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# Giving my circuit symetrical drive??

Discussion in 'Homework Help' started by Denny1234, Oct 29, 2008.

1. ### Denny1234 Thread Starter Member

Feb 17, 2008
27
0
Hi I've done a really easy inverter design which i have attached (sorry about the quality). However after making it my tutor has told me I have to give it symetrical drive?

I've read about it somewhere because I know it means giving the same delay for pull up or pull down but I dont have a clue how to implement that practically, what would have i have to do to my design to give it symetrical drive?? I've looked on google and my textbooks and there is surprisingly very little about it.

Thanks for any help

• ###### Inverter.bmp
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Last edited: Oct 29, 2008
2. ### Ron H AAC Fanatic!

Apr 14, 2005
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681
Here is a hint: P-channel carrier mobility is about 1/3 that of n-channel.

3. ### scubasteve_911 AAC Fanatic!

Dec 27, 2007
1,202
1
To expand on Ron's comments, the key to creating symmetrical gates is understanding the proportion of mobility.

Typically, mobility is ~25uA/V^2 for NMOS and 10uA/V^2 for PMOS. So, the hole mobility of PMOS is about 40% of the electron mobility of NMOS.

You can relate this to the W/L ratio of the transistor. You would size either the NMOS or PMOS transistor for a rated output current, which is inversely proportional to Ft. Anyways, if you chose a W/L of 10/1 for NMOS, then you would need to use a 25/1 W/L ratio for PMOS to make it symmetrical. For other arrangements, it gets more complex.

Steve

Last edited: Oct 30, 2008
4. ### Denny1234 Thread Starter Member

Feb 17, 2008
27
0
Thanks for that, we were given the mobility of the pmos and nmos on my worksheet and I should have realised that they would have been a reason for it!

Cheers.