General DC Procedure The procedure for solving BJT circuits at DC is very

General dc procedure the procedure for solving bjt

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General DC Procedure The procedure for solving BJT circuits at DC is very similar to that of the MOSFET. As we saw in the previous (simple) example, the general steps are to: 1. Assume an operating mode, using any known information if possible. 2. Analyze the circuit given this assumption. (3). Check conditions to verify that assumptions were correct. 4. Iterate solution if necessary. Notice that we have added a fourth step iteration. Often, it will be difficult to directly solve for the final currents in a BJT circuit. We will typically make simplifying assumptions and iterate our solution once or twice to get a final result. In such cases, we skip checking conditions (Step 3) until the final iteration only. The best way to learn this process is to jump right into some examples. The following are a few simple examples of BJT DC analysis.
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229 BJT DC Example 1 For the following circuit, find all transistor voltages and currents, and determine / verify the operating mode. Assume that for the B E junction is 0.7V, , and . Solution TO begin, we assume an operating mode, as per the solution procedure. In the absence of other information, it is a good idea (like with the MOSFET) to assume active mode much like we always assume saturation for the MOSFET. Given this mode, we know that we have (equivalently) a forward-biased diode from base to emitter with : (1) With this information, we can easily calculate the emitter current given the known resistor, : (2)
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230 We can now easily determine base and collector current using the determined equations for active mode of the BJT: (3) Knowing all of the currents, we can lastly determine the unknown collector voltage, and then verify the operating mode: (4) Since the determined value for is greater than the minimum level of , then the transistor is indeed operating in active mode. If we had found , we would switch the transistor mode and repeat the problem. Also, no iteration was needed in this example because no simplifying assumptions were made. It is very important to note that because we simplified the model for saturation mode for the BJT, there is the potential that, for a BJT operating very near the boundary of saturation and active mode, you will find contradictory results. In particular, results may indicate that the BJT meets neither the criteria for active nor saturation mode, but is still clearly ‘ON’ ( ). In such cases (in the real world), we would need to use our more advanced model to resolve this contradiction. For this course, we will never give a problem with this issue on a test or exam; if you encounter it, then something went wrong elsewhere in your calculations.
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231 BJT DC Example 2 For the following circuit, you are given . Determine the DC bias point (find all transistor currents and voltages, and confirm the mode of operation).
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