Please help me with this amplifier circuit.

[Titanium]

Could you please help me through this problem:

In figure 3a) there is shown an amplifier circuit based on a npn-transistor and figure 3b) illustrates its small-signal model.

(a) Explain what is the small-signal model and what is it used for. What is
descriped by the symbols (r_π, C_π, etc.) in figure 3b)? Is there anything
missing?

(b) Determine the gain of the system A = Vo/Vs at low frequencies (C_C1
and C_C2 more significant than the other capacitances) assuming RB》r_π. Explain qualitatively what happens at very high frequencies and at
middle frequencies.

Figure：http://i645.photobucket.com/albums/uu173/lxazy/ji.jpg?t=1257527280

Thanks a lot for looking and helping!

[Titanium]

Please help!
Thank you!

[bertus]

Hello,

The link does not work.

You can post the picture here by using the manage attachment button:

Attachments and Images How do I attach a file to a post?
To attach a file to your post, you need to be using the main 'New Post' or 'New Thread' page and not 'Quick Reply'. To use the main 'New Post' page, click the 'Post Reply' button in the relevant thread.
On this page, below the message box, you will find a button labelled 'Manage Attachments'. Clicking this button will open a new window for uploading attachments. You can upload an attachment either from your computer or from another URL by using the appropriate box on this page. Alternatively you can click the Attachment Icon to open this page.
To upload a file from your computer, click the 'Browse' button and locate the file. To upload a file from another URL, enter the full URL for the file in the second box on this page. Once you have completed one of the boxes, click 'Upload'.
Once the upload is completed the file name will appear below the input boxes in this window. You can then close the window to return to the new post screen.
What files types can I use? How large can attachments be?
In the attachment window you will find a list of the allowed file types and their maximum sizes. Files that are larger than these sizes will be rejected. There may also be an overall quota limit to the number of attachments you can post to the board.
How do I add an image to a post?
If you have uploaded an image as an attachment, you can click the arrow next to the 'Attachment Icon' and select it from the list. This will be inserted into your post and can be located where you want it displayed.
To include an image that is not uploaded as an attachment and is located on another website, you can do so by copying the full URL to the image, (not the page on which the image is located), and either pressing the 'Insert Image' icon or by typing [img] before the URL and [/img] after it, ensuring that you do not have any spaces before or after the URL of the image. You can insert pictures from your albums (?) in this way too.

Greetings,
Bertus

[Titanium]

Thank you for your attention!
Here is the figure:

Originally Posted by bertus

Hello,

The link does not work.

You can post the picture here by using the manage attachment button:

Greetings,
Bertus

[Titanium]

Please help, indeed!

[bertus]

Hello,

I am not familair with this kind of network representation of the amplifier.
I have made notes as I see the parts in the network.



Greetings,
Bertus

[Titanium]

I see, thanks anyway!

[hobbyist]

The small signal model, represents the signal (AC) voltages work at, it does not consider the DC bias voltages, but only what refers to AC currents only.

The resistances have been thevenized and such, if you look into the input this will be what the input signal see's, it sees R1 and R2 as a single resistance, in parrallell with the series resistor RE and re, denoted as R pi.

The output current B*ib see's Rc and Rl in parrallel, depending on the frequency of this input signal, would determine the phase shift the signals see with the capacitors, as well as the reactances these capacitors offer to the signals, if the signal frequency is high enough, then sometimes the capacitors can be represented as shorts across them as the signal is not Aaffected by them, but that again deals with the frequencies involved.

This setup is useful when doing signal analysis on an amp stage.

[hobbyist]

Titanium:
Please ask questions on the thread, instead of PM.
Because the forum is for all to benefit from in learning by questions asked.

Also If I give a wrong answer to a question someone more knowledgebale would have an opurtunity to interject and correct any wrong advise I may give.

With that being said:

Everything I wrote in my last post is about all I really understand about this circuit analysis.

Here is the question asked
If someone can help with his question it would be greatly appreciated.
Thankyou.


[QUOTE=Titanium]Thanks a lot for your explanations, but regarding my question, could you please show me the correct solution?
Thank you in advance!