Amplifier Circuit Diagrams

# Low Power Audio Amplifier using 555 Timer

In venues like auditoriums or similar halls, conventional audio amplifier techniques typically employ high-power circuits to power large loudspeakers. Nevertheless, for applications demanding the use of compact loudspeakers with relatively limited range needs, we can fulfill these requirements by constructing a low-power amplifier designed for a low output current, approximately 200 mA.

This article provides an explanation of the principles, design, and functioning of a low-power audio amplifier utilizing a 555 Timer. The 555 Timer generates a carrier signal that is modulated by the amplified audio source, resulting in a modulated signal that drives a small loudspeaker.

### Outline

• Low Power Audio Amplifier Circuit Principle:
• 555 Timer as an Amplifier Circuit Diagram:
• Circuit Design of Low Power Audio Amplifier:
• Low Power Audio Amplifier using 555 Timer Circuit Simulation:
• How 555 Timer as an Amplifier Circuit Works?
• 555 Timer as an Amplifier Circuit Applications:
• Limitations of Audio Amplifier Circuit:

## Low Power Audio Amplifier Circuit Principle:

This design relies on the concepts of audio amplification and pulse width modulation, employing a 555 Timer. The audio signal is amplified through the use of the TL071, a low-noise, high-input operational amplifier, before it is directed to the control pin of the 555 Timer. The 555 Timer serves as an astable multivibrator, producing an oscillating signal. This audio signal modulates the oscillating signal, creating pulse width modulation, where the width of the output pulse adjusts according to the voltage at the control pin, representing the audio signal.

## Circuit Design of Low Power Audio Amplifier:

The circuit design process is straightforward, comprising just two key steps: constructing the preamplifier segment and devising the astable multivibrator segment.

We’ve chosen the TL071 low-noise JFET input operational amplifier, notable for its low input bias current and a swift slew rate of approximately 13V/s. To provide a 6V voltage to the non-inverting terminal of the OPAMP, we’ve devised a voltage divider network employing two 47K resistors. Assuming a desired gain of roughly 22 (V/V) or 27.2dB and an approximate value of 1K for one of the feedback resistors, we’ve determined that the value of the other resistor should be approximately 22K. Given the low output impedance of this amplifier, we’ve connected a 1K resistor at the output to link it to the control pin of the 555 Timer.

The subsequent phase in the design process is the 555 timer astable circuit. In this configuration, we’ve employed two resistors for both charging and discharging the capacitor in a typical 555 Timer setup as an astable multivibrator circuit. However, in place of one of the resistors, we’ve opted for a 1N4007 diode to expedite the discharge process. For an output frequency target of around 145 KHz and a capacitor value of approximately 10nF (considering the forward resistance of 1N4007 at around 1 Ohm), we can calculate that the threshold resistor should be approximately 1K.

## Low Power Audio Amplifier using 555 Timer Circuit Simulation:

Once the circuit is designed, the next step involves timer circuit simulation. Here we follow a series of steps to simulate the circuit using Multisim software.

1. Under the Simulate option, select the microphone simulation model from the LabView instruments.
2. The required parameters are specified in this manner (The time of recording and sampling rate).
3. The software is used to build the circuit, and the microphone is attached as an input to the circuit.
4. Under the Simulate menu, a loudspeaker model is selected from the LabView instruments and linked as an output to the circuit.
5. The interactive simulation is activated by setting the end time to be equal to or greater than the recording time.
6. The loudspeaker’s “Play” button is greyed out while the circuit simulation is running, and it is enabled once the simulation is over.

## How 555 Timer as an Amplifier Circuit Works?

The circuit’s operation can be divided into two main phases: pre-amplification, which involves amplifying the electric signal, and pulse width modulation. The amplification task is carried out by the low-noise operational amplifier, the TL071. The process begins with a microphone sensing an incoming audio signal, converting it into a low-voltage electric signal. This low-voltage AC signal is then directed to the non-inverting terminal of the operational amplifier via a 1uF electrolyte capacitor, effectively blocking any DC current from the audio signal.

The operational amplifier then amplifies this signal, with the degree of amplification determined by the values of the feedback resistors. Through the feedback network, the operational amplifier operates in linear mode, ensuring that the voltage at the non-inverting terminal matches the output voltage. The amplified signal, after being processed through a capacitor (for DC component elimination) and a resistor, is supplied to the control pin of the 555 Timer. In this setup, the 555 Timer functions in astable mode, and the output signal’s frequency is governed by the combination of resistors R1 and C1.

However, the output pulse width varies due to the presence of the control voltage in this configuration. The audio voltage modulates the carrier output signal of the 555 Timer, resulting in a modulated signal that drives the loudspeaker. Interestingly, the loudspeaker responds to the DC value of the modulated signal rather than the high-frequency component, effectively amplifying the audio signal.

## 555 Timer as an Amplifier Circuit Applications:

1. This application can be used to develop low power music systems used in vehicles.
2. It can be used in classrooms with limited areas.

## Limitations of Audio Amplifier Circuit:

1. This circuit is suitable only for low power loudspeakers.
2. 555 Timer doesn’t produce 50% duty cycle signal.
3. This circuit is theoretical and may require changes in hardware implementation.

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