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Unformatted text preview: ECE 323 Winter 2009 BJT Approximations for DC Bias Calculations February 16, 2009 When finding dc voltages and currents in circuits that contain bipolar junction transis- tors (BJTs) it is often convenient to approxi- mate the transistor as a collection of simpler linear devices much like we have done with diode circuits. To use the simpler models for a BJT, we have to first guess the oper- ating mode for the transistor. We then re- place the transistor in the circuit with the model that corresponds to the assumed op- erating mode and analyze the circuit to ver- ify that the chosen mode is correct. If the conditions that describe the assumed mode are not consistent with the terminal voltages and currents found with the simpler model in the circuit, then another mode must be selected and tested. The actual mode of op- eration is found in this manner by process of elimination. It is often helpful to know the application for the transistor in the circuit Table 1: Conditions for operating modes of an npn transistor. Mode Conditions Application cut off V BE < . 7 V logic gates V BC < . 7 V active I B > amplifiers I E > I C > V CE > . 2 V saturated I B > logic gates I E > I C > I C < βI B when guessing the operating mode. For ex- ample, transistors in linear amplifier circuits must be in active mode. Transistors used in V BEQ 0.7 V I BQ I CQ I EQ V CEQ I BQ β + _ ACTIVE npn base emitter collector Figure 1: Approximate models of npn transistors for calculating dc currents and...
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