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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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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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).

- Spring '17