Re: RF power chain mods and improvements..


Howard Fidel
 

Jerry:
When you use a transformer with the push pull configuration like Q93, Q97. the collector sits quiescent at the supply voltage. Then you add the + signal swing on to that to get the maximum voltage the device sees. So with a +2 V supply, you can't go lower then 0 volts, so you can't go higher then 2+2= 4 volts. That leaves a 1/2 volt margin for derating the device.

Howard

On 6/9/2018 11:54 AM, Jerry Gaffke via Groups.Io wrote:
Howard,

Thanks for posting that!
I find it interesting that you got away without a transformer at T10.
You say flat to 30mhz, have you breadboarded it, or is that a result from simulation?

Those parts are likely a bit too expensive for hfsignals, but way more cost effective for most of us
than spending weeks trying to get some $0.15 parts to do the job.

>  The MMIC has a absolute maximum voltage of 4.5 VDC.
>  Assuming you use a transformer (actually you must to get the drive level  you need for the output stage)
>  you an only use a +2 volt supply for the device for a 4 v p-p swing.

Mouser summary for the BGA616 mmic says a 6v supply, but turns out that's the rail
feeding the resistor, the device sees 4.5v max.
Most of the new devices are shooting for very low power supply voltages. 
I can't quite follow the logic regarding the statement  "you can only use a +2 volt supply".
With a transformer between the mmic and the load it would seem the mmic supply could be most anything.
Data sheet suggests it would typically be a 33 ohm resistor from 6v, device drawing 60ma,
so 6v - 33*0.060 = 4.0v at the device if trying to use the BGA616 mmic.

At 18dBm, that mmic might just barely replace Q911,912.
But not drive the IRF510's.
Not a good choice for inclusion in the uBitx.
But still, an interesting and very cheap mmic.

Jerry


On Sat, Jun 9, 2018 at 06:00 am, Howard Fidel wrote:
Attached is my driver it goes between VR1 and the outputs of T10 you need to remove C84 and T10 and insert this circuit. I am using the AMP signal to reduce the overall power output when driving a linear amp which needs 5W not 10W. U7 is a current driver, I chose it because I have a number of them, I probably could select something better, or use an emitter follower but it should do the job at 0 cost for me. The same with the T13-1 transformer, I have them. The opamp was selected for its speed slew rate voltage and current drive capability. C45 is there because U7 needs a minimum load capacitance for stability. I will see if I need to add something there later. TX is the switched 12 volts that is there in transmit only.
I am working on a PCB with this being one part of a daughter board for the uBitx. I would be interested in any feedback. I am not suggesting that this is a cost effective way of improving the uBitx for production. I will post the entire schematic in a few days.


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