# Input Impedance of a MOSFET circuit

• Engineering
• CoolDude420
In summary, the conversation is discussing how to calculate the input impedance of a circuit, specifically the series combination of C1 and C2. The person initially thought it would be the series combination, but after doing the calculation, they realized they needed to use superposition and zero out the current source in the small-signal model. The solution ultimately involves using KCL to solve for the input impedance.
CoolDude420

## Homework Statement

(From Razavi's Design of Analog CMOS Integrated Circuits - Chapter 6)

## The Attempt at a Solution

Not too sure about calculating the input impedance of this. Initially I thought that it would be the series combination of C1 and C2, since we are neglecting Cgs, so

But after I actually did it on paper, I got something like this,

Usually, I would try and use superposition for this but I don't see how that would work since Vx would dissapear. Honestly, I have no idea what to do now?

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Delta2
EDIT: I just realized my mistake. When calculating input/output resistances, all independant sources have to be zeroed just like in Thevenin's Theorem, so that current source there shouldn't be there in my small-signal model.
From that it's just one KCL and job done.

## What is input impedance?

Input impedance is a measure of the resistance that a circuit presents to a signal source, and it is typically expressed in Ohms. It represents the amount of resistance a circuit exhibits to an incoming signal and is an important parameter in electronic circuits.

## How is input impedance calculated?

Input impedance can be calculated by measuring the voltage and current at the input of a circuit and using Ohm's law (Z = V/I) to determine the impedance. In the case of a MOSFET circuit, the input impedance can also be determined by analyzing the circuit's equivalent input circuit and using the appropriate equations.

## What factors affect the input impedance of a MOSFET circuit?

The input impedance of a MOSFET circuit is affected by several factors, including the gate-source voltage, the biasing of the MOSFET, and the physical characteristics of the MOSFET itself. Additionally, any external components, such as resistors or capacitors, can also impact the input impedance of the circuit.

## Why is input impedance important in MOSFET circuits?

The input impedance of a MOSFET circuit is important because it determines the amount of current that can be delivered to the circuit. A high input impedance allows for efficient signal transfer, while a low input impedance can cause signal loss and distortion. It is also a critical parameter in designing and analyzing circuits to ensure proper functionality.

## How can the input impedance of a MOSFET circuit be optimized?

The input impedance of a MOSFET circuit can be optimized by selecting the appropriate biasing and operating conditions for the MOSFET, as well as choosing external components that will provide the desired input impedance. Careful design and analysis of the circuit can also help to optimize the input impedance for optimal performance.

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