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Unformatted text preview: EE 323 Winter 2009 Analysis of Transistor Amplifiers January 29, 2009 When transistors are used for amplifica- tion, we take advantage of the characteris- tic operating mode in which one terminal current is primarily controlled by the volt- age between the other two terminals. For BJT amplifiers, we use the fact that in active mode, the collector current is an exponential function of the voltage between the base and emitter. i C I S exp parenleftbigg v BE kT/q parenrightbigg (npn) i C I S exp parenleftbigg v EB kT/q parenrightbigg (pnp) For MOSFET amplifiers, we use the analo- gous pinch off mode in which the drain cur- rent is a quadratic function of the gate-source voltage. i D K ( v GS V t ) 2 (NMOS & PMOS) In either case, when a small time-varying control voltage is applied along with a dc off- set (called a bias point or operating point), the exponential or quadratic control equation can be linearized for treating the time vary- ing component. For the exponential BJT control, the linearization follows the form exp x 1 + x (for | x | 1) and for the quadratic MOSFET control, the linearization follows the binomial approxi- mation (1 + ax ) k 1 + kx (for | x | 1) Although the response to a small time varying control signal can be linearized, in general the dc biasing must follow the rules for active (BJTs) or pinchoff (MOSFETs) modes. To analyze transistor amplifier cir- cuits, the traditional method is to separate the ac and dc analysis, using different models to replace the transistor in the two analysis steps....
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