[Amps] LDMOS Solid State Amplifiers

Roger (K8RI) k8ri at rogerhalstead.com
Sat May 5 11:35:46 PDT 2012

On 5/5/2012 11:30 AM, Tom Thompson wrote:
> Bob,
> Vdd was fed in at the U point on the brass tube, single turn, point.  If
> I switch back to that transformer, I'll try the bifilar choke feed and
> report.  I did some IMD measurements this morning.  I fed the amp with
> two Norcal 40 QRP transceivers on 40 m.

If I have followed this correctly:
As this is a single, PP amp fed with two independent exciters how do you 
maintain proper phasing which is critical? Even if the exciters are 
synchronized a tiny difference in path length can make a difference.


Roger (K8RI)

>    The input IMD due to the
> combiner isolation was -44 dBt where dBt means below one of the two tone
> peaks instead of the carrier which if present would be 6 dB higher.  The
> contribution from the input IMD is given by B = 20Log(1+10^(-A/20))
> where A is difference in dB between the input IMD and the output IMD.  I
> measured an output IMD3 of -32 dBt at 100 watts on the amp under test.
> That makes A = 12dB therefore B = 2dB which makes IMD3 = -34dBt.  IMD5
> was -60dBt and IMD7 was -52dBt.
> 73   Tom   W0IVJ
> On 5/4/2012 11:59 PM, Bob Henderson wrote:
>> Tom
>> Interesting.  Thanks.
>> With your brass tube transformer, how was Vdd fed to the drains?
>> The 10db reduction in H3&   H5 is a significant improvement but I am
>> wondering how much is due to the bifilar choke feed of Vdd and how much due
>> to transition to a TLT?  It would have been interesting to see what change
>> resulted from adding the bifilar choke feed to your brass tube transformer
>> set up.
>> H3&   H5 at -22dBc or better is easily good enough.  After that, it's all
>> about IMD performance.
>> 73 Bob, 5B4AGN
>> On 4 May 2012 23:43, Tom Thompson<tlthompson at qwest.net>   wrote:
>>> Bob,
>>> I am experimenting with the 300 W Freescale part.  Using the brass tube
>>> output transformer with a single turn on the primary and 2 turns on the
>>> secondary without a harmonic filter I measured the following:
>>> Vdd = 50 V
>>> Id = 10.5 A
>>> Po = 200 W
>>> 3H = -11.8 dBc
>>> 5H = -20.5 dBc
>>> I then followed Manfred's suggestions and used a 4:1 transmission line
>>> transformer wound with 30 ohm coax, a bifilar wound power combiner to
>>> supply drain voltage, and a choke balun on the output of the transmission
>>> line transformer.  I then measured the following with no harmonic filter:
>>> Vdd = 50 V
>>> Id = 7.5 A
>>> Po = 200 W
>>> 3H = -22dBc
>>> 5H = -30 dBc
>>> When I reduced the power output to 100 W, 3H went to -30 dBc.  In all
>>> cases the total Idq was 1.5 A.
>>> I hope this helps.
>>> 73,  Tom  W0IVJ
>>> On 5/3/2012 8:21 PM, Manfred Mornhinweg wrote:
>>>> Bob,
>>>>    My problem area was the extent of harmonics generated within the
>>>>> device.  H3 was within a dB or two of fundamental energy levels and
>>>>> H5 only marginally better.
>>>> That typically happens when your output network isn't correctly done.
>>>> There is an incredible amount of equipment, including HF ham
>>>> transceivers comemrcially made today, that have incorrectly implemented
>>>> power amplifiers, due to their designers not understanding of the basic
>>>> principles under which transformers operate.
>>>>    A serious problem.  My output arrangement focused largely upon a 1:9
>>>>> coax wound RF2000 from RF Parts as used in the Granberg designs at
>>>>> the 1kW level.
>>>> Granberg apparently was the one who "invented", or at least popularized,
>>>>     the wrong output network. Several of his papers contain the mistake,
>>>> but others do not. It seems to me that he really didn't understand this
>>>> issue, at least not when he published those old papers.
>>>> How are you feeding the drains?  If you are using a bifiliar choke,
>>>> designed in such a way that it can act as a balancing autotransformer,
>>>> then that should be fine, and you have to look elsewhere for the reason
>>>> of the high harmonics. But if you are using two individual chokes, then
>>>> that's wrong, and if you are feeding the drains through some sort of
>>>> center point on the transformer, then there is a pretty good chance that
>>>> it's wrong too!
>>>> Typical symptoms of the incorrect output configuration are: Extremely
>>>> high distortion (harmonics, IMD), horrible waveform at the drains, that
>>>> includes peaks well above twice Vdd, low efficiency, low gain, and a
>>>> sort of gain breakpoint: Up to a certain power the amp is easy to drive,
>>>> and from that point up it gets suddenly very hard to drive further.
>>>>    Harmonics were not a consequence of transformer saturation
>>>> That could hardly ever happen at HF. Before you saturate a ferrite core
>>>> at HF, you will melt it down with the losses!
>>>> But DC saturation can happen, in very tricky situations, specially if
>>>> you have hugely more inductance than needed.
>>>>    No problem in a single frequency amp but I am way short of clever
>>>>> enough to figure out a scheme which will handle that over 5 octaves.
>>>> Use either an output transformer that has a true center point, or a
>>>> bifiliar choke to supply power. Note that the typical RF power
>>>> transformers made from two ferrite tubes, with a single-turn primary, DO
>>>> NOT HAVE A CENTER POINT. The junction of the two metal tubes is NOT a
>>>> center point! Using this junction as a makeshift center point causes
>>>> endless trouble, and many amplifiers, based on some of Granberg's
>>>> designs, contain exactly this mistake.
>>>> With transmission line transformers, a center point is usually also
>>>> unavailable, but some transmission line configurations can have one.
>>>> The basic point is this: Class B or class AB push-pull amps MUST, I
>>>> repeat _MUST_ have something that provides balance around a true center
>>>> point. It cannot work in pure differential mode, because each FET
>>>> conducts for half of each cycle, and is in high impedance during the
>>>> other half cycle. You cannot draw current between one transistor that is
>>>> on and another that is off! That's why balun or balbal type output
>>>> transformers only work correctly in conjunction with a bifiliar feed
>>>> choke that provides the center point.
>>>> Class A push pull amps do not have this restriction, and can work well
>>>> in pure balanced mode.
>>>> So, check your feed arrangement, maybe that's where your problem is!
>>>> Manfred
>>>> ========================
>>>> Visit my hobby homepage!
>>>> http://ludens.cl
>>>> ========================
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