
Testing the Power Amplifier Circuit:
After constructing and simulating the circuit on Multisim, the input is sourced from an AC signal voltage by connecting it to the coupling capacitor in the preamplifier stage. The input voltage is configured to be 4Vpp with a frequency of 1kHz. To measure the output power, a Wattmeter is connected, with its voltage terminals bridging an 8Ohm load resistor and its current terminals positioned between the output terminal and the load resistor. It’s evident that the maximum output power reaches approximately 200W.
Applications of Power Amplifier Circuit:
- This circuit can be used to drive a loudspeaker of low input impedance, in audio amplification.
- We can also use this circuit to drive high power antennas for long range transmission.
- This circuit is theoretical and the output contains distortion.
- The use of linear devices like BJTs cause more power dissipation, thus reducing the efficiency of the system.