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Tamil Nadu HSE Second Year Basic Electronics Engineering study materials
Tamil Nadu HSE Second Year Basic Electronics Engineering study resources

Tamil Nadu HSE Second Year Basic Electronics Engineering: Field Effect Transistors and Amplifiers

Tamil Nadu HSE Second Year Basic Electronics Engineering · Work through your exam, one topic at a time.

All guide sections

Field Effect Transistors and Amplifiers

This domain tests voltage-controlled, unipolar devices: JFET and MOSFET characteristics, the transfer curve, transconductance and the common-source amplifier.

Core concepts

  • A JFET controls channel width by reverse-biasing the gate-channel junction, so gate current is tiny and input resistance is very high (10^8 Ω or more).
  • Key parameters: I_DSS (drain current at V_GS = 0) and V_P or V_GS(off). For an n-channel JFET, V_P is negative.
  • Transfer characteristic: I_D = I_DSS(1 − V_GS/V_P)², valid for V_GS between V_P and 0.
  • Transconductance g_m = ΔI_D/ΔV_GS = (2 I_DSS/|V_P|)(1 − V_GS/V_P).
  • Enhancement MOSFETs have no channel at V_GS = 0 and need V_GS > V_th. The insulated gate gives extremely high input resistance but is easily damaged by static.
  • Common-source amplifier: A_v = −g_m(R_D ∥ R_L); the minus sign means 180° phase inversion.

Worked example

I_DSS = 8 mA, V_P = −4 V, V_GS = −2 V. Then I_D = 8(1 − 0.5)² = 2 mA. g_m = (2 × 8/4)(1 − 0.5) = 2 mS. With R_D = 4.7 kΩ and no load, A_v = −2 mS × 4.7 kΩ = −9.4, so a 50 mV input gives about 0.47 V output, inverted.

Traps

Dropping the sign of V_P; using V_GS beyond pinch-off; confusing mS (mA/V) with µS; assuming the MOSFET gate draws steady current; ignoring that a FET is unipolar and therefore has no thermal runaway.