In electronics, "gain" means the factor by which a device multiplies a signal's voltage. For a unity gain component, this factor is one -- not increasing or decreasing an input voltage, but keeping it the same. Although signal processing typically involves amplification, or a voltage increase, a unity gain circuit isolates sensitive circuit elements from one another, preventing interference and optimizing performance.
Amplification and Attenuation
Electronic amplifier circuits can have almost any gain factor, from a small fraction of one to millions or billions. Sound systems, radio and television typically have a gain of 10 or more. A radio, for example, amplifies signals from the antenna in the range of microvolts to millivolts; to bring this to a more useful several hundred millivolts, the amplifier must have a gain of about 1,000 -- much greater than unity. Conversely, an attenuator reduces a signal that's too strong for a circuit; for example, a 2,500-volt signal is too great for an oscilloscope to measure directly, but an attenuator will reduce the signal to 25 volts, making it safe to measure. In this case, the attenuator has a gain of .01 -- much lower than unity.
Electronic designers frequently choose devices called operational amplifiers as easy-to-use gain elements in circuits. You can set them up for nearly any gain number -- one, one-half or 1 million. In an op amp circuit, the ratio of the feedback resistor's ohm value to that of the input resistor determines the gain value. For example, if the input and feedback resistors are both 1,000 ohms, the ratio is 1000/1000 and the circuit has unity gain.
A unity gain amplifier may alter some aspects of an incoming signal without affecting the voltage. One example is the integrator, a circuit that limits the speed of voltage changes. For example, the input signal changes at a rate of 10 volts per second, but the integrator's output limits this to 1 volt per second. An integrator is useful for averaging the value of rapidly-changing measurements, such as a sound-level meter in a noisy factory.
Some circuit elements have low drive current; when you connect other parts to them, they become unstable or might shut down altogether. To solve this problem, you can connect a buffer between the sensitive component and the other parts. A buffer is a unity-gain amplifier that delivers sufficient current to the other circuit parts. The sensitive elements send a voltage to the buffer, which produces an identical voltage at its output but with greater drive current. This improves the stability and reliability of the whole circuit.
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