Bjt

[beenuseren]

Could anyone tell me about the re model of a transistor? How is the transistor represented with the re parameters? How to arrive to that model?

Help me please.

 

[hjames]

I don't remember this particular terminology, but google yielded this page:
**broken link removed**

The general idea is to linearize the B-E diode and use Hfe to calculate the resulting collector current. Obviously this is only for small-signals...

 

[beenuseren]

Before I understand the re model I have another doubt.
Let us consider ,man NPN transistor with CE configuration.
If we connect a load across the collector and the ground, does it change the collector to ground voltage?

I mean does the load current effect the collector current, Vce, and base current?

 

[hjames]

Before I understand the re model I have another doubt.
Let us consider ,man NPN transistor with CE configuration.
If we connect a load across the collector and the ground, does it change the collector to ground voltage?

I mean does the load current effect the collector current, Vce, and base current?

It depends on the load - is it resistive, inductive, capacitive, or some combination of the three? If you just change the output impedance attached to the collector, and keep the collector current the same, what do you think will happen to the voltage at the collector?

The other question you need to be asking is : "If I change the load impedance, do I need to re-bias the transistor?"

 

[beenuseren]

It depends on the load - is it resistive, inductive, capacitive, or some combination of the three? If you just change the output impedance attached to the collector, and keep the collector current the same, what do you think will happen to the voltage at the collector?

The other question you need to be asking is : "If I change the load impedance, do I need to re-bias the transistor?"

So does it mean in a properly biased transistor the biasing conditions donot change even when a resistive load is connected? i.e Vce and Ic remain the same?

Then how does the addition of the load current to the transistor circuit, does not bring any changes in the Vce? For a particular input how Vce remains the same value, even with the addition of load current to the transistor?

thanks

 

[hjames]

Arg, language issue. I was asking *you* what would happen if you changed the value of a resistor while holding the current constant. V=I*R, and obviously if R changes, V will change.

 

[beenuseren]

hjams, I am quite confused. Here is my doubt:

For a voltage divider biased circuit( CE config), the operating point doesnot change with the addition of load to the collector and ground. How is it possible? Addition of load should alter the collector current, Vce
and voltage drops across Rc, Re, theoretically.

What I think is , if we donot want to change the operating point, then the load impedence should be very high when compared with the source impedence.(Here the source impedence is the output impedence of the BJT).
Am I right?

If so, how will we know, what load value we can use, for a any biased BJT ?
Is there any calculation?

 

[beenuseren]

Also what does ro signifies in the small signal model ?

 

[hjames]

hjams, I am quite confused. Here is my doubt:

For a voltage divider biased circuit( CE config), the operating point doesnot change with the addition of load to the collector and ground. How is it possible? Addition of load should alter the collector current, Vce
and voltage drops across Rc, Re, theoretically.

What I think is , if we donot want to change the operating point, then the load impedence should be very high when compared with the source impedence.(Here the source impedence is the output impedence of the BJT).
Am I right?

If so, how will we know, what load value we can use, for a any biased BJT ?
Is there any calculation?

So the deal is that the output of a CE configuration acts like a current source. If you have a resisistive load connected to that current source, it will develop a voltage. If you change the resistance to something else, the output voltage will change since the current is fixed. Now since this is a real circuit, if the resistance gets large enough, the transistor will stop acting like a current source and start saturating(Vce approaches 0V) - at which point the Re transistor model fails and you need to go to another model. If you know how big the input signals are, you can use Hfe to calculate what the largest load resistance you can use.

ro stands for output resistance

 

[beenuseren]

So it does mean depending upon the transistor parameters, there is a limit for the maximum and minimum resistance we can use as load?

Why is sometimes the ro value is omitted in the calculations?