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72598Re: [softrock40] Re: GM3SBC QRO Modifications

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  • Sid Boyce
    Mar 5, 2013
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      Thanks Warren,
      It's worth a try so I'll have a closer look at it.

      Out of 10 BS170's in the pack from RS Components only 3 matched closely within 0.01V and the 2 removed were in the same ball park.

      The spread on the 10 were 3.13V - 3.18V. Whether the 3.18V ones would be OK to mix with 3.13V is the question.
      73 ... Sid.
       
      On 05/03/13 15:10, warrenallgyer wrote:
       

      Sid

      Off my soapbox and responding directly to your suggestion: for the original design concept to work you need would need to tightly couple the 4 PAs and the bias FET. The key to the original circuit is the tight thermal coupling between the PAs and the bias FET. Once I realized this I took each of mine apart and applied a THIN layer of thermal grease on the backside of the heatsink. The bias FET must "feel the pain" of the two PAs in order to be stable under extended periods of high load.... like WSPR operations.

      Under full load the negative feedback generated by the source resistance, together with the thermal adjustment of the bias, makes a very stable circuit. The two botom side PAs in the QRO design operate outside this thermal feedback loop and rely on air flow to keep them under control. This SHOULD work.... but they have no way to protest if it does not.

      Warren Allgyer - W8TOD

      --- In softrock40@yahoogroups.com, Sid Boyce wrote:
      >
      > Hi Warren,
      > Looks like an alternative could be to mount the 4 FET's on a separate
      > larger heatsink with 3 short leads from each pair to the board to
      > address the thermal issue? Ferrite beads on the gates?
      > 73 ... Sid.
      >
      > On 05/03/13 10:29, warrenallgyer wrote:
      > >
      > > It remains a controversial subject.... I have not seen any evidence of
      > > the need but I am sure willing to be proven wrong.
      > >
      > > One possible weakness with Ed's design, and a possible reason for
      > > worrying more about the match, is the added two PAs on the bottom of
      > > the board are not thermally coupled to the bias BS170. This thermal
      > > coupling is largely responsible for the temperature stability of the
      > > original design. As the temperature of the assembly rises the bias is
      > > adjusted to compensate so they do not run away. The bottom two FETs
      > > are not coupled and potentially, I suppose, could run away without the
      > > bias FET being aware. The forced air cooling should prevent this but
      > > it does not have the benefit of the closed loop that the original
      > > design has.
      > >
      > > As far as matching the bias FET.... I have posted my findings and
      > > opinions on that in the past and I have not had reason to change that
      > > position. To me it is "chicken soup"..... "it couldn't hurt".
      > >
      > > Warren Allgyer - W8TOD
      > >
      > > --- In softrock40@yahoogroups.com
      > > , "Alan" wrote:
      > > >
      > > >
      > > > ----- Original Message -----
      > > > Subject: [softrock40] Re: GM3SBC QRO Modifications
      > > >
      > > >
      > > > > Hi Alan
      > > > >
      > > > > Yes I have seen a wide variance in Gate Threshold Voltage in the
      > > devices I have tested. I have not however been able to translate
      > > > > that into a measurable impact on power output, inter-modulation,
      > > harmonic content, or stability. As I noted, I defer to Ed based
      > > > > on his extensive testing but I would be interested to know if
      > > anyone has observed any measurable operating impact from this
      > > > > variance.
      > > > >
      > > >
      > > >
      > > > I see Ed says 4 BS170s need to be matched but surely the one used as
      > > a bias source is also best matched?
      > > >
      > > > The last instance of bad matching was due to a mis-matched Q5,
      > > causing the bias to be so low it cut off the PA devices. If all are
      > > > matched then the bias should be correct without requiring any
      > > adjustment.
      > > > And maybe no change to R41?
      > > >
      > > > I cannot comment on the tolerable amount of mis-match, I'd guess a
      > > BS170 might be non-linear if it were cut off with no drive?
      > > >
      > > > 73 Alan G4ZFQ
      > > >
      > >
      > >
      >
      >



      -- 
      Sid Boyce ... Hamradio License G3VBV, Licensed Private Pilot
      Emeritus IBM/Amdahl Mainframes and Sun/Fujitsu Servers Tech Support
      Senior Staff Specialist, Cricket Coach
      Microsoft Windows Free Zone - Linux used for all Computing Tasks
      
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