EECS 242 BJT High Frequency Distortion High Frequency Distortion in BJTs Meyer BIAS Assume current drive Large Circuit Equivalent Circuit vi vo neglect Distortion Due to Diffusion Capacitor CB is a non linear diffusion capacitor diffusion cap Cje is emitter base depletion capacitor Assume constant depletion cap If fT of a BJT is constant with IC then we have no high frequency distortion in the device for current drive and Linear Differential Equation fT constant F constant Kirk Effect Flat region Kirk Effect HighLevel Injection Due to Cje 0 Governing Differential Eq BJT Series Expansion BJT Series Expansion cont Substitute Memoryless Terms Non linear differential equation for vo vs ii where null always add Memory Terms In put First Order Second Order Memory Second Order Third Order Memory Third Order No null because a3 is negative IM2 in BJT at High Frequency Independent of frequency at high frequency IM3 in BJT at High Frequency Again Example 2 GHz LNA Front End 2 9V 750 mA 2 9V BIAS for intercept point into 50 6 dBm output IP3 Very close to SPICE and measurements 150 mA LNA Example cont Note Low compared to r LNA with Emitter Degen Analysis of BJT transconductor as a nonlinear Gm cell Include emitter degeneration KCL Equations IM3 Calculation These terms can resonate For small offset frequencies Typical Example References UCB EECS 242 Class Notes Robert G Meyer Spring 1995 K L Fong and R G Meyer High Frequency Nonlinearity Analysis of Common Emitter and Differential Pair Transconductance Stages JSSC pp 548 555 vol 33 no 4 April 1998
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