rare to need more than two iterations including the one driven by the initial

# Rare to need more than two iterations including the

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rare to need more than two iterations (including the one driven by the initial guess) when the values of are large. The exceptions to this rule are (of course) low transistors, and configurations where the value of is large, regardless of . Overall, one can see that this is a very simple method we did not even need to solve a system of equations. One-Shot Solution This circuit can also (relatively) easily be solved using a single-stage solution. This process becomes exponentially more difficult as the circuit complexity increases. We would begin our process in the same manner as the previous solution, by writing a node equation for , and re-arranging to yield the same update equation as before: (1)

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234 From here, we need to either replace with a term using , or vice versa. Typically, it is easier to replace the current, as it often has alternate equations due to resistors elsewhere in the circuit (typically emitter resistors or collector resistors). In this case, we trace the path of through both transistors to the resistor before finding an alternate equation. Working backwards, we see a similar path as in the iterative solution: (2) Substituting and solving yields a direct solution for : (3) Confirming BJT operating modes: (4)
235 The answer is identical to what we obtained after iterating. In general, both methods entail roughly the same set of steps, though the iterative method typically makes these sequential so that we do not need to solve the circuit all at once. Many people find it easier to work through in this way, but the choice is yours. This ends the basic examples on BJT DC calculations. More examples are available in the solutions to the past exams. I will also provide one additional extremely difficult (challenge) problem; as before, if you can solve this, you are very prepared for this section. Our last work for the course will cover the BJT small signal model. As we will see, it is very similar to the MOSFET model. The only addition will be a resistor from base (formerly gate) to emitter (formerly source). Otherwise, everything will be basically identical. This means that all of the small-signal circuit knowledge you have from MOSFET circuits will still apply. Source: LAST YEAR’S EXAM In-Class Break Exam Art

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236 This set of notes provides an extra practice problem for BJTs at DC. Current Topics: More DC BJT Examples Next Topics: SS Model, SS Analysis The following example is significantly more complex than you would find on a test or exam. If you are comfortable with this example, then you should have no problem with anything in an actual test situation. BJT DC Example 3 The following is the output stage of a large-signal BJT amplifier. It is actually part of a 20W audio amplifier which you can easily build yourself, if interested. The only other major component is an op- amp and some resistors.
• Spring '17

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