[ARC5] AM linear amplification

[Dennis Monticelli]

FYI.  The Doherty amp is pretty much the defacto standard architecture in
wireless base stations used around the world.  As you probably guessed,
they need high linearity combined with good efficiency.  The competing
scheme is called envelope tracking.  In the latter the power supply is
modulated to follow the amplitude envelope of the data modulation.  It
sounds simple but DSP is involved.....i.e. don't try this a home  :-)

Dennis AE6C

On Mon, Dec 19, 2016 at 1:00 PM, Richard Knoppow <1oldlens1 at ix.netcom.com>
wrote:

>     Please note that if your transmitter does not have controlled carrier
> modulation using a linear amp is not similar.  Linear amplifiers were
> standard practice for broadcast transmitters until about the late 1930s
> when high level plate modulated transmitters began to displace them.  A
> linear amp has very low overall efficiency for an AM signal. The efficiency
> of a Class-B linear is about 33% for the carrier, a Class AB will be
> slightly lower. This is because the level of the signal must be kept low
> enough to allow linear amplification of the modulation peaks. The actual
> overall efficiency varies with the modulation. A linear handling SSB will
> have an average efficiency of around 70%. There were several special
> amplifier circuits developed to try to get average efficiency increased for
> AM broadcast use to lower operating cost. The only successful one was the
> Doherty amplifier, made by Western Electric and a similar amplifier now
> made by Continental Electronics.  This amplifier uses two sections, one
> operating at Class B when the carrier only is present and both tubes
> contributing when the carrier is modulated. The overall efficiency is
> relatively constant with modulation at around 70% These amps require phase
> shift networks so they are difficult to make when the transmitter must
> cover different frequencies although such transmitters were made.
>    There is nothing wrong with using a Class AB or B amp for AM but you
> must put up with low overall efficiency and make sure the plate dissipation
> is adequate. Older books on transmitters will have a ton of both
> theoretical and practical design information.
>
>
> On 12/19/2016 12:36 PM, Kenneth G. Gordon wrote:
>
>> On 20 Dec 2016 at 9:05, AKLDGUY . wrote:
>>
>>
>>> My BC-AO-230 puts out about 5W and I'm interested in boosting that with
>>> a 6146B Class AB1
>>> linear amp. The ARRL Handbooks say that AM linear amplification is
>>> similar to controlled
>>> carrier modulation and caution that efficiency is low, about 33%, but
>>> don't give explicit
>>> operating conditions.
>>>
>>> I assume that the (6146B) stage cannot at any time be driven to the same
>>> output that it is
>>> capable of in SSB amplification. That's because with no audio applied,
>>> the level of the AM carrier
>>> drive must be carefully set so that the tube's plate is dissipating half
>>> of its rated dissipation. The
>>> application of audio drives the stage to a level where the plate
>>> dissipation is approached but not
>>> exceeded. This level is far below the SSB PEP input.
>>>
>>> Anyone have any comments?
>>>
>>
>> I have nothing to add. You have about covered it.
>>
>> Using any linear amp in AM service requires keeping the plate dissipation
>> at a "reasonable"
>> level.
>>
>> You obviously understand that.
>>
>> I would expect no more than about 25 watts output from your amp. Still,
>> that is 5X the
>> un-amplified output. I think that would be well worth the effort.
>>
>> Actually, the 807 or 1625 has a slightly greater plate-dissipation rating
>> than the 6146. Some
>> books show it as having a PD rating about 5 watts greater than a 6146.
>>
>> In addition, controlled-carrier modulation has greater AVERAGE efficiency
>> than straight AM,
>> because the carrier falls to a much lower level in the absence of
>> modulation, while it
>> remains steady for AM.
>>
>> CC modulation of a 6146 results in around 35 watts output.
>>
>> Ken W7EKB
>>
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> --
> Richard Knoppow
> 1oldlens1 at ix.netcom.com
> WB6KBL
>
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