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Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback

Subject: Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback
From: "" <>
Date: Wed, 28 Dec 2005 06:22:49 GMT
List-post: <>
Hi Tom,
   A couple of points for you to ponder.
   A single OPA-695 won't make 5V RMS.   However, it's capable of a voltage 
gain of 10X, 20dB power, so you only need a single stage to go from 5mW to 
500mW.   A pair of them connected differentially will output 5V RMS, and 
provide some 2nd harmonic cancellation as a bonus, but, they probably shouldn't 
drive less than a 200 ohm load for best distortion numbers.

   Or... a single '695 could be followed by a +/-15V rail part, a THS3001 (or a 
more recent part from that family) for instance, to output an easy 5V RMS... or 
10V from a differential pair.  Again, this scheme may not be happy driving into 
50 ohms.

   Note: Both of these op-amp approaches only exhibit exquistely low distortion 
on the low bands.  At higher freqs., their distortion numbers rapidly 

   As the 4CXwhatever grid is a high impedance point, a tuned matching network 
might be better fit than a wideband transformer for single band operation.  

   Your proposed 20dB negative feedback around the driver-final means that you 
will require more grid drive, say somewhere around 50V RMS to the driver, to 
maintain the same overall power out.  The op-amp pre-driver may not provide 
enough oomph in this instance since a 10-1 voltage gain, employing a tuned 
matching network sourced from 200 ohms, would imply a grid impedance of 20K 
ohms.  Where's that free lunch?  :-)

73 & Good evening,
   Marv  WC6W


-- "Tom Cathey" <> wrote:
Hi Bob,

Thanks for the suggestion.

You make a good point of possibly eliminating the 6146 pre driver. I thought 
about that myself, but didn't have enough knowledge to know if I would have 
enough gain left to reduce the overall gain with  20 db of negative feedback 
afterwards.  Figured I'd just try it and see.

Well, I know that the 4CX-XXXX final [they're all similar] will need about 
300 volts of swing. And the OPA-695 will do 10db gain. That's 5mW to 50mW. I 
figured I'd put two OPA-695's  in cascade for 20db = 500mW.  Into 50 ohms, 
500mW =  5V.    So we have 5V to drive the 4CX-250 driver.  And, the 9:1 
transformer will make it X3 = 15V, right?  This is a 9:1 impedance ratio = 
3:1 voltage ratio...?

Anyway, with 20 db of negative feedback from the final to the 4CX-250 driver 
will mean a 20db reduction in gain.  I suppose a lot depends upon the gain 
of the 4CX-250 driver and the transformer.  This is where I am not sure how 
to calculate it and proceed. [20 Log db for voltage, etc.]

ie, Will an OPA-695 putting out 500mw into an X? step up ratio transformer 
that drives a 4CX-250, that drives 300v into a 4CX-XXX X final  with 20db of 
negative feedback have enough system gain?  And, will the 9:1 voltage step 
up transformer have absolutely no affect on the low distortion goal... even 
if a small space charge current is pulled? What if we need more than 9:1 
step up, can it be done with cascaded transformers without ill effects? 
BTW, I'm curious why you feel the transformer will have bandwidth 
limitations... are we talking of a broadband unun torroidal type?


Tom, K1JJ


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