Amplification

Amplification is the process of making a signal larger, using energy from an external power supply, while preserving the information the signal carries. A weak voltage from a microphone, an antenna or a heart-monitor electrode goes in, and a copy with greater voltage, current or power comes out. The word names the concept; the circuits and devices that perform it are covered separately under amplifier, and the active element that does the real work is usually a transistor or, in older equipment, a vacuum tube.

Amplification never creates energy; it controls a larger flow with a smaller one. A transistor in an amplifier acts as a valve: the small input signal modulates how much current is let through from a steady supply, so the output is a stronger version of the input shape. This is why an amplifier needs power even when it produces no sound, and why the output power can exceed the input power without breaking conservation of energy, since the difference is drawn from the supply and the rest is dissipated as heat. Efficiency, the fraction of supply power delivered to the load, is therefore a key figure, and it differs enormously between amplifier designs.

Gain can be measured as a voltage, current or power ratio, and engineers usually quote it in decibels. Voltage gain is the output voltage divided by the input voltage, current gain is the corresponding current ratio, and power gain is the output power over input power, which equals the product of the voltage and current gains. A decibel is ten times the base-ten logarithm of a power ratio, or twenty times the logarithm of a voltage ratio, so a voltage gain of 10 is 20 decibels, a gain of 100 is 40 decibels, and doubling the voltage adds about 6 decibels. Stages in a chain multiply as plain ratios but simply add in decibels, which is the whole reason the unit is used. A gain of zero decibels means the output equals the input, and a negative number means attenuation.

Useful amplification is linear, and the limits of the linear region define distortion. An ideal amplifier multiplies every instant of the input by the same constant, so the output is a scaled copy. Real devices are linear only over a range: if the input is too large, the output hits the supply voltages and is flattened, which is clipping, and even short of clipping a gently curved transfer characteristic adds harmonics that were not in the input. Distortion is usually quoted as total harmonic distortion, a percentage of unwanted harmonic content, and in radio systems nonlinearity also makes two signals mix into spurious products. Negative feedback is the standard cure: trading away some gain for flatness and lower distortion, a method first described by Harold Black at Bell Labs in 1927.

Bandwidth is the other limit: gain holds only over a band of frequencies. Transistor and wiring capacitances roll the gain off at high frequencies, and coupling capacitors cut it at low ones. The usual rule is that the gain-bandwidth product of a feedback amplifier is roughly constant, so a stage set for ten times the gain has about one tenth of the bandwidth, and the frequency at which a transistor's gain falls to one, called the transition frequency, sets the ceiling. Audio needs about 20 hertz to 20 kilohertz, while a mobile-phone radio front end works at hundreds of megahertz to several gigahertz, so the design choices are quite different. Noise is also amplified along with the wanted signal, and the first stage, which sets the noise of the whole chain, is chosen with particular care.

The history of amplification is the history of electronics. The triode vacuum tube, patented by Lee de Forest as the Audion in 1906, made long-distance telephony and radio broadcasting possible by repeating and strengthening faint signals. The transistor, demonstrated at Bell Labs on 23 December 1947, did the same task without a hot filament, and the integrated op-amp later put a high-gain amplifier into a small package that feedback turns into a precise building block. Today amplification is everywhere: sensors in phones, hearing aids, radio receivers and medical instruments all depend on it, and each digital logic gate is in effect an amplifier driven to its limits, restoring a degraded signal to clean levels at every step.

Read amplification as the controlled borrowing of energy from a supply to enlarge a signal faithfully: its quality is judged by gain, bandwidth and distortion, and its limits are set by the supply, by the devices and by noise.