Question

How can you calculate the input impedance and voltage gain of a common-drain MOSFET amplifier?

Answer 1

To calculate the input impedance and voltage gain of a common-drain (CD) MOSFET amplifier, also known as a source follower or emitter follower configuration, you can follow these steps:

Circuit Configuration:
The common-drain MOSFET amplifier is characterized by the MOSFET being connected in a source follower configuration, with the input applied to the gate and the output taken from the source terminal. The load resistor is connected to the source terminal, and a voltage source is applied to the gate terminal.

Small-Signal Model:
Assuming the amplifier operates in the small-signal AC region, we can use the small-signal equivalent model of the MOSFET. For the common-drain configuration, the small-signal model looks like this:

scss
Copy code
+Vdd
  |
  R_load
  |
  |
 [MOSFET]
  |  |
  |  G (Gate)
 [Source]

Calculating Input Impedance (Zin):
The input impedance of the common-drain MOSFET amplifier is high, typically on the order of several hundred kilohms to megohms. It is primarily determined by the resistance seen at the gate terminal (reverse-biased PN junction) and is given by the formula:

Zin = 1 / jωCgs

where:
Zin = Input Impedance
j = imaginary unit (√(-1))
ω = Angular frequency of the input signal
Cgs = Gate-to-Source capacitance of the MOSFET in the small-signal model

Calculating Voltage Gain (Av):
The voltage gain of a common-drain MOSFET amplifier is less than 1 (unity gain or slightly less than unity) due to the voltage drop across the output impedance (Rs) and finite channel resistance (rd). The voltage gain is approximately given by:

Av ≈ 1 - gm * (Rs + rd)

where:
Av = Voltage Gain
gm = Transconductance of the MOSFET (g_m = ΔID/ΔVG, where ID is the change in drain current, and VG is the change in gate-to-source voltage in the small-signal model)
Rs = Source resistance (load resistor)
rd = Drain resistance (channel resistance) of the MOSFET in the small-signal model

It's important to note that the voltage gain might vary slightly depending on the operating point (biasing) of the MOSFET and the MOSFET's transconductance value, which in turn depends on the operating point and the MOSFET's parameters.

Remember, these calculations are based on the small-signal model and assume that the signal is small enough not to drive the MOSFET into saturation or cutoff regions. For large-signal analysis, additional considerations would be necessary.