Lecture 12 - Lecture 12: Introduction to electronic analog...

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Lecture 12: Introduction to electronic analog circuits 361-1-3661 1 Our aim is to develop the transistor small-signal model for high frequencies and to find the high-frequency response of elementary transistor circuits. We will do this for the BJT transistors only; the analysis of JFET and MOSFET transistor circuits is very similar. 8.1. Transistor small-signal model for high frequencies At high-frequencies, the impedances of the parasitic capacitances related to the transistor junctions (see Fig. 1) become comparable to the values of the corresponding h - parameters of the transistor model, which was obtained for dc and relatively low frequencies. Thus to adjust the transistor model for high frequencies, we include in it the parasitic capacitances of the transistor emitter and collector junctions, C π and C μ , and also the ohmic resistance of the base, r b . We also rename some of the transistor small-signal parameters to allow the h ie parameter represent the input impedance of the new model: ϖ r r j i j v h b b be ie + = = 0 ) ( ) ( , (1) where e fe r h r ) 1 ( + = . (2) Unity-gain frequency (GBP) For a given transistor, the ohmic resistance r b of the base and the capacitance C of the collector junction can be found analytically. The capacitance C of the emitter junction is usually measured experimentally. The experimental setup is shown in Fig. 2. The aim is to measure the frequency ω t ("omega test"), at which the CE amplifier with the collector, which is short-circuited for small signals, has the current gain |A i ( j t )|=1. The unity-gain frequency t is then translated into C . Note from Fig. 2 that in the small-signal analysis, r b can be neglected because it is connected in series with the current source i s and, hence, does not affect the base current. The transistor output resistance, r o , can also be neglected because it is short-circuited. The capacitor C can be connected in parallel with C , provided that the current flowing through C is much smaller than the current generated by the dependent source: v E p+ n++ C p Eo p Co i C n i B V CE i B i R i E n Bo B r B B' C π j C d C C d Q d T be BE V v V Bo e n / ) ( + T BE V V Bo e n / v Q C j j d d v Q C d d d d v Q C j d d E B ' C B ' C C r B B C E B' i b h fe i m v r o v ce C B B' r B C i b h fe i m v r o v ce C v r B i B' r B C C (1 + h fe ) i i
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This note was uploaded on 01/14/2012 for the course EE 361-1-3711 taught by Professor Prof.eugenepaperno during the Fall '11 term at Ben-Gurion University.

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Lecture 12 - Lecture 12: Introduction to electronic analog...

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