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Question
Consider the circuit arrangement shown in figure (a) for studying input and output characteristics of npn transistor in CE configuration.
 (a) |
 (b) |
Select the values of RB and RC for a transistor whose VBE = 0.7 V, so that the transistor is operating at point Q as shown in the characteristics shown in figure (b). Given that the input impedance of the transistor is very small and VCC = VBB = 16 V, also find the voltage gain and power gain of the circuit making appropriate assumptions.
Solution
According to the problem, at point Q, from graph VBE = 0.7 V, VCC = VBB = 16 V and VCE = 8 V
IC = 4 mA = 4 × 10–3 A
IB = 30 μA = 30 × 10–6 A
Since, VCC = ICRC + VCE
`R_C = (V_(CC) - V_(CE))/I_C`
= `(16 - 8)/(4 xx 10^-3)`
= `(8 xx 1000)/4`
= 2 kΩ
Similarly, VBB = IBRB + VBE
`R_B = (V_(BB) - V_(BE))/I_B`
= `(16 - 0.7)/(30 xx 10^-6)`
= `510 xx 10^3` Ω
= 510 kΩ
Current gain, `β = I_C/I_B = (4 xx 10^-3)/(30 xx 10^-6)` = 133.3
Voltage gain = `β (R_C)/R_B = (133 xx 2 xx 10^3)/(510 xx 10^3)` = 0.52
Power gain = β × Voltage gain = 133 × 0.52 = 69
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