FET high input impedance wideband amplifiers
Posted on Dec 11, 2010 5373
FET has high input impedance, high input impedance amplifier can be made. However, FET amplifying device, the distributed capacitance and the Miller effect input capacitance of
the high frequency is very high. In addition, with the rising operating frequency high input impedance FET will also be gradually reduced. Therefore, without negative feedback common source FET amplification is not possible to obtain a very high input impedance over a wide frequency range. So a high input impedance FET production of wideband amplifier, plus a series of negative voltage feedback amplifiers should be used or the use of source follower plate. Figure 8-11 is a FET, and a bipolar transistor group into a wideband high input impedance preamplifier. Figure, FET common source amplifier VTi composition, its drain output of the transistor is coupled directly to Cape bamboo base o VT2 VT2 collector output and after Rs and R3 voltage divider is fed back to the flooded VTi pole is formed series voltage deep negative feedback, which greatly improved the frequency response of the amplifier, and increase its input impedance. Figure each component values, the circuit voltage gain Kv 2.7, the output impedance equal to 80n, pass -band of several megohms. Rs can be improved to increase the overall amplifier gain, Kv may be greater than 100-fold. But with K, mention high frequency response, along with the decrease.