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Solved Examples · Example 4

Q.Find the Q point for the circuit of figure 2.1.11 considering transistor to be of silicon.

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Figure 2.1.11
Figure 2.1.11

[!TLDR]

The operating point is Q(10.8 V, 0.39 mA)Q(10.8\ \text{V},\ 0.39\ \text{mA}).

The Q point (quiescent/operating point) is the zero-signal point on the load line, with coordinates Q(VCEQ,ICQ)Q(V_{CEQ}, I_{CQ}). For voltage divider bias it is computed from the potential-divider output voltage across R2R_2.

Divider voltage:

V2=VCCR2R1+R2=12×(1×103)11×103=1211=1.09 VV_2 = \frac{V_{CC} R_2}{R_1 + R_2} = \frac{12\times(1\times10^{3})}{11\times10^{3}} = \frac{12}{11} = 1.09\ \text{V}

Quiescent collector current (with IE≈ICI_E \approx I_C and silicon VBE=0.7 VV_{BE} = 0.7\ \text{V}):

ICQ=V2−VBERE=1.09−0.71×103=0.39 mAI_{CQ} = \frac{V_2 - V_{BE}}{R_E} = \frac{1.09 - 0.7}{1\times10^{3}} = 0.39\ \text{mA}

Quiescent collector-to-emitter voltage:

VCEQ=VCC−ICQ(RC+RE)=12−(0.39×10−3)(2×103+1×103)=12−1.17=10.8 VV_{CEQ} = V_{CC} - I_{CQ}(R_C + R_E) = 12 - (0.39\times10^{-3})(2\times10^{3} + 1\times10^{3}) = 12 - 1.17 = 10.8\ \text{V} …

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