- #1
Pushoam
- 962
- 51
Homework Statement
Homework Equations
The Attempt at a Solution
The equivalent circuit is drawn below.
It is assumed that the potential drop across the pn junction in forward bias is 0 V(I can't take it 0.7 V as the input voltage is 0.5 V). In this case the p-n junction is equivalent to a connecting wire-piece.
Since ##\alpha ## is nearly equal to 1, ##I_B# is taken to be 0.
So, the current flowing through ##R_{in}## is ##I_E = \frac{500~mV}{20~\Omega} = 25 ~mA##
In fact, the above figure is wrong.Right?
It is the p-n junction in forward bias which should get replaced by Rin and np junction in reverse bias which should get replaced by Rout.
Then, the potential drop across the diode in forward bias is equal to the potential drop across the Rin i.e. 500 mV and the potential drop across the reverse bias is the potential drop across the Rout.
Potential drop across Rout = ## I_C R_{out} = 2500 V ##........(1)
Using Kirchoff's Voltage loop law, this voltage drop should be equal to the voltage drop across## R_C##.
But, due to Ohm's law, voltage drop across ##R_C = I_C R_C = 25V##......(2)
So, there is a contradiction.
In (1), I have considered the n-p junction as a linear element. I think this is wrong. The n-p junction could not be replaced by Rout. Then , in this case, there is no need to know Rout. Right?
So, does it mean that , in general, in transistor, the forward bias p-n junction could be taken as a linear element, but the reverse bias p-n junction has to be taken as a non-linear element?
The voltage amplification is : ## \frac{25V}{0.5V}=50##