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Tuned Bandpass Filter Homework

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Jul 12, 2011
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Hi everyone, this is my first post.

I have a some problem encountered during my homework. I have no idea where to post it so if its not in the appropriate sub-forum please help me move it mods =)

Our lecturer gaved us a semester homework. The circuit looks like below

He asked us to calculate the values. Those labeled components cannot be changed.

What really confused me is the inductor in the source channel. Both the resistor R1 & R2 looks like a voltage divider. C2 is a bypass cap. The FET looks more to me like a variable resistor. So where should i start?

The requirements are:
1. gain must be more than 20dB
2. Q factor must be above 20 and as high as possible.
3. centre frequency 680kHz from a range of 500k to 1.2M
4. input is a sine wave.

Thanks for any reply in advance! If possible please let me know the guidelines on how you calculate or is there any theory involved! Thanks!

You mean tuned bandpass, not tunable? That's a kind of pleonasm, I think. I know tuned amplifiers, but a bandpass will be always tuned.

To calculate gains, one would need the FET parameters, e.g. Vgth and Idss according to an ideal FET characteristic.

of course, nobody but you should do the main work - however, perhaps some remarks are helpful:

* To solve the task, you must have some basic knowledge on FET amplifiers (operating point, gain calculation)
* You also should know something on tuned circuits (resonant effect, bandwidth, center frequency, Q factor)
* The inductor is not in the source path but a part of the "working" load impedance that is connected to the drain node.
* Question: Can you recognize that (and why) - in principle - the circuit acts as a bandpass?
* Do you know which part determines (limits) the maximum gain?
My recommendation: Before starting to solve the task, try to answer these points.

Good luck

---------- Post added at 22:14 ---------- Previous post was at 21:31 ----------

One additional point: You should try to find out, which reactive parts mainly determine the bandpass characteristics (and which are less important - and why).
Thanks for your reply guys.

I will try to sort out the answers and the small little things a.s.a.p

I will come back here if there are still any problems. :p


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