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https://designers-guide.org/forum/YaBB.pl Design >> RF Design >> PA Linearity Question https://designers-guide.org/forum/YaBB.pl?num=1145563756 Message started by jxbvt on Apr 20th, 2006, 1:09pm |
Title: PA Linearity Question Post by jxbvt on Apr 20th, 2006, 1:09pm hi....my understanding is that in Pin-Pout curve when the output power saturates (starts to level off) it is mainly because of output waveform clipping. so, as you increase Pin, Pout can't increase because the swing can't go any bigger. in other words, the gain starts to compress. gain is vo/vi but vo is limited and can't increase but vi is increasing so compression occurs. now, here is my question: how is this related to linearity? meaning, how is this related to harmonics, etc. because I don't see a realationship. why people say that the output power is saturating because of nonlinearities---to be it is because of physical limitations of transistor (clipping) and not harmonics, etc. usually (in HBT) they say that linearity is mainly in gm and Ic but as i have been saying above i don't see gm and Ic having higher order power (harmonics) being the reason of output power saturation. any feedback?? |
Title: Re: PA Linearity Question Post by aaron_do on Apr 21st, 2006, 1:24am I'm actually not that familiar with PA design, but anyway...... Harmonics arise as a result of the gain compression, but do not actually cause gain compression in the desired band unless they pass through a second nonlinear stage (then you get intermodulation). For example if you input a sine wave and get out a square wave then you now have the fundamental frequency plus a bunch of harmonics. But the gain compression in the desired band is not due to the harmonics, they are a result of the nonlinearity. As for the nonlinearity, the nonlinearity is basically a result of the gain compression due to the limited supply voltage. It isn't really a result of the transistor's intrinsic gm (where gm is taken assuming an unlimited supply voltage or zero output resistance). For example a MOSFET is ideally only second order nonlinear so there shouldn't be any gain compression in the fundamental frequency. But the gm of the transistor will change according to the limited supply voltage. If you have an output y=a1x(t) + a2x^2(t) - a3x^3(t) and you input only a fundamental x(t)=A1cos(wt) then you will get y(t) = a1A1cos(wt)-3/4a3A1^2cos(wt) at the fundamental plus a whole bunch of harmonics. The -3/4 term is the gain compression. So basically, 1)supply causes gain compression, 2)gain compression changes transistor gm, 3)harmonics are an effect not a cause Sorry for taking so long... |
Title: Re: PA Linearity Question Post by aaron_do on Apr 21st, 2006, 1:54am BTW...i'm not really an expert so feel free to correct me... regards, Aaron |
Title: Re: PA Linearity Question Post by jxbvt on Apr 21st, 2006, 11:47am hi aaron thanks for the reply...that's exactly my understanding too, that compression is causing harmonics and not the other way around. one thing though: you said that gm changes because of limited supply voltage, can you explain that? gm is a function of Ic (the dc bias collector current) in BJTs and I don't see why gm would be changing because the bias would ideally remain constant. but your explanation did confirm what i was thinking. |
Title: Re: PA Linearity Question Post by milkdragon on Apr 21st, 2006, 5:36pm I think it both the transistor and the output impedance contribute to the non-linearity. For sure because of the nonlinearity effect of the transistor, the output will be distorted at certain input level. However, for PA design, impedance at the output play an important role: Pout = (Vrms^2)/Rout Let assume you can use the full swing at the output @ Vdd, then the max output power (saturated power) u can get is a function of your Rout, the smaller the Rout, the higher the saturated power. |
Title: Re: PA Linearity Question Post by aaron_do on Apr 22nd, 2006, 12:09am I agree that both the transistor and the supply contribute to the nonlinearity, but the supply voltage is responsible for the gain compression which cannot be explained by a square law (MOSFET) or exponential law (BJT) model. As for the changing gm...the gm of a transistor is measured without a load => ideal voltage source so the output never saturates. In the actual design, the output voltage is gm.rout so since Rout is basically constant, it must be gm that is changing at high input levels (that's my thinking but you may want to confirm it elsewhere). Sorry for the confusion... cheers, Aaron |
Title: Re: PA Linearity Question Post by jxbvt on Apr 22nd, 2006, 7:35am it's right that smaller Rout==>higher Pout but as aaron said the gain will still start to compress after Pin value that makes the output waveform go the maximum level. aaron, yeah i checked up on that and you're right about gm changing. i saw a paper that identified Cpi and Ic (in BJTs) as the cause of nonlinearity...so i think gm changes because there is nonlinearity in Ic, Vbe and this clarifies a lot....because of this compression at the output harmonics are being created (in Ic also) and this causes gm to vary too. |
Title: Re: PA Linearity Question Post by milkdragon on Apr 24th, 2006, 12:39pm I have a side question: What if gm is still linear at high input level but gmRout > Vdd? Given a supply limitation, the signal can swing from 0 to 2Vdd (assuming you are using inductive load) or the signal has to be distorted for even higher output power (which means more harmonics). There are some cases you can swing higher than that (Class D,E or F) using switching mode, which it is not linear. My understanding is that there are two main types of limiation regarding to linearity, one is output limited, the other is input limited. For the output limited case, which is determined by Rout (or supply voltage); In contrast, the transistor gm is the dominant factor if it is input limited. |
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