FET high input impedance broadband amplifier

 Field effect transistors have very high input impedance and can be made into high input impedance amplifiers. However, in field effect tube amplifiers
, the distributed capacitance and Miller effect capacitance at the input end have a great impact on high frequencies. In addition, as the operating frequency increases
, the input impedance of the field effect transistor will gradually decrease. Therefore,
it is impossible to obtain a high input impedance over a wide frequency range using a common-source FET amplifier without negative feedback . Therefore, to use field effect tubes to produce high input impedance
wide-band amplifiers, amplifiers with series voltage negative feedback should be used or source plate followers should be used. Figure 8-11 is a high input impedance wide-band preamplifier composed of
    field effect tubes and bipolar transistors . In the figure, the field effect transistor VTi forms a common source amplifier, and its drain output is directly coupled to the base of the transistor VT2. The collector output of VT2 is divided by Rs and R3 and fed back to the electrode of VTi, forming Deep series voltage negative feedback greatly improves the amplifier's frequency response and increases its input impedance. According to the values ​​of each component in the figure, the circuit's voltage gain Kv=2.7, the output impedance is equal to 80n, and the passband reaches several megaohms. Increasing Rs can increase the total gain of the amplifier, and Kv can be greater than 100 times. But as K,, increases , the frequency response decreases.