Question 3.SP.10: The Si transistor of Fig. 3-14 is biased for constant base c......
The Si transistor of Fig. 3-14 is biased for constant base current. If β = 80, V_{CEQ} = 8 \text{V}, R_C = 3 kΩ, and V_{CC} = 15 \text{V}, find (a) I_{CQ} and (b) the required value of R_B. (c) Find R_B if the transistor is a Ge device.
(a) By KVL around the collector-emitter circuit,
I_{CQ} = \frac{V_{CC} – V_{CEQ}}{R_c} = \frac{15 – 8}{3 × 10^3} = 2.333 \text{mA}
(b) If leakage current is neglected, (3.2) gives
β(≡ h_{FE}) ≡ \frac{α}{1 – α} ≡ \frac{I_C – I_{CEO}}{I_B} (3.2)
I_{BQ} = \frac{I_{CQ}}{β} = \frac{2.333 × 10^{-3}}{80} = 29.16 μ\text{A}
Since the transistor is a Si device, V_{BEQ} = 0.7 \text{V} and, by KVL around the outer loop,
R_B = \frac{V_{CC} – V_{CEQ}}{I_{BQ}} = \frac{15 – 0.7}{29.16 × 10^{-6}} = 490.4 kΩ
(c) The only difference here is that V_{BEQ} = 0.3 \text{V}; thus
R_B = \frac{15 – 0.3}{29.16 × 10^{-6}} = 504.1 kΩ