在红外辐射计、压电检测器,使用电容传声器的超声检测装置,零值指示器和低电平换能器等高灵敏度电子仪器中,信号源通常具有较高的内阻。前置放大器除要求其噪声要很小之外,还必须具有尽可能高的输入阻抗,提供一定的增益,并把信噪比提高到一定的水平。用普通三极管制成的前置放大器,即使在元件和电路结构上给以细致的考虑,也不易很好地满足以上的要求。但使用场效应晶体管,由于它本身是一种电压控制元件,同时其电流噪声特别小,所以在这种场合使用最为合适,已经得到十分广泛的应用。本文简述这种放大器的原理和几个实际使用的线路。理想的放大器对信号应不附加任何噪声。因此,把一个实际的放大器化为理想的放大器时,在理想放大器的入口就应包括两部分噪声 In infrared radiometers, piezoelectric detectors, ultrasonic testing devices that use condenser microphones, high-sensitivity electronic instruments such as zero-value indicators and low-level transducers, signal sources often have high internal resistance. In addition to requiring less noise, the preamplifier must have the highest possible input impedance, provide some gain, and raise the signal-to-noise ratio to a certain level. Preamplifier made with ordinary transistor, even in the components and circuit structure to give careful consideration, it is not easy to well meet the above requirements. However, the use of field-effect transistors, because it is a voltage control element itself, while its current noise is particularly small, so the most appropriate use of this occasion, has been very widely used. This article briefly describes the principle of this amplifier and several actual use of the line. The ideal amplifier should have no noise on the signal. Therefore, to turn an actual amplifier into an ideal amplifier, the entrance to the ideal amplifier should include two parts of noise