In common-source JFET amplifier, the output voltage is ________

Question

Examoogle · Accepted Answer

180° out of phase with the input

Explanation: In a common-source JFET amplifier, the output is taken from the drain terminal while the input is applied to the gate. Due to the inherent transconductance property of the FET, an increase in gate voltage leads to an increase in drain current, which consequently increases the voltage drop across the drain load resistor, causing the drain-to-source voltage to decrease.

Working Principle: The JFET acts as a transconductance amplifier where $I_D$ is controlled by $V_{GS}$. When $V_{GS}$ increases, the channel conductivity decreases (in depletion mode) or increases (in enhancement mode, depending on type), but fundamentally, the drain current $I_D$ follows the input signal. The output voltage $V_{out} = V_{DD} - I_D R_D$ creates an inverse relationship: as $I_D$ rises, $V_{out}$ falls, resulting in a 180° phase shift.

Key Formulas: $A_v = -g_m R_D$ — Voltage gain of the common-source amplifier; $V_{out} = V_{DD} - I_D R_D$ — Output voltage relationship

Key Points: The gate-source junction is reverse biased, leading to very high input impedance.; Common-source configuration provides both voltage and current gain.; The negative sign in the gain formula $A_v = -g_m R_D$ mathematically signifies the 180° phase inversion.

Advantages: High input impedance; High voltage gain

Disadvantages: Miller effect capacitance limits high-frequency response; Output impedance is relatively high (equal to $R_D$)

Applications: Preamplifiers; RF amplifiers; Sensor signal conditioning

Additional Info: Option B is incorrect because non-inverting operation occurs in common-drain (source follower) configurations.. Option C is physically impossible for a simple resistive load amplifier.. Option D is incorrect as taking output at the source creates a common-drain configuration with unity gain.

Answer: A is correct — The common-source JFET amplifier acts as an inverting amplifier, introducing a 180° phase shift between the input signal at the gate and the output signal at the drain.

Exam Tip: Always remember that common-gate configurations are non-inverting (in phase), while common-source configurations are inverting (180° phase shift).

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