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https://designers-guide.org/forum/YaBB.pl Design >> RF Design >> Bandwidth in Linear PA https://designers-guide.org/forum/YaBB.pl?num=1278414763 Message started by purplewolf on Jul 6th, 2010, 4:12am |
Title: Bandwidth in Linear PA Post by purplewolf on Jul 6th, 2010, 4:12am whats the relation between bandwidth of a single ended linear PA vs diffrential PA .. There is no specific LC tank in the diffrential PA like class B as compared in single ended design.. is diffrental offers twice the BW as compared to single to single ended????????i dont get it? |
Title: Re: Bandwidth in Linear PA Post by purplewolf on Jul 19th, 2010, 2:23am in the single ended design i have classical formulas for LC tank design with which i can adjust my bandwidth.. but in diffrential i have transformer only doing impedance transformation from source to load... how do i come to know whats the BANDWIDTH of my load network |
Title: Re: Bandwidth in Linear PA Post by pancho_hideboo on Jul 19th, 2010, 8:23am purplewolf wrote on Jul 19th, 2010, 2:23am:
purplewolf wrote on Jul 19th, 2010, 2:23am:
So it is same as "classical formulas for LC tank design", although I don't know what you mean by "classical formulas for LC tank design". Also see http://www.designers-guide.org/Forum/YaBB.pl?num=1240229676 |
Title: Re: Bandwidth in Linear PA Post by purplewolf on Jul 19th, 2010, 11:59am Q = R/wL R = 50 Q = f/BW wL = wc From TH Lee book. |
Title: Re: Bandwidth in Linear PA Post by pancho_hideboo on Jul 20th, 2010, 9:55am purplewolf wrote on Jul 19th, 2010, 11:59am:
In your formula, Q is proportional to R. It is very funny. |
Title: Re: Bandwidth in Linear PA Post by Ken Kundert on Jul 20th, 2010, 1:45pm In an LC tank when the resistance is in parallel with tank, the Q is proportional to the resistance. |
Title: Re: Bandwidth in Linear PA Post by purplewolf on Jul 20th, 2010, 5:15pm yups... Ken is absolutely correct |
Title: Re: Bandwidth in Linear PA Post by spring on Jul 20th, 2010, 6:13pm purplewolf wrote on Jul 6th, 2010, 4:12am:
Hi ,purplewolf Can you post you schematic? |
Title: Re: Bandwidth in Linear PA Post by RFICDUDE on Jul 20th, 2010, 7:30pm Your original question asked about single ended versus differential impedance as it applies to amplifier matching to some desired load (I assume this is what you meant?). The only difference between differential and single ended matching is that under the same conditions the differential parasitic capacitance of the amplifier may be reduced compared to the single ended amplifier. Why? because a differential amplifier looks like two single ended amps driven differentially. The size of the two devices is likely to be smaller than the single ended equivalent (although it does not have to be the case). The shunt capacitances of the two single ended amplifiers appear to be in series for the differential amplifier thus making the capacitance look smaller for the diff amp. There is no real advantage to this over the single ended amplifier. The resistive losses may be a little less too because the resistance of each half of the diff amp looks like they are in series for the differential output. So, it may be that the resistance across the tuned tank is less for the differential case and perhaps this is why some may consider the Q to be higher for the diff amp? The paradox seems to be that BW decreases as Q increases for a resonant circuit, so I too am perplexed by the assertion that BW is wider for a differential amp versus single ended. However, there can be a LC tank for class B PA. Most textbook differential class B circuits are pseudo differential with a center tapped transformer. The center tapped transformer represents an equivalent half circuit for the class B amp, so the circuit really looks like two single ended circuits driven differentially. The transformer usually offers some RF common mode reject such that there still is some difference between the differential and common mode gain of the amp. I'm not sure if this helps answer your brief question :-/ |
Title: Re: Bandwidth in Linear PA Post by pancho_hideboo on Jul 20th, 2010, 8:16pm purplewolf wrote on Jul 20th, 2010, 5:15pm:
It seems your Q is for parallel RLC-tank. Q=ω0*C/G=R/(ω0*L) But if you consider parallel LC-tank where R means loss of L, Q is never proportional to R. However your equation is still funny. purplewolf wrote on Jul 19th, 2010, 11:59am:
purplewolf wrote on Jul 6th, 2010, 4:12am:
Why can differential PA give twice BW as compared to single to single ended ? Ideally differential PA with no LC tank should show lowpass characteristics, unless there are DC-Block capacitors. If you use two Single to Single ended Class-B Amplifies with LC tank as differential PA, BW is same as one Single ended Class-B Amplifies whose frequency characteristics is bandpass. purplewolf wrote on Jul 19th, 2010, 2:23am:
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Title: Re: Bandwidth in Linear PA Post by purplewolf on Jul 21st, 2010, 2:15am for individual inductor the Q = wL/R, where R account for the losses of inducto.. but the loaded quality factor of tank is Q = f/BW =R/wL=50/wL wL =wC <=> 1/(ω0*L)=ω0*C <=> 1/(ω0*C)=ω0*L. .. makes no difference. Dont mix up the two Q's... I just assume that the bandwidth is either twice (or may be half) incase of diffrential as compared to single ended since it is combination of two single ended..I havent found any mathematical proof about its BW in literature .. Long story short ,I have absolutely no idea about it. |
Title: Re: Bandwidth in Linear PA Post by purplewolf on Jul 21st, 2010, 2:30am here are the two schematic |
Title: Re: Bandwidth in Linear PA Post by purplewolf on Jul 21st, 2010, 2:31am . |
Title: Re: Bandwidth in Linear PA Post by purplewolf on Jul 21st, 2010, 3:49am change the file extension to .tiff if theres problem in viewing |
Title: Re: Bandwidth in Linear PA Post by pancho_hideboo on Jul 21st, 2010, 7:25am purplewolf wrote on Jul 21st, 2010, 2:15am:
Consider a definition of resonator's Q. Resonator's Q is defined as ratio of reactive energy and loss energy at resonance for any resonator. Q of parallel LC-tank resonator is Q=ω0*L/R, here R means loss of L. This Q is not inductor's Q. Q of parallel LCR-tank resonator is Q=ω0*C/G, here G means parallel conductance of LC-tank. purplewolf wrote on Jul 21st, 2010, 2:15am:
Do you think "wL =wC" is surely correct ? I think your "wL =wC" makes clear difference compared to 1/(ω0*L)=ω0*C <=> 1/(ω0*C)=ω0*L. Your "diffrential.jpg" uses ideal transfomers which are not physical. So your "diffrential.jpg" should show lowpass characteristics, since there are no DC-Block capacitors and no resonators. But actual transfomer is mutual coupled inductors which forms double resonator, that is, coupled LC-tank. purplewolf wrote on Jul 21st, 2010, 2:15am:
Study characteristics or behaviors of double resonators or coupled resonators Generally BW will be narrower if you use double resonators. But if you permit two peak characteristic over passband, you can increase BW using double resonators. purplewolf wrote on Jul 21st, 2010, 2:15am:
Such things are decribed in any RF books. Sorry I don't have international books which are written in English. Read true classical-RF circuit text books which are oriented for discrete RF circuits not RF-IC's. Or search old application notes on discrete PA by Motorola(=Currently On-Semi). |
Title: Re: Bandwidth in Linear PA Post by Markaka Electronics on Jul 21st, 2010, 9:14am agree with ken |
Title: Re: Bandwidth in Linear PA Post by purplewolf on Jul 21st, 2010, 11:11am thnx pancho... |
Title: Re: Bandwidth in Linear PA Post by purplewolf on Jul 25th, 2010, 9:08am very good document... |
Title: Re: Bandwidth in Linear PA Post by pancho_hideboo on Jul 25th, 2010, 9:40am I don't have international books of this topics which are written in English. So I searched relative useful documents in English. See http://www.oersted.dtu.dk/upload/institutter/_oersted/uddannelse/emi_kurser/31415/Ch2Rfcirct05.pdf purplewolf wrote on Jul 25th, 2010, 9:08am:
I have more good text books in my native language. |
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