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 R_B and R_C for a transistor whose V_BE = 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 V_CC = V_BB = 16 V, also find the voltage gain and power gain of the circuit making appropriate assumptions.

Answer 1

According to the problem, at point Q, from graph V_BE = 0.7 V, V_CC = V_BB = 16 V and V_CE = 8 V

I_C = 4 mA = 4 × 10^–3 A

I_B = 30 μA = 30 × 10^–6 A

Since, V_CC = I_CR_C + V_CE

`R_C = (V_(CC) - V_(CE))/I_C`

= `(16 - 8)/(4 xx 10^-3)`

= `(8 xx 1000)/4`

= 2 kΩ

Similarly, V_BB = I_BR_B + V_BE 

`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