Amplifier
An amplifier is a circuit or device that takes a weak signal and delivers a stronger one, drawing the extra power from a supply. The word names the physical object, from a single transistor stage to a rack-sized radio transmitter, while amplification names the underlying idea of enlarging a signal. An amplifier is built around one or more active devices, such as vacuum tubes or transistors, plus resistors, capacitors and often feedback networks that fix its gain, frequency range and input and output impedance.
The simplest transistor amplifier is the common-emitter stage, and it shows how the parts cooperate. A bipolar transistor with its emitter grounded (or tied to ground through a small resistor) has the input signal applied to the base and the output taken from the collector, which is connected to the supply through a collector resistor. A rise in input voltage raises the collector current, increasing the drop across the resistor and lowering the output voltage, so the output is a larger, inverted copy of the input. Voltage gain is roughly the collector resistor times the transconductance, and bias resistors hold the transistor at a mid-range operating point so that the signal can swing both ways. Other single-transistor arrangements, the common-collector (emitter follower) and common-base stages, trade voltage gain for impedance buffering or high-frequency performance.
The operational amplifier packages high gain into a general-purpose block that feedback turns into precise functions. An op-amp is a differential amplifier with enormous open-loop gain, typically 100,000 or more, and two inputs whose voltage difference it amplifies. Connect part of the output back to the inverting input and the closed-loop gain depends almost entirely on two resistors, so a ratio of 10 to 1 gives a gain of 10 regardless of the transistor spread inside. Devices such as the Fairchild 741, introduced in 1968, made this block cheap and ubiquitous. Op-amps are used for sensor preamplifiers, filters, active audio circuits, voltage regulation and analogue computing, and their inputs, built from field-effect transistors in many modern parts, draw almost no current.
Power amplifiers are classified by how long each transistor conducts, which sets the balance of fidelity against efficiency. In class A the transistor conducts for the whole cycle and sits at a high steady current, giving very low distortion but a theoretical efficiency of only 25 percent with a resistive load, so most of the supply power becomes heat. Class B uses two devices, each conducting for half of the cycle, and can approach about 78 percent efficiency, but the hand-off between them creates crossover distortion near zero volts. Class AB biases both slightly on, so the handover is smooth, and is the usual compromise for audio, often built from a complementary pair of n-p-n and p-n-p transistors. Class D turns the output devices fully on or fully off at a high rate, with a modulated pulse train that a low-pass filter converts back to the signal, giving efficiencies above 90 percent, which is why it dominates battery-powered speakers and phones.
Negative feedback is the idea that made accurate amplifiers practical, and it came from the telephone network. Harold Black of Bell Labs conceived the negative feedback amplifier in 1927, seeking to reduce the distortion of the repeaters that boosted long-distance telephone lines, and his patent was granted in 1937. By feeding a portion of the output back, inverted, to the input, a high-gain amplifier gives up gain in exchange for stable, predictable and low-distortion behaviour, and the result depends on the passive feedback components rather than the active device. Harry Nyquist and Hendrik Bode then supplied the stability theory that tells when feedback becomes oscillation. A common pitfall is underestimating that risk: feedback with too much delay at high frequencies turns the amplifier into an oscillator, and layout, capacitance and loads all matter.
Different jobs call for very different amplifiers, and the term covers the whole range. Audio amplifiers need flat gain over roughly 20 hertz to 20 kilohertz and low distortion; radio-frequency amplifiers work at hundreds of megahertz to many gigahertz, where tuned circuits, matching networks and low noise are central, with a low-noise amplifier at the receiver input and a power amplifier at the transmitter output. Every phone contains several of these, usually built from gallium arsenide, silicon-germanium or silicon transistors. Instrumentation amplifiers pull microvolt signals out of strain gauges and bioelectric sensors, and inside digital chips countless small amplifier stages, from sense amplifiers reading memory cells to line drivers, restore signals and drive loads. A common misconception is that more power or more gain means a better amplifier; in practice, noise, bandwidth, linearity and stability decide quality.
Read an amplifier as the engineered object that performs amplification: a transistor or tube operated at a chosen bias, wrapped in feedback, and sized to its job, whose class and architecture express how it trades fidelity, bandwidth and efficiency.