[time-nuts] 5>10 doubler

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Feb 3 06:29:18 EST 2015


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



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