MOSFET Switch and Amplifier: Application Note

This MOSFET application note covers a basic description of the MOSFET Switch and MOSFET Amplifier.

MOSFET Switch

The following circuit diagram illustrates a MOSFET being used as a switch.

Figure 1: MOSFET switch

Figure 1 depicts an Enhancement mode type N-channel MOSFET device. As shown, the MOSFET is switched ON when a positive voltage is applied as $V_{GS}$. This will drive the LAMP connected on the Drain terminal to be powered ON. When zero voltage is applied to the device at $V_{GS}$, it will be powered OFF. Hence, the Lamp will be powered OFF at this stage.

MOSFET Amplifier

In many applications, MOSFETs are used as small signal linear amplifiers.

Figure 2: MOSFET amplifier

Figure 2 depicts a MOSFET used as an amplifier. Typically, in amplifier circuits, FETs work in the saturation region. In this region, the current does not depend on the drain voltage. The current is a function of the Gate voltage only. In MOSFET amplifiers, the operating point is normally in the saturation region.