N_BJT_N12 - Bipolar Junction Transistors(BJTs(12 1 BIPOLAR...

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1 Bipolar Junction Transistors (BJTs) (12)
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2 BIPOLAR JUNCTION TRANSISTORS (BJT’s) § 5.2.3 Dependence of i C on the Collector Voltage -- The Early Effect …
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3 BIPOLAR JUNCTION TRANSISTORS (BJT’s) § 5.4 Analysis of Transistor Circuits at DC (Page 421). We are now ready to consider the analysis of BJT circuits to which only dc voltages are applied. In the following examples we will use the simple model in which, I V BE ] of a conducting transistor is 0.7 V and I V CE I of a saturated transistor is 0.2 V, and we will neglect the Early effect. Better models can, of course, be used to obtain more accurate results. This, however, is usually achieved at the expense of speed of analysis, and more importantly, it could impede the circuit designer's ability to gain insight regarding circuit behavior. Accurate results using elaborate models can be obtained using circuit simulation with SPICE, as we shall see in Section 5.11. This is almost always done in the final stages of a design and certainly before circuit fabrication. Computer simulation, however, is not a substitute for quick pencil-and-paper circuit analysis, an essential ability that aspiring circuit designers must muster. The following series of examples is a step in that direction. As will be seen, in analyzing a circuit the first question that one must answer is: In which mode is the transistor operating? In some cases, the answer will be obvious. In many cases, however, it will not. Needless to say, as the reader gains practice and experience in transistor circuit analysis and design, the answer will be obvious in a much larger proportion of problems. The answer, however, can always be determined by utilizing the following procedure:
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4 BIPOLAR JUNCTION TRANSISTORS (BJT’s) § 5.4 Analysis of Transistor Circuits at DC (Page 421) …. Assume that the transistor is operating in the active mode, and proceed to determine the various voltages and currents that correspond. Then check for consistency of the results with the assumption of active-mode operation; that is, is v CB of an npn transistor greater than -0.4 V (or VCB of a pnp transistor lower than 0.4 V)? If the answer is yes, then our task is complete. If the answer is no, assume saturation-mode operation, and proceed to determine currents and voltages and then to check for consistency of the results with the assumption of saturation mode operation. Here the test is usually to compute the ratio I C ,/I B and to verify that it is lower than the transistor β; i.e., β force < β. since β for a given transistor varies over a wide range, one should use the lowest specified β for this test. Finally, note that the order of these two assumptions can be reversed.
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This note was uploaded on 01/29/2011 for the course EE 203 taught by Professor Gp.(r)muzaffarali during the Fall '10 term at College of E&ME, NUST.

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N_BJT_N12 - Bipolar Junction Transistors(BJTs(12 1 BIPOLAR...

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