What is dynamic resistance of FET?

Dynamic Resistance of FET

The dynamic resistance of a field-effect transistor (FET) is the ratio of the change in drain-source voltage (V_{DS}) to the change in drain current (I_D) when the gate-source voltage (V_{GS}) is held constant. It is a measure of the FET's ability to amplify a signal.

In the ohmic region of the FET's transfer characteristics, the dynamic resistance is given by:

$$r_d = \frac{\Delta V_{DS}}{\Delta I_D}$$

The dynamic resistance of a FET is typically very high, typically in the megaohms to gigaohms range. This makes FETs very efficient amplifiers, as they can produce a large output signal with a small input signal.

Calculation of Dynamic Resistance

The dynamic resistance of a FET can be calculated from its transconductance (g_m), which is the ratio of the change in drain current to the change in gate-source voltage when the drain-source voltage is held constant:

$$g_m = \frac{\Delta I_D}{\Delta V_{GS}}$$

The dynamic resistance is then given by:

$$r_d = \frac{1}{g_m}$$

Factors Affecting Dynamic Resistance

The dynamic resistance of a FET is affected by a number of factors, including:

• Gate-source voltage: The dynamic resistance decreases as the gate-source voltage increases. This is because the higher gate-source voltage increases the drain current.
• Drain-source voltage: The dynamic resistance also decreases as the drain-source voltage increases. This is because the higher drain-source voltage causes the FET to operate in the saturation region, where the drain current is less sensitive to changes in the gate-source voltage.
• Channel length: The dynamic resistance decreases as the channel length decreases. This is because the shorter channel length allows the FET to operate at higher drain currents.
• Channel width: The dynamic resistance increases as the channel width increases. This is because the wider channel allows the FET to carry more current.