Field effect transistors voice amplifier, the input impedance of the transistor is low, only lkCZ, requiring signal source provides a constant current signal to work. The field
effect transistor is not, it is a voltage-controlled device, within the normal working range of Si, the gate G a few take almost no current. Field effect transistor has a very high input impedance (107n), low noise, large dynamic range and high magnification, use it usually only composed of a voltage amplifier with level to meet the requirements. It is widely used in various electronic sub-circuits. Sub-junction field effect transistor and an insulated gate type two, this section describes the junction field effect transistor transistor. Figure is a gate voltage divider type common source amplifier, similar to those in Figure 9-18 common emitter VT1 transistor amplifier. It works as follows: transistor amplifier and similar circuits, must always be first gate (G) of a source (S) and the gate (G) - drain (D) two IN junction is reverse biased state (for N-type tube ), such as the drain current gate current k change varies, if the drain resistance Rn great, you can get a lot of voltage changes from f and electrical amplification. Its gate bias resistor bias current is not made to its gate, and the gate bias voltage is obtained. To do this in the circuit, it is common bias resistor adjustable resistor RP, easy to obtain when appropriate quiescent point in the circuit to adjust the gate bias.