Common Source Amplifier

Small-Signal Analysis - AC Equivalent Circuit: 

The ac equivalent circuit is constructed by assuming that all capacitance have zero impedance at the signal frequency and dc voltage sources are ac ground.

• Input voltage is applied to the gate terminal
• Output signal appears at the drain terminal
• Source is common to both input and output signals. Thus circuit is termed a Common-Source (C-S) Amplifier.
• The terminal gain of the C-S amplifier is the gain from the gate terminal to the drain terminal

CS Amplifiers Input Resistance and Signal-Source Gain:

Define RiG as the input resistance, looking into the base of the transistor. Rin is the resistance presented to Vi.

The signal source voltage gain is:

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Common-Source Amplifiers Design Estimate : "Rule of Thumb"

Our rule-of-thumb estimate for the Common Source amplifier: the voltage gain equals the power supply voltage. Note that, this is 10 times smaller than that for the BJT!

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Example: CS Amplifiers Voltage Gain

 

Problem Statement: Calculate voltage gain, input resistance and maximum input signal level for a common-source amplifier with a specified Q-point. Given data: Kn = 0.50 mA/V2, VTN = 1 V, λ = 0.0133V-1, Q-point is (0.241 mA, 3.81 V)

Solution: 

Assumptions: MOSFET is in the active region. Signals are low enough to be considered small signals.

 

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