What is the input impedance of the MOSFET

Basics of radar technology

MOSFET

Figure 1: MOSFET circuit symbol
(the circle can also be omitted)

Figure 1: MOSFET circuit symbol
(the circle can also be omitted)

MOSFET is the abbreviation for M.et al Oxide S.emiconductor (also: S.ilicon) F.ield E.ffect Transistor. In order to take into account the fact that in modern processes no more oxide is used under the gate, the neutral term is also used IGFET For I.nsulated Gate FET used. The MOSFET is another type of field effect transistor and has several advantages over the JFET. The input impedance of the MOSFET is increased again compared to the JFET (10 to 100 MΩ). This extremely high input impedance no longer creates any load on upstream components and, together with the high voltage gain, makes the MOSFET an ideal RF amplifier element, also for measuring devices and laboratory circuits.

As with the bipolar transistor, there are two complementary designs: n-channel MOSFET and p-channel MOSFET. These can be divided into self-locking (enhancement transistor, "enrichment type") and self-conducting (depletion transistor, "impoverishment type") designs. It has at least three connections electrodes: gate, drain, source and with some designs the additional connection substrate (or bulk) is led to the outside.) A circuit diagram symbol is shown in Figure 1.

Figure 2: Schematic structure of an n-channel MOSFET

Figure 2: Schematic structure of an n-channel MOSFET

A MOSFET is a voltage controlled transistor. The voltage between gate and source controls the flow of current between drain and source. Virtually no current flows through the gate. As a result of this structure, the gate connection, insulating layer and substrate connection form a capacitor which is charged when a voltage is applied between the gate and the substrate. In the process, the minority carriers (in the case of p-silicon electrons) migrate to the boundary layer in the substrate and recombine with the majority carriers (in the case of p-silicon holes). This has the effect of displacing the majority bearers and is called “impoverishment”. From a certain voltage Uth (Threshold voltage) the displacement of the majority charge carriers is so great that they are no longer available for recombination. There is an accumulation of minority carriers as a result of which the actually p-doped substrate becomes n-channel conductive close to the insulating layer. This condition is called strong "inversion". The resulting thin n-channel conducting channel now connects the two n-regions source and drain, which means that charge carriers can flow (almost) unhindered from source to drain.

Use of a MOSFET as an amplifier

Fig. 3: Amplifier circuit with a MOSFET

Fig. 3: Amplifier circuit with a MOSFET

Figure 3 shows a standard circuit for an amplifier stage with a self-blocking n-channel MOSFET of type BS170. This amplifier stage is characterized by a high input impedance and a good frequency response and is often used as an impedance converter. C1 and C2 are the input and output capacitors for DC decoupling. R1 and R2 form an input voltage divider and determine the gain factor. The gate bias is achieved here by R3. This form is called "dynamic bias" and at the same time creates negative voltage feedback that improves the frequency response but reduces the gain. R4 is the operating resistance, which, as with all amplifier circuits, determines the quiescent current and thus the operating point. This circuit also causes a phase change of 180 °, as already described for the JFET.