Whilst the output signal of the barely class A JFET amplifier has a lower unwanted harmonic content and thus requires less filtering to achieve a given suppression of unwanted harmonics and/or subharmonics, the question of the flicker phase noise penalty incurred by the barely class A amplifier approach remains unresolved.
Bruce On Tuesday, February 03, 2015 01:12:41 AM Charles Steinmetz wrote: > Andrea wrote: > >>But, what is the advantage between it and a couple of diode- connected > >>transistors with a full A-class (more linear, so less spurs) > >>amplifier in front > >>of it? > >> > >>If it's so, why use a nonlinear (or barely linear) gain stage to rectify? > >>Using just one stage means in general less phase noise output (but with > >>probably more spurs that can be filtered out), versus a more stage linear > >>amplifier (perhaps with strong negative feedblack) followed by a > >>rectifier? > > I replied: > >The "barely Class A" push-push doubler does not rectify the signal > >-- it creates the second harmonic because of the primarily > >second-order transfer characteristics of the JFETs. The design goal > >is to map the DC bias and the input signal to the portion of the > >FETs' characteristic curve that has the best fit to a second-order > >transfer function, while at the same time holding noise below the > >design requirement. > > Perhaps some pictures would be helpful (see below). Figure 1 (top) > shows an ideal full-wave rectified sine wave, similar to what is > produced by a full-wave diode rectifier, a bipolar transistor > push-push doubler, or a FET doubler driven into pinchoff (Class > B). Obviously, it is extremely rich in harmonics. The second > harmonic of the output (doubled) frequency is only 14dB below the > desired signal, and a series of even harmonics stretches as far as > the eye can see, diminishing only very slowly with increasing > harmonic number. (In practice, there will be a HF rolloff that makes > things slightly better. However, there will also be odd-order > components, which an ideal full-wave rectifier would not produce.) > > Figure 2 (bottom) shows waveforms from the simulation of my "barely > Class A" push-push doubler, using a matched pair of J111 FETs (J310s > perform almost identically, with the appropriate change in the bias > resistor). I purposely introduced a 10mV gate voltage imbalance in > the simulation to model imperfect matching. The red and magenta > traces are the currents in the two FETs, showing a primarily > second-order transfer characteristic. When these currents are added > by the push-push connection and put through a 4:1 (turns ratio) > transformer into a 50 ohm load, the green trace results. This trace > shows the simulated raw output, without any traps. Obviously, this > is very much closer to a clean 10MHz signal than the rectified signal > in Figure 1. > > The 5MHz component is ~40dB below the desired 10MHz signal. This > depends strongly on how well the FETs are matched and on the layout > and shielding. J111s or J310s from the same lot, matched to within > 1mV, should do better than this (the 5MHz component from my > breadboard circuit is below -45dBc, without any traps). The other > visible distortion products, and their levels, are: > > 15MHz -75dBc > 20MHz -45dBc > 25MHz -100dBc > 30MHz -75dBc > 35MHz -100dBc (all figures are approximate). > > The breadboard circuit performs similarly (the 15MHz component is > about 10dB lower from the breadboard, so I needed traps only at 5, > 20, and 30MHz to get all spurious responses below -80dBc). > > As I noted before, the "barely Class A" circuit is not materially > noisier than a FET push-push doubler that is run into Class AB or B, > but it has MUCH lower spurious outputs and, therefore, does not need > the sort of aggressive filtering the Class AB/B circuits need, > avoiding the increase in phase noise and other problems associated > with aggressive filters. > > Best regards, > > Charles _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
