Chapter 3
Q. 3.4
As you saw earlier, a 2N3904 transistor has a β_{DC} range from 100 to 300. Assume a 2N3904 is used in the collector-feedback biased circuit shown in Figure 3–17 . Compute the minimum and maximum collector current based on this specification.
Step-by-Step
Verified Solution
Substitute the values given for β_{DC} = 100 into Equation (3–4).
I_{C} = \frac { {V_{CC} – V_{BE} } }{R_{C} + R_{B}/\beta _{DC} } (3–4)
I_{C(min)} = \frac { {V_{CC} – V_{BE} } }{R_{C} + R_{B}/\beta _{DC} } = \frac {12 V – 0.7 V }{2.0 kΩ + 150 kΩ/300} = 3.2 mA
Repeat the calculation for β_{DC} = 300.
I_{C(max)} = \frac { {V_{CC} – V_{BE} } }{R_{C} + R_{B}/\beta _{DC} } = \frac{12 V – 0.7 V}{2.0 k\Omega + 150 k\Omega /300} = 4.5 mA
Note that a 300% change in β_{DC} resulted in only a 40% change in collector current for this case, which is a considerable improvement over the base-bias case in Example 3–3 .