The inherently low input impedance of the transistor makes it necessary for special techniques to be used to raise this low impedance to values in the range of 1 to 30 megohms. This paper reviews a few of the techniques that allow high input impedance amplifiers to be realized. The main design criteria for these amplifiers are (1) high input impedance, (2) thermal stability, and (3) low noise figure. The various circuits that are presented will be discussed with respect to criteria (1) and (2) above. General comments will be made on the factors influencing noise figure.
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