Where does an amplifier get the energy to make a signal larger?

A microphone's small electrical signal can control a loudspeaker output far more powerful than the microphone could supply alone. An amplifier makes this possible by using energy from a power supply. The input determines the changing pattern of the output; it does not provide all the output energy. That distinction is the heart of electronic amplification.

A transistor can operate in a region where a small change at its input changes a larger current elsewhere in the device. A surrounding circuit establishes an operating point, or bias, so the varying signal can move above and below that point without immediately reaching a limit. Other components convert the controlled current into a useful output voltage or provide the current needed by a load. An amplifier can be built from many transistors, but the principle remains controlled energy transfer.

Voltage gain compares the change in output voltage with the change in input voltage. If a circuit has a gain of ten within its operating range, a 0.1-volt signal variation produces a 1-volt output variation. Current gain and power gain describe related but different quantities. A large unloaded voltage is not necessarily enough to drive a low-resistance speaker: the output must supply the required current as well.

Real amplifiers have limits set by supply voltages, internal voltage requirements, output-current capability and heat. Increasing the input beyond the useful range can flatten the tops of the waveform, called clipping. The output no longer has the same shape as the input, so an audio signal becomes distorted. An amplifier cannot make an unlimited output simply because a gain formula says it should. The illustration places the intended larger waveform beside the clipped result to show where the linear relationship fails.

Negative feedback returns a portion of the output in a way that opposes the input difference. In a properly designed amplifier, this can establish a predictable gain and reduce some distortion. It also involves trade-offs in frequency response and stability. Feedback does not create energy or remove the physical supply limits. The amplifier's power stage still draws energy from its supply, and some of that energy becomes heat.

An amplifier also enlarges unwanted signals that enter its useful range. Noise already present at the input does not become a cleaner message merely because it is louder, and the electronics add noise of their own. Careful design therefore considers gain, bandwidth, distortion, noise and the intended load together. The small signal is a control pattern; the power supply is the energy source. Keeping those roles separate makes amplification understandable across microphones, instruments, radios and sensors.