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## 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. ## 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 .