Voltage Regulators - Problem 4.a

anhnha

Joined Apr 19, 2012
905
Actually I had a big problem in understanding diodes before because we only used constant voltage drop model for diodes. So a lot of contradiction happened. For example in BE junction of transistor, what will happen if the biasing voltage across BE is large than the ideal voltage model.
 

Jony130

Joined Feb 17, 2009
5,487
But in hand calculation It does not matter. It is impossible to know the exact Vbe value. So the only thing we can do is assume some value.
 

anhnha

Joined Apr 19, 2012
905
But in hand calculation It does not matter. It is impossible to know the exact Vbe value. So the only thing we can do is assume some value.
I had many problems with it. With we use ideal model here when biasing voltage across BE is larger than forward voltage drop the current is infinite. That is wrong,
 

anhnha

Joined Apr 19, 2012
905
Can you show this kind of example?
With a voltage divider biasing for transistor, we usually assume that there is no base current. So the voltage across BE will be Vcc*R2/(R1 + R2). If this voltage is larger than 0.7V (say the voltage drop for ideal diode model) and so the current ib should be infinite.
However with method using Thevenin I have no problem.
 

Jony130

Joined Feb 17, 2009
5,487
With a voltage divider biasing for transistor, we usually assume that there is no base current. So the voltage across BE will be Vcc*R2/(R1 + R2). If this voltage is larger than 0.7V (say the voltage drop for ideal diode model) and so the current ib should be infinite.
Because without Re resistor you cannot use this method. And even if your circuit has Re resistor, this method still gives you only rough approximation of a Ic current. But sometimes even Thevenin is not a big help for us.
Try find Ic for this circuit? If β = 150.
1.PNG
 

anhnha

Joined Apr 19, 2012
905
Vth = 0.6V
Rth = R1||R2 = 0.9k
With this Thevenin voltage I think we need to base on Ib vs Vbe characteristic and using graphical method to solve for Ib.
After getting Ib, Ic = β*Ib.
 
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