The use of field effect tube

The main purpose of field effect tube in circuit is amplification, variable resistance, electronic switch, impedance transformation and constant current, etc.

1. Zoom in

The field effect tube is the same as the bipolar transistor, the main purpose is to amplify.

Figure 7-5 shows the FET amplifier circuit, VT is an N-channel junction FET, RD is the drain load resistance, and RG and RS form a self-sufficient negative gate voltage bias circuit.

The input signal Ui is coupled to the gate of the field effect transistor VT via C1, and is superimposed with the original negative bias voltage of the gate, so that the drain current lD changes accordingly, and a voltage drop is generated on the load resistance RD, after the DC isolation by C2 Output, the amplified output voltage Uo is obtained at the output end. Since the input impedance of the field effect tube is very high, the coupling capacitors C1 and C2 can be small-capacity capacitors.

2. Variable resistance

Another main purpose of the field effect tube is to be used as a variable resistor.

In the circuit shown in Figure 7-6, the N-channel junction field effect transistor VT is used as a variable resistor and forms a voltage divider with R1, which divides the input signal Ui and sends it to the amplifier for amplification. The magnitude of the equivalent resistance of VT is determined by the absolute value of its negative gate bias (-UG). Changing the magnitude of the negative bias voltage of the gate can change the magnitude of the equivalent resistance of VT to achieve the purpose of adjusting the input signal voltage of the amplifier.

3. Electronic switch

Used as an electronic switch is also one of the main uses of field effect tubes.

Figure 7-7 shows the electronic switch of the field effect tube to control the light-emitting diode circuit, and VT is an N-channel junction field effect tube. When the negative bias voltage (-UG) of the VT grid is 0V, the VT is turned on and the light-emitting diode VD emits light. When the negative bias voltage (-UG) of the VT grid is -UP, the VT is turned off and the light emitting diode VD does not emit light.

4. Impedance transformation

The high input impedance of the FET is very suitable for impedance transformation.

Figure 7-8 shows the FET source follower circuit. The circuit structure is similar to the transistor emitter follower, but because the FET is a voltage-controlled device, the input impedance is extremely high, so the FET source follower has a higher input impedance Zi and a lower output impedance Z0. It is often used in the high-impedance input stage of a multi-stage amplifier for impedance conversion.

5. Constant current

The field effect tube can easily constitute a constant current source.

Figure 7-9 shows a constant current source circuit composed of N-channel junction field effect transistors with constant current, where UP is the pinch-off voltage of the field effect transistor. If the drain current ID through the field effect tube increases for some reason, the negative gate voltage formed on the source resistor RS also increases, forcing the ID to fall. If the drain current ID through the field effect tube decreases for some reason, the negative gate voltage formed on the source resistor RS will also decrease, forcing the ID to rise, and finally keeping the ID constant.

The commonly used packages for SMD MOS tubes include SOT-23, SOT-523, SOT-323, TO-252, TO-263, etc. The MOS tube brands supplied by Pingshang Technology generally include: Changdian Fairchild VISHAY IR ON, etc. Other tubes are also available, if you need to know other electronic products, please consult online customer service.