Fixed bias circuit

Fixed bias circuit

(1) Biasing method of field effect transistor (FET) amplifier circuit
    ① Fixed bias circuit. In field effect tube amplifiers, it is sometimes necessary to add an external gate DC bias power supply. This method is called a fixed
bias circuit as shown in the figure.
    cl and c2 are the input coupling capacitor and the output coupling capacitor respectively. +Vcc is added to the drain of the VT tube through the drain load resistor Rz
, and the source of the VT tube is grounded. - vcc is a dedicated bias DC power supply for the gate, which is a negative polarity power supply. It is added to the
gate of the VT1 tube through the gate bias resistor Ri, so that the gate voltage is lower than the source voltage, thus establishing the normal bias of the VT tube. Set voltage.
    In the circuit, the input signal M is coupled to the gate of the field effect transistor VT via C1, and is superimposed with the original negative gate bias. The field effect transistor is affected by the gate
. The drain current I changes accordingly, and a voltage drop occurs on the load resistor R2. After being isolated from DC by c2, it is output, and
the amplified signal voltage 乩 is obtained at the output end. l2 is in phase with U, and Uo is in anti-phase with U. The advantage of
    this bias circuit is that the VT operating point can be selected arbitrarily and is not restricted by other factors. It also makes full use of the drain DC power
supply +Vcc, so it can be used for low-voltage power supply amplifiers. The disadvantage is that it requires two DC power supplies.