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https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> High IIp3 amp https://designers-guide.org/forum/YaBB.pl?num=1182847323 Message started by aaron_do on Jun 26th, 2007, 1:42am |
Title: High IIp3 amp Post by aaron_do on Jun 26th, 2007, 1:42am Hi all, I recently read a paper where the baseband amp used unrestricted biasing to get high IIP3: i.e. no tail current source. The improvement in IIP3 is tremendous. The only problems i could see with the design were: 1) the circuit had no immunity to process variation 2) due to variation its possible the bias point would change and therefore the biasing of the subsequent stage would be affected. These problems can be worked around though. So is there a really good reason why we would want to add a tail current given the huge loss in IIP3? thanks, Aaron |
Title: Re: High IIp3 amp Post by aaron_do on Jun 26th, 2007, 3:18am just realised you need it for IIP2...... |
Title: Re: High IIp3 amp Post by RFICDUDE on Jun 26th, 2007, 8:36pm What was the circuit topology (citation)? A pair of common source amps isn't that much better for IP3 than a differential pair; however, there are transconductor circuits which are extremely linear over a wide swing range but the noise performance isn't very good. As you noted, IP2 is an issue and common mode rejection is too if you are worried about substrate coupling. Thanks |
Title: Re: High IIp3 amp Post by aaron_do on Jun 27th, 2007, 1:32am I just double checked...you're right. Actually i made a mistake in the circuit i was simulating. The IIP3 was higher than i expected but not much. thanks |
Title: Re: High IIp3 amp Post by chase.ng on Jun 27th, 2007, 1:41am Isn't it the bipolar differential pair will normally have better linearity compared to common emitter configuration? I will think that the same applies to FET circuit also. chase |
Title: Re: High IIp3 amp Post by aaron_do on Jun 27th, 2007, 4:16am well i was talking about IIP3. The reason is because a MOSFET is a square law device so in theory, there is no third order non-linearity. If you use a tail current though, the limiting action of the tail current causes 3rd order non-linearity. I'm not that familiar with bipolar... cheers, Aaron |
Title: Re: High IIp3 amp Post by Visjnoe on Jun 28th, 2007, 1:52am Dear Aaron, here are some other reasons why you don't want to omit the tail current source: 1) PSRR becomes (much) worse 2) CMRR becomes (much) worse If you want to linearize a CMOS differential pair, you could also use resistors (in the source region). Regards Peter |
Title: Re: High IIp3 amp Post by aaron_do on Jun 28th, 2007, 6:09am thanks. I find that using unrestricted biasing does wonders for my IIP3 though...I figured that I could follow the amp with a buffer with a tail current to improve the CMRR. Not sure about PSRR though...thanks for your input Aaron |
Title: Re: High IIp3 amp Post by Visjnoe on Jul 2nd, 2007, 11:34am Dear Aaron, placing a buffer with a tail current source after your amplifier (without current source) will not improve your PSRR/CMRR performance, since once the 'disturbance' is added to your nominal/ideal signal, it will proceed through the system (through your buffer etc.) so you can not get rid of it this way. I have seen this kind of things before (e.g.omitting current sources in CML logic) and it's true, there are some benefits, but PSRR/CMRR are very important when you integrate your macro in a SoC. Succes! Regards Peter |
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