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Design >> Analog Design >> CMOS buffer
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Message started by aaron_do on May 10^th, 2007, 12:58am

Title: CMOS buffer
Post by aaron_do on May 10^th, 2007, 12:58am

hi all,

i'm trying to design a buffer two convert a differential signal to single ended. At the same time, the buffer must contribute low noise and be able to drive a 50 ohm load while providing 0 dB gain from diff to single.

Anybody got any suggestions?

thanks,
Aaron

Title: Re: CMOS buffer
Post by rajeee1000 on May 10^th, 2007, 11:27pm

Hello Aaron,

I feel it can be done something like this:

A simple diff par with 50 Ohm resistive loads with just one of it's output tapped would proviode 50Ohm output impedance. A 25 Ohm equivalent source degenration to get 0 dB diff to single-ended gain (The gm of the input transistors must be sufficiently large). The source degeneration also helps in maintaining the 50 Ohm output impedance.

Regards
Rajesh

Title: Re: CMOS buffer
Post by aaron_do on May 13^th, 2007, 10:26pm

thanks Rajesh,

I guess that would sort of work. I really want to aim for exactly 0 dB gain. Also there's a problem with that design because the tail current noise would not get cancelled since the plus and minus don't cancel.

What I'm using is a kind of source follower. The bottom NMOS is connected to V+ and the top is connected to V-. Although the gain is not exactly 0 dB, it is quite exact so that's good enough. The common mode signals are also cancelled. Unfortunately, the gm's have to be very close to get good cancellation of common mode. Plus you need huge current to drive the 50 ohm loads. Its just for testing so i'm not too worried about the current consumption.

thanks,
Aaron