MOS as Diode, Switch and Active Resistor

A simple N-channel MOSFET can be used as a diode, Switch and Active resistor. This presentation is a part of course of Analog CMOS Design, based on textbook of same title by Allen Holberg.

1. MOS Diode & Active Resistor Sudhanshu Janwadkar

2. MOS as Diode: When the gate of the MOSFET is connected to the drain, it acts like a diode with characteristics similar to a pn -junction diode.

3. Circuit Connection

4. Region of Operation When the gate is connected to the drain of an enhancement MOSFET, the MOSFET is always in the saturation region.

5. Since V T is always greater than zero for an enhancement device, then V DG = 0 satisfies the conditions for saturation.

6. Expression for Current/Voltage

7. The comparison is basically the difference between an exponential and a square-law function . However, if the designer is willing to spend W/L, the comparison becomes more interesting as shown below How Does the MOS Diode Compare with a pn Diode ?

8. Small Signal Model

9. Small signal resistance of MOS Diode

10. Applications of MOS diode The MOS diode is a component of Current Mirror circuits The MOS Diode is used for level translation

11. Application(To generate a Bias Voltage)

12. MOSFET as Active Resistor The MOSFET Switch can be viewed as a resistor, where the transistor’s Drain and Source form the two terminals of a floating resistor.

13. MOSFET acts in non-saturation region. The Resistance can be given as:

14. Review Questions: 1. With the help of suitable schematic and necessary expressions, explain MOSFET as switch, diode and active resistor.