# MOSFET amplifier design, any help appreciated

Discussion in 'Homework Help' started by Renton, Apr 22, 2008.

1. ### Renton Thread Starter Member

Apr 21, 2008
12
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Hi, everybody! I have an engineering project to finish that requires me to design a MOSFET based amplifier using given specifications. Here is the project:

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Design a single or multistage MOSFET based amplifier:

Voltage gain of at least 200. Normal or inverted output acceptable.

Small signal input resistance greater than 1 megaohm

Small signal output resistance less than 200 ohms

Allow output signal of 8 volts peak-to-peak into a load resistor of 1000 ohms

Use no more than 5 active (transistor) devices

Use standard 5% valued resistors of no more than 100K in value

Given transistor parameters:
threshold voltage = 2 V; device transconductance, K = 1 mA per volt
squared; channel modulation parameter (lambda) = 0.02 per volt.
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I tried to first figure out how to get the output resistance, and I think it has something to do with the resistors parallel to the transistor. I know what a standard amplifier with one or two capacitors, 3 or so resistors, and one transistor looks like, but I don't know how to manipulate it to fit these specifications.

I'm really sorry for not giving more of my attempts to solve this, but I'm just so stuck. If anyone can give me any clue on how to begin, it would be greatly appreciated. Thanks!

2. ### Renton Thread Starter Member

Apr 21, 2008
12
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Is it possible to achieve the specifications I listed using the common emmitter amplifier design? Thanks for any help!

3. ### Caveman Active Member

Apr 15, 2008
471
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I'm sorry that I don't have time to look into it further, but here are some hints. And I'll try to give you reasons so you can understand better why.

You will likely need a multiple stage amplifier. This is for two reasons. First of all it will be hard to get the gain you want with a single stage with FETs. Secondly, you will probably use a common emitter to get that gain, but it will be difficult to get the output impedance low with a common emitter stage, so you probably want to add a stage that is better for that. Typically, you would use a common collector (also called a source follower).

4. ### Renton Thread Starter Member

Apr 21, 2008
12
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Thank you so much for the help, Caveman! I didn't have a good idea where to start, so you have REALLY helped me out! I will try to move forward on the project now, and maybe I can post my progress at a later time. Thanks again for taking the time to help!

5. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
You'll have better luck searching "common source" instead of "common emitter." This may seem like nitpicking, but this small difference in search terms will yield big differences in search results.

6. ### Renton Thread Starter Member

Apr 21, 2008
12
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Thanks, thingmaker3! Any comments are appreciated!

7. ### Renton Thread Starter Member

Apr 21, 2008
12
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Hi, everybody. I just had a question about PSPICE. For this project, I think I want to use the MbreakN3 as my transistor, but what is the difference between MbreakN3 and MbreakN3D? Thanks for any help.

Apr 25, 2008
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9. ### Renton Thread Starter Member

Apr 21, 2008
12
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Thanks for the link, Digi Dave! But is there any place on that site that helps with PSPICE parameters of the MbreakN mosfet?