Question

Consider the circuit arrangnent shown in Fig. for studying input and output characteristics of npn transistor in CE configuration.
Select the values of `R_` and `R_` for a transistor whose `V_(BE)=0.7V`, so that the transistor is operating at point Q as shown in the characteristics shown in Fig.
Given that the input impedance of the transistor is very small and `V_(C C)=V_(BB)=16V`, also find the voltage gain and power gain of circuit making appropriate assumptions.

Answer 1

Given, `V_(BE) = 0.7 V, V_(C C) = V_(BB) = 16 V`
`V_(CE) = 8V`
`I_(C) = 4 mA = 4 xx 10^(-3) A`
`I_(B) = 30 mu A = 30 xx 10^(-6) A`
For the ouput characteristic at `theta`,
`V_(C C) = I_(C) R_(C) + V_(CE)`
`R_(C) = (V_(C C) - V_(CE))/(I_(C)) = (16 - 8)/(4 xx 10^(-3)) = (8 xx 1000)/(4) = 2 k Omega`
Using the relation, `V_(BB) = I_(B) R_(B) + V_(BE)`
`R_(B) = (V_(BB) - V_(BE))/(I_(B)) = (16 - 0.7)/(30 xx 10^(-6))`
`= 150 xx 10^(3) Omega = 510 k Omega`
`beta = (I_(C))/(I_(B)) = (4 xx 10^(-3))/(30 xx 10^(-6)) = 133`
Voltage gain `= beta (R_(C))/(R_(B)) = (133 xx 2 xx 10^(3))/(510 xx 10^(3)) = 0.52`
Power gain `beta xx` Voltage gain `= 133 xx 0.52 = 69`