A power amplifier circuit is the one with minimum output impedance, used to drive loads like a speaker, which require high power at low impedance. Here we designed a power amplifier circuit using push pull class AB configuration to derive a power of 150W to drive a load of 8 Ohms (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:
- Limitations:
- Power Amplifier Circuit Design:
Principle Behind Power Amplifier Circuit:
The multiple ways of biassing a bipolar junction transistor is the main theory behind this circuit. A microphone’s electric signal output is quite modest. The CE configuration of a BJT biassed in class A mode amplifies this low voltage signal to a level that can be sustained. The output is an inverted amplified signal in this mode. This is a very low-powered signal. The power level of this signal is amplified by two Darlington power transistors in a class AB arrangement. This transistor is driven by a transistor designed in class A mode.
Theory Behind Power Amplifier Circuit:
Class AB amplifiers and class A voltage amplifiers are two crucial components of this circuit. The output signal of a transistor biassed in class AB mode is amplified for just half of the input signal. An AB amplifier is made up of two matched transistors, one of which conducts for half of the input signal and the other for the other half. To prevent cross over distortion, a practical class AB amplifier uses diodes to give biassing to the two transistors. A transistor with a common emitter design drives this amplifier.
A transistor with a class bias An inverted version of the input signal is produced by a mode.
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.
Design of 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.
| COMPONENT | VALUE |
|---|---|
| R1 | 25K |
| R2 | 3K |
| R3 | 10K |
| R4 | 2K |
| R5 | 200Ohms |
| R6 | 3K |
| R7 | 3K |
| R8 | 40Ohms |
| R9, R10 | 0.33Ohms |
| C1,C2,C3 | 10uF, electrolyte |
| C4 | 0.01uF, electrolyte |
| Q1 | TIP141, NPN |
| Q2 | TIP147, PNP |
| Q3 | TIP41 |
| Q4 | BC546 |
| Vcc | +/-50V |
Testing the Power Amplifier Circuit:
The input is provided by connecting an AC signal voltage source to the coupling capacitor of the preamplifier stage after the circuit has been built and drawn on Multisim. The input voltage is set to 4Vpp and the frequency is set to 1kHz. The output is calculated by connecting a Wattmeter with the voltage terminals across an 8Ohm load resistor and the current terminals between the output terminal and the load resistor. We can see that the highest output power is roughly 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.
Limitations:
- 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.
