The amplifier remains a transformative invention for being able to receive the human voice and boost it so thousands of people can hear it at once. Without the microphone, politics itself may have evolved differently; the device has been ubiquitous in everything from public protests to political rallies. Amplifiers have also transformed entertainment, astronomy, search and rescue, medical diagnostics, and every human endeavour where some physical change can be converted to an electric signal. For simplicity’s sake, let’s focus here on audio signals.
What is a microphone?
A microphone is a type of transducer — a device that converts energy in one form to another, in this case from sound to electrical. There are different microphones for use in specific contexts, varying in terms of how they collect sound energy and how they convert it to electric signals.
Microphones in telephones and voice recorders are also called condenser microphones because they use capacitors to transduce audio signals (‘condenser’ is an old name for capacitors). A simple capacitor consists of two plates facing each other, separated in between by an air gap — or any suitable dielectric material — but connected on either side to a common circuit, including a power source. As the current enters one plate and builds up, it becomes depleted from the other. The difference creates an electric field between the plates that stores energy within the dielectric medium.
The ability of a capacitor to store energy is called its capacitance. A condenser microphone replaces one of the plates with a vibrating diaphragm. When a person speaks into the microphone, the diaphragm vibrates, altering the capacitance, which is converted to audio signals.
How is the sound amplified?
Once an audio signal is available, it is transported to a preamplifier (or preamp). Say there is an audio signal in the form of a varying voltage.
The preamp will acquire the voltage and supply it with more energy from an external power source while minimising noise. The ratio of the output of this process to the input is called the preamp’s gain.
The key to minimising noise is a step called impedance-matching. Impedance is a measure of how much a given circuit resists, or impedes, the flow of electric current (in a direct-current circuit, the impedance is equal to the resistance.)
During impedance-matching, the impedance of the preamp’s circuit is matched to that of the circuit that emitted the audio signal. If there is a mismatch, the signal will become noisy, reducing the preamp’s gain.
The amplification itself is performed by an active component like a transistor.
How is voltage amplified?
A bipolar junction NPN transistor consists of a base, a collector, and an emitter.
First, the small incoming voltage is applied between the base and the emitter — that is, the transistor is forward-biased — such that electrons flow out of the emitter.
The base is configured so that most of the electrons diffuse into the collector, leading to a large collector current. In a typical transistor, the collector current is at least 50-times higher than the base current.
Next, the collector is supplied with a high voltage from an external source, like a battery, through a resistor. When the collector current flows across this resistor, it creates a strong voltage. This voltage when measured at the collector is the output voltage, that is, the amplified version of the input signal.
A signal can be drawn as a sinusoidal wave; an amplified signal will have the same shape but bigger, in this case meaning the voltage swing will be higher.
Before the preamplified signal goes to a loudspeaker, it also passes through the driver and power stages. The driver stage uses transistors to keep the voltage constant while increasing the current while the power stage further increases both the voltage and the current.
Power amplifiers come in multiple classes, each for specific applications. Class A and B amplifiers are used in basic sound systems and small studios. Home theatres and more intensive use cases prefer Class AB amplifiers. Antennae emitting radio-frequency signals at a single carrier frequency use Class C amplifiers whereas public announcement systems use Class D amplifiers.
Some of these groups have sub-classes that, while sharing many of the same features, deviate in particular ones like being able to handle very high power or enabling high-frequency signals.
How is loud sound produced?
The loudness of the sound produced at the loudspeaker is a function of the power of the input signal.
The loudspeaker has three basic components: voice coil, external magnet, and diaphragm. The voice coil is a coil of wire placed within the magnet. When the input voltage is applied across the wire, a large current is produced, which in turn produces a strong magnetic field around the coil. This field interacts with that of the external magnet, causing the coil to move as the input voltage varies.
The coil is connected to the diaphragm and the coil’s motion vibrates the diaphragm’s membrane, producing sound. The higher the power of the input signal, the louder the sound produced by the diaphragm.
To further enhance this process, the external magnetic field is made stronger, the diaphragm larger, and the input signal more powerful. Loudspeakers can also be built with smaller diaphragms with faster-moving membranes to facilitate high frequency sounds and larger diaphragms to facilitate low frequency ones.
Published – February 04, 2025 06:00 am IST
