[Amps] LDMOS availability

[Roger (K8RI)]

Like Bill and Jim(s) I disagree.  You really aren't running class C all 
the time. You are varying the bias from class to change from class C to 
AB 1 or 2 either continuously or in steps. I question the effect this 
would have on the actual linearity.  With the bias / class changes, the 
drive required changes and the gain changes.

Even with normal amps, is the output a faithful reproduction of the 
input? With the current state of the art for ham gear, I doubt it even 
in class AB1. Many of the signals I hear on the bands went through some 
stage that wasn't linear. Dynamic predistortion we have finally 
surpassed the signal quality of the old tube rigs. HOWEVER, as I've 
asked before,  how much deviation can we accept in the output from being 
a faithful reproduction of the input before we no longer see it as being 
linear.

73, Roger (K8RI)


On 6/10/2017 Saturday 1:48 AM, Jim Thomson wrote:
> Date: Fri, 9 Jun 2017 22:02:53 -0600
> From: Jim Garland <4cx250b at miamioh.edu>
> To: amps group <amps at contesting.com>
> Subject: Re: [Amps] LDMOS availability
>
> <There are two issues, here, one being the definition of Class C, and the
> <other being linearity. I'm inclined to agree with Bill on both. It's not
> hard to imagine modulating the bias, so that on voice peaks, for
> example, the amp moves in the direction of Class B, but regresses to
> Class C at low drive levels.In that case, the class of operation varies
> with the drive. That said, Class C is by definition non-and that occurs
> whenever the device doesn't conduct over a full RF cycle.
>
> I have a homebrew amp with two-tiered bias. On SSB, in pauses in speech,
> the amp is biased at a very low resting current (approaching Class C) to
> cut down on heat, but on voice peaks where the linearity is needed, it
> switches to Class B. One has to be careful playing that trick, however,
> to avoid switching transients.
>
> 73,
>
> Jim W8ZR
>
> ## Another amp config that switches about, is the infamous G2DAF circuit.
> I have a pair of  4-400s in that config.  Done right, they operate just fine,  but the
> component values have to be chosen very very carefully..esp the screen caps.  Nobody
> has ever complained of  imd...and cant tell the difference between the G2DAF amp, and the
> GG amps..when I toggle between them...while listening off freq.   A lot of G2DAF amps are not
> configured correctly, and I dont recommend  the design  for any new construction projects.
> I have seen the G2DAF config used on 4-400, 4-1000,  4CX-1000, 4CX-250B.
> They essentially switch from class C-B-AB..and back again  from AB-B-C.
>
> ##  I looked into the  two-tiered bias scheme back in the 70s. When it 1st came out
> in QST, its function was to cut off idle current  between dots and dashes  for qsk cw.
>
> ##  To avoid the dreaded switching transients, esp on ssb,  the three-tiered bias scheme
> was introduced.  Cut off on RX, the  bias V is high...like 40-60 vdc.   With PTT activated,
> the  bias V is reduced, like 15-20 vdc, so idle current is real low,  like 20-40 ma.   With RF
> detected, bias V toggles to its normal linear state, like  5-8 vdc,  such that normal idle current occurs.
> While talking on ssb, then the bias is just toggling between  say 15-20 vdc...and 5-8 vdc.
> With the two-tiered  scheme, the bias is toggling between say  40 - 60  vdc and    5-8 vdc.
> That takes time..and the amp ends  up being non linear while the drive level increases..esp
> at low levels.  If you have any amount of back ground noise, their will be enough RF from  the xcvr,
> even with just a few milliwatts, to toggle the bias,  defeating the purpose of the bi or ...tri-state bias scheme.
> A simple noise gate..or downward expander, would solve that issue. A noise gate is just a high ratio
> downward expander.
>
> ##  Even with the three – tiered scheme, its still not fast enough to toggle between syllables, so normal
> say 5-8 vdc bias stays on for the duration of the word phrases. It will however, toggle between words..
> sometimes.
>
> ##  I played  with it..and finally gave up.   Where it  sorta  works good is if using PTT-Footswitch SSB.
> For VOX  ssb,  it does not buy you anything..... except between the last word spoken....and when the vox drops out.
> Which is of such short duration, with  the typ 300-500 msec  vox delay.
>
> ###   For  CW use, I found that its just as easy to simply increase the bias Voltage a bunch, such that idle current
> is really low, like  15-40 ma.   Then you get the heat reduction between  dots + dashes.  Idle power is WAY down, heat
> is WAY down.   Tank eff increases a little bit,  1-3 %,  further reducing dissipation when driven full bore.  On any GG
> amp, you cant cut off the idle current, and run it into class B  or  Class C.....other wise you will get key clix.   It has to
> draw a little bit of idle current, hence the  15-40 ma.   15-40 ma is a helluva lot better  vs  150-220 ma,  heat wise.
> 14-40 ma also works good on FM.
>
> Jim   VE7RF
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