JFET AC Analysis JFET ac Analysis Transistor Model

(JFET) – AC Analysis

JFET ac Analysis Transistor Model • Transistor can be redrawn using HYBRID π MODEL. Hybrid (Giacoletto) equivalent model.

JFET ac Analysis Transconductance, gm • FET: dc gate-to-source voltage (VGS) controls the level of dc drain current (ID) • Relationship between output and input: VP=VGS(off) ID (m. A) IDSS • gm=gfs=yfs • Unit for gm: Siemen or Ω-1 VGS (V) VP@VGS(off) 0

Transconductance, gm ID (m. A) • Function at Q : slope of the tangent line Q 2 Q 1 ID 1 0 VGS (V) VP@VGS(off) Where gm 0 also known as gm max VGS 1 ID 2 VGS 2

• Effect of ID Transconductance, gm ID (m. A) Q 2 0 ID 1 Q 1 VGS (V) VGS(off) VGS 1 ID 2 VGS 2

JFET ac Analysis Resistance, rd (rds) • At VGS constant, • On specification sheet, output impedance of FET always appear as yos ( S)

JFET ac Analysis Common-source amplifier with bypass capacitor CS +VDD R 1 RD C 2 + + C 1 Vi RL R 2 RS CS - - Zi Vo ZO

JFET ac Analysis Common-source amplifier with bypass capacitor CS ii g + vgs + gmvgs RTh Vi io d rd RD RL s - Zi Ac equivalent network Vo - Zo

Hybrid- π parameters: ,

Input impedance: Output impedance:

Voltage gain: Current gain:

If rd >> RD (rd= ) Input impedance: Output impedance:

If rd >> RD (rd= ) Voltage gain: Current gain:

Example (1): +VDD R 1 RD C 2 + C 1 Vi RL R 2 RS CS - Zi ZO • Given: IDSS = 16 m. A, VP = -3 V, VDD = 18 V, R 1 = 110 MΩ, R 2 = 10 MΩ, RD = 1. 8 kΩ, RS = 150 Ω, RL = 100 kΩ, VGSQ = • -0. 35 V and IDQ = 7. 6 m. A. Assume rd = ∞ • Find; – Determine gm and compare to gm 0 – Sketch the ac equivalent network – Find Zi – Calculate Z 0 – Find Av

To be continued