>>>The main question Allison is do you agree with the splitting of the second feedback resistor as I did to do the maths.? ?
It is not my analysis beyond eyeball.? Not a debate for me.
>>Indeed for the? AD 8009 the datasheet? recommends? about a? 50R resistor in the output drive line to? reduce chance of instability in the amplifier.?
>THe datasheet is a bit coy on the conditions that lead to such instability.? Yes I refer to the? 8009.? ?IN the case in question? the OP resistors have been put in? parallel to? ensure that can dissipate? up to? >0.5? W? that is what makes me suspect that those reisistors are there not only for stability reasons but also as? a form of power dump if the? RHS transformer is left unterminated.?
Unterminated transformer has a reflected condition that with dependencies will be reactive and op amps dislike reactive loads.?
Adding resistance makes it less so.? IF your driving a mosfet reactive in the normal case.
?
>Are there any? dual? Video amps of this type (two to a package )? that you know of? which? can do a similar duty at HF? to? 50 Mhz drive? I get the impression that this OPA is? pretty much unique in this >regard.?
NOt studied and likely the LT1254 series (dual 1253) that are fairly old likely qualify.? That was the driver use by
Elecraft in the K2.
>I will include a? photo of a commercial application that uses this type of? Tx chip but I have some considerable reservations about what is shown.? Specifically there appear to be be cappies in the lines >from the? driver transformer to the OPA . This would mean that? ?there is no way that the signal bias can get to the op amp.. Clearly there could be components under the PCb but given there is a heat >sink there , that seems unlikely.? The alternate is that the UKR Mafr has published a false photo and what is supplied does work and is subtly? different from the photo. This design has zero filtering post >the? RF power amp mosfets.? ?I include it mainly because it shows how the transformers? were implemented as hand wound designs and not? commericial designs as per schematic supplied (or a completely different application).
As to filtering the board is too small and a wideband design so that would be on the input and output sides
as needed.
The picture does not obviously agree with what you say.? The backside of the board and whats under the part is not
shown. heatsink is not a limitation.? ?Is it more than a two layer board?? ?I find when I do my own work I
get better answers.? I always find a copy of a bad copy is generally bad.? ?Best case for that is many of
the Motorola apnotes are not a predictable nor reliable when copies are made and subtle changes added.
As a note generally those are at best guides or advertising.? I prefer to look at all with a critical eye.
Generally I do not use those opamps due to other factors like generally high power
consumption and higher noise figures.
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Allison
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