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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.

3.17

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 .