So there seems to be some difference of opinion as to whether the Heath SB-220
grid-grounding implementation actually suipplies negative feedback or not,
actually reduces IMD components in the output or not.
We all want to transmit the cleanest possible signal. At the end of the day,
those of us who have SB-220s need to decide whether to remove the Heath
circuitry and ground the grids directly with short heavy conductors, or leave
it
alone. In the absence of incontrovertible engineering evidence (i.e., showing
the math), perhaps somebody with an SB-220 and a spectrum analyzer needs to do
an IMD profile using each grid circuit and SEE which one produces the cleaner
output.
Bill W5WVO
Bill, W6WRT wrote:
> ORIGINAL MESSAGE:
>
> On Thu, 30 Jul 2009 11:28:38 -0400, Bill Fuqua <wlfuqu00@uky.edu>
> wrote:
>
>> vious message applied to the negative feedback in a grounded grid
>> amplifier due to the cathode and drive impedances.Similar to the
>> negative feedback due to an un-bypassed emitter resistor in a common
>> emitter amplifier. I was not referring to a un-bypassed grid
>> resistor at all.
>
> REPLY:
>
> An un-bypassed emitter resistor is not negative feedback, it is just
> gain reduction. For true NFB you have to take a sample of the output
> and feed it back to the input, out of phase with the input.
>
> True NFB always reduces the input impedance because the fed-back
> signal opposes the input signal and therefore requires more drive. In
> the emitter resistor case, the input impedance is actually increased.
>
> For a full discussion of feedback amplifiers, both negative and
> positive, see:
>
> http://en.wikipedia.org/wiki/Negative_feedback_amplifier
>
> Caution: This may be more information than you wanted. :-)
>
> 73, Bill W6WRT
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