buffer amplifiers

This class of amplifier is designed to follow low level stages, one example is a crystal oscillator. An oscillator for optimum performance can NOT be loaded down, it needs an intermediate stage following. This will then present a sufficiently high enough input impedance so it is not considered a significant load to the oscillator. The intermediate
buffer amplifiers - schematic

or buffer stage, while not representing a load must then have a sufficiently low output impedance to drive successive stages. If these concepts of impedance confuse or worry you then look at my other page " what is impedance " it`s a rough and ready explanation, followed by a more technical explanation of impedance matching but hopefully you will come back with a considerably better understanding. From what I said above, two desirable traits of a buffer amplifier are high input impedance and low output impedance. Obviously succeeding stages should also be linear because we don`t want to introduce distortion. As a matter of interest a buffer amplifier is not solely limited to following an oscillator. You could have a requirement of wanting to sample the output of the first mixer for what is called a "Panadaptor", a means of visually seeing adjacent signals on an oscilloscope. Also a high quality agc system should be derived from the last i. f. stage, it should be buffered before going to the agc amplifier and then on to the agc detector. Other examples are low level outputs of audio amplifier stages such as microphone inputs where the microphone is a high impedance type. Although not so common now, phono inputs to audio ampliers also needed buffering. Here I`m going to use a very practical example where one of my readers has a requirement for a voltage controlled oscillator operating at 1. 8 - 2. 0 Mhz (amateur radio band...

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