field effect transistor amplifier circuit

1. The bias circuit has two methods: self-generated bias and hybrid bias. Circuit I in Table 1 uses the IdRs generated by the drain voltage ID through Rs as the generated bias voltage, that is, Ugs=-IdRso can stabilize the operating point. The larger |IdRs|, the better the stability, but an excessively negative bias voltage will cause the tube to pinch off and fail to work. The above defects can be overcome by using hybrid bias circuits such as Table 2 and Table 3. They are hybrid biases composed of self-generated bias and external bias. Since the external bias EdRp (Rp is the voltage division coefficient) increases the gate potential, it is easier to select a larger IdRs to stabilize the operating point. Circuits 2 and 3 The function of Rg is to increase the input resistance of the circuit 2. Graphical method Use the graphical method to find the static operating point of the circuit as follows: Table 1 Commonly used field effect transistor amplifier circuits 1 2 3 Circuit Graphical method Equivalent circuit (1) Write the DC load line The equation is: Uds=Ed-Id(Rd+Rs)=15-3.2Id Let ID=0, then UDS=15 volts, mark N point on the abscissa, and let UDS=0, get ID=4.7 mA , mark point M on the ordinate, connect M and into a straight line, then MN is the DC load line. (2) Draw the gate-drain characteristics (transfer characteristics): According to the intersection coordinates of the load line and each drain characteristic curve, draw the ID=f(UGS) curve shown on the left side of Figure B below, which is called the gate-drain characteristic. (3) Draw a gate loop load line with Tga=1/Rs through the origin of the gate-drain characteristic coordinates. It intersects with the gate-drain characteristic at Q. Then draw a horizontal axis parallel line passing through the Q point and intersects with the gate-drain load line at Q' . Read from the static operating points Q and Q': IDQ = 2.5 mA, UGSQ = -3 volts, UDSG = 7 volts, the graphical method in Table 1 is the same. 3. Equivalent circuit analysis method The micro-variation circuit of the field effect transistor is shown in the figure below. For specific examples of equivalent circuits written from the field effect tube amplifier circuit, please refer to Table 1. The method of finding the voltage amplification factor and input and output resistance based on the equivalent circuit is the same as for a transistor circuit.