In one line
In a common-source FET amplifier, the FET is biased in its saturation (pinch-off) region; a small AC gate voltage varies the drain current through the transconductance \( g_m \), producing an amplified, phase-inverted voltage across the drain resistor.
Explained simply
A FET amplifier works like a BJT amplifier, but the control is by voltage instead of current. Since the gate draws practically no current, the amplifier hardly loads the signal source, like a very sensitive listener who does not interrupt the speaker. This makes FET stages ideal as input stages for weak signals from microphones, sensors and antennas.
Technically, the common-source (CS) configuration is the FET equivalent of common emitter: input between gate and source, output between drain and source, source common to both. Its voltage gain depends on \( g_m \) and the drain resistor \( R_D \).
How it works
- Biasing: for a JFET, self bias with a source resistor \( R_S \) makes \( V_{GS} = -I_DR_S \) (negative). For an enhancement MOSFET, a voltage divider gives \( V_{GS} > V_T \). A large gate resistor \( R_G \) keeps input resistance high.
- The Q-point is placed in the saturation (pinch-off) region where \( I_D \) depends only on \( V_{GS} \).
- The AC input \( v_{gs} \) changes drain current by \( i_d = g_mv_{gs} \).
- Output \( v_o = -i_dR_D \): when \( v_{gs} \) rises, \( I_D \) rises and drain voltage falls, so output is inverted by 180°.
- A bypass capacitor across \( R_S \) keeps the AC gain high; coupling capacitors pass the signal and block DC.
Other configurations: common drain (source follower, gain about 1, used as a buffer) and common gate.
Key formula
\( A_v = -g_mR_D \) (approx., \( R_S \) bypassed, \( r_d \gg R_D \)), \( g_m = \dfrac{\Delta I_D}{\Delta V_{GS}} \), for a JFET \( g_m = \dfrac{2I_{DSS}}{|V_P|}\left(1 - \dfrac{V_{GS}}{V_P}\right) \)
\( A_v \) = voltage gain, \( g_m \) = transconductance (siemens, S, often mS), \( R_D \) = drain resistance (\( \Omega \)), \( r_d \) = drain resistance of the FET itself (\( \Omega \)), other symbols as in Shockley's equation.
Quick example
With \( g_m = 2 \) mS and \( R_D = 5 \) kΩ, \( A_v \approx -10 \). This is less than a typical BJT stage, but the input resistance can be many megaohms, which is why FET input stages are used in electret microphones and measuring instruments. (Illustration only.)
Common mistakes
- Biasing a JFET with positive gate voltage.
- Saying a FET amplifier has no phase inversion in CS configuration.
- Giving \( g_m \) the unit ohm; it is siemens (A/V).
How MU asks it
Numericals are not expected in this module (MU syllabus), so questions are theory with diagrams. Typical: "Explain the application of FET as an amplifier" (5 marks). For full marks draw the CS circuit (self-biased JFET or voltage-divider MOSFET) with coupling and bypass capacitors, explain biasing in the saturation region, define \( g_m \), write \( A_v = -g_mR_D \), show inverted waveforms, and mention the high input impedance advantage.