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#1
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Triode Feedback
For the curious, please look over at
alt.binaries.pictures.radio I will now get out of the way!! Cheers, John Stewart |
#2
Posted to rec.audio.tubes
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Triode Feedback
Hi flipper and John
A link to this article has already been posted on the thread "On the absence of feedback in triodes". Note the summary. It's a fiction. cheers, Ian "flipper" wrote in message ... On Wed, 11 Jan 2006 09:20:48 -0500, John Stewart wrote: For the curious, please look over at alt.binaries.pictures.radio I will now get out of the way!! Cheers, John Stewart I haven't been abused lately so I'll take a shot at it. It strikes me that the article "inherent feedback in triodes" should be called, more precisely, feedback when a triode is driving a resistive load (or any load that alters plate voltage) because that's what's sampling the output and causing the feedback. The output of the triode itself is current, a function of grid and plate voltage. The external plate resistor samples that current and converts it to a voltage that is then subtracted from B+. I.E. ra-Anode-Zb is the feedback summing junction. The feedback is influenced by ra so there is an 'internal' component, which shows in his "equivalent control voltage" (1/u)dVb term. He calls 1/u the "feedback transmission coefficient" but, clearly, if there is no Zb there is no dVb and no feedback. One should be able to operate a triode close to the no feedback condition with a solid state bipolar current mirror in the plate circuit since voltage change across Vbe would be negligible compared to B+. The analysis is valid for the traditional triode amplifier circuit, however, as there will be "feedback" from ra-Anode-Zb. |
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