To calculate the input impedance and voltage gain of a common-drain (CD) MOSFET amplifier, also known as a source follower, you need to consider the circuit configuration and characteristics of the MOSFET. A common-drain amplifier has the input applied to the gate and the output taken from the source, with the drain connected to the power supply.
Here are the steps to calculate the input impedance and voltage gain:
Input Impedance (Zin):
The input impedance of a common-drain MOSFET amplifier is given by the impedance seen at the gate terminal.
a. Find the gate resistance (Rg):
This is the resistance connected in series with the gate terminal. In many cases, the gate of a MOSFET is directly connected to the input signal source, so Rg is often negligible (close to zero).
b. Find the gate-source capacitance (Cgs):
This is the capacitance between the gate and source terminals. It's an important parameter as it affects the high-frequency response and input impedance.
The input impedance (Zin) can be approximated as:
Zin ≈ Rg + (1 / jωCgs)
Voltage Gain (Av):
The voltage gain of a common-drain MOSFET amplifier is the ratio of the change in output voltage (Vout) to the change in input voltage (Vin).
a. Find the drain-source resistance (Rd):
This is the resistance connected in series with the drain terminal. In practice, the resistance of the MOSFET itself can be considered as Rd.
b. Find the transconductance (gm):
The transconductance is a measure of how much the drain current changes for a given change in the gate-source voltage.
c. Calculate the voltage gain (Av):
The voltage gain of the common-drain amplifier is approximately equal to unity, as it is a voltage follower. This means the output voltage follows the input voltage without significant amplification.
Av ≈ 1
Keep in mind that the calculations above are based on simplifications and are applicable to ideal common-drain amplifiers. In real-world scenarios, you might need to consider additional circuit elements and parasitic effects, such as MOSFET channel resistance, output load resistance, and various capacitances for more accurate results.
Also, note that different MOSFET models have slightly different characteristics, so make sure to refer to the specific datasheet and model parameters when performing detailed analyses.