# How to solve BJT- Small signal equivalent circuit?

Discussion in 'Homework Help' started by HeWasOnly3, Sep 2, 2010.

1. ### HeWasOnly3 Thread Starter New Member

Sep 2, 2010
12
0
Hi there gurus apparently I have encounter a question regarding drawing small signal equivalent circuit. I can't solve this question. Is it possible for you guys to show me to drawings and calculations as well? This is because normally the question that I did has load resistor connected to the BJT. However this BJT doen't have any load resistor connected. Help is appreciated thank you

File size:
185.8 KB
Views:
34
2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
The effective load is normally considered as Rload in parallel with RC - bearing in mind that hoe is normally part of the calculation but ignored in this example. Does that help your thinking if the effective load is therefore just RC?

3. ### HeWasOnly3 Thread Starter New Member

Sep 2, 2010
12
0
So all I need to assume that there is no Rload? Hence output impedance Zo= Rc Only?

4. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
Yep! That's all there is!

HeWasOnly3 likes this.
5. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
Why not draw up the h-parameter equivalent circuit including the relevant external components, RB & RC.

Then submit what you have worked out.

If you are not sure about the h-parameter model you might try Wikipedia.

HeWasOnly3 likes this.
6. ### HeWasOnly3 Thread Starter New Member

Sep 2, 2010
12
0
Here is the H-parameter equivalent circuit now how to get Av and Ai?

File size:
206.8 KB
Views:
25
7. ### HeWasOnly3 Thread Starter New Member

Sep 2, 2010
12
0
Here is my solution for Av I'm not sure whether it is correct. Now how do I solve Ai? Hmmm.....

File size:
211.8 KB
Views:
20
8. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783

The amplifier current gain is the output current divided by the total input current. The input current also includes the current flowing into RB which doesn't flow in the base - i.e. Ib. So Iin=Ib+IRB

HeWasOnly3 likes this.
9. ### HeWasOnly3 Thread Starter New Member

Sep 2, 2010
12
0
Hmm okay then what is the output current for this BJT?

10. ### HeWasOnly3 Thread Starter New Member

Sep 2, 2010
12
0
Okay I have found the output current I(output)=Ic=(hfe)(Ib) is it correct? If it is, then the last step is to find IRB. I have no idea how to find that ><

11. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
Have a look at your schematic in post #6

It should be reasonably clear that Iin=Vs/Zi

You have calculated Zi.

You know Ib=Vs/hie

And you have Iout=hfe*Ib

From there you can find Ai ....

HeWasOnly3 likes this.
12. ### HeWasOnly3 Thread Starter New Member

Sep 2, 2010
12
0
Many thanks dude! You are awesome! Finally i have solved it thank you so much guys