Solid-State Devices & Circuits 11. CE Amplifiers at High Frequencies ECE 342 Solid-State Devices & Circuits 11. CE Amplifiers at High Frequencies Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois jschutt@emlab.uiuc.edu

BJT Capacitances Base: Diffusion Capacitance: Cde (small signal) where Qn is minority carrier charge in base where tF is the forward transit time (time spent crossing base)

BJT Capacitances Base-emitter junction capacitance: Cjeo is Cje at 0 V. Voe is EBJ built in voltage ~ 0.9 V

BJT Capacitances In hybrid pi model, Cde+Cje=Cp Collector-base junction capacitance Cmo is Cm at 0 V. Voc is CBJ built in voltage ~ 0.9 V Cp is around a few tens of pF Cm is around a few pF

High-Frequency Hybrid-p Model

CE - Three Frequency Bands

CE High-Frequency Model

CE High-Frequency Model

CE High-Frequency Model

CE High-Frequency Model

Bipolar Miller Effect The left hand side of the circuit at XX’ knows the existence of Cm only through the current Im  replace Cm with Ceq from base to ground

Bipolar Miller Effect

Bipolar Miller Effect

Bipolar Miller Effect (cont’)

Short-Circuit Current Gain

Short-Circuit Current Gain (Cont’) Define hfe as short-circuit current gain

Short-Circuit Current Gain (con’t) Define hfe has a single pole (or STC) response. Unity gain bandwidth is for: In some cases, if Cm is known, then

Short-Circuit Current Gain (con’t) From which we get

BJT-CE – Miller Effect – Exact Analysis

BJT-CE – Miller Effect – Exact Analysis

BJT-CE – Miller Effect – Exact Analysis We neglect the terms in s2 since Miller If we multiply through by

BJT-CE – Miller Effect – Exact Analysis If Ri = 0