Amplifier

What Is an Amplifier?

An amplifier, electronic amplifier, or (informally) amp is an electronic device that can increase the power of a signal (a time-varying voltage or current). It is a two-port electronic circuit that uses electric power from a power supply to improve the amplitude of a signal applied to its input terminals. It, therefore, produces a proportionally greater amplitude signal at its output. Its gain measures the amplification an amplifier provides: the ratio of output voltage, current, or power to input. An amplifier is a circuit that has a power gain higher than one. It can be a separate equipment or an electrical circuit within another device.

Categorization

Amplification is fundamental to modern electronics, and amplifiers are widely used in almost all electronic equipment. Amplifiers can be categorized in different ways. One is by the frequency of the electronic signal being amplified. For example, amplifiers amplify signals in the audio (sound) range of less than 20 kHz. RF amplifiers amplify frequencies in the radio frequency range between 20 kHz and 300 GHz. Servo amplifiers and instrumentation amplifiers may work with very low frequencies down to the direct current. Their physical placement can also categorize amplifiers in the signal chain; a preamplifier may precede other signal processing stages, for example. The first practical electrical device which could amplify was the triode vacuum tube. Lee De Forest invented it in 1906, which led to the first amplifiers around 1912. Today, most amplifiers use transistors. People wrote a great deal about the configuration of generic amplifier circuit design, though they usually overlooked the subtlety and quiet effectiveness of the topology. The input differential pair implements one of the few forms of distortion cancelation that engineers can rely upon to work reliably without adjustment. This is because the transconductance of the input pair is determined by the physics of transistor action rather than the matching of ill-defined parameters such as beta; the logarithmic relation between lc and Vbe is proverbially accurate over some eight or nine decades of current variation.

Voltage

The voltage signal at the voltage amplifier stage (VAS) transistor base is typically a couple of millivolts, looking instead like a distorted triangle wave. Fortunately, the voltage here is of a little more than academic interest. The circuit topology consists of a transconductance amp (voltage-difference input to current output) driving into a trans resistance (current-to-voltage converter) stage. The differential-pair action straightens the exponential Vbe/lc law in the first case. In the second, LF's global (overall) feedback factor is sufficient to linearise the VAS. The HF shunts negative Feedback, though Cdom conventionally takes over VAS-linearization while the overall feedback factor falls.

Source Texts

• Amplifier