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Negative feedback to differential pair driver

Doing this at the driver stage could cause problems with common-mode distortion, unless you arranged negative feedback to both sides. CM distortion is less of a problem when NFB is applied at the input stage, as signal levels are much lower. Applying NFB to both sides carries the risk of extra phase shift, as you might no longer have a minimum phase system.
 
Just wondering if anyone has used negative feedback to the differential pair used as CC drivers for the power tubes. I don't mean the phase inverter configuration, but the long tailed pair with two in- and two outputs.

It's done frequently in the Circlotron OTL topology. No different, really, than how it's done with SS op-amps. The main thing to watch out for is that you buffer inputs that could be connected to different source impedances, as these add to the Rs resistor(s) and can unbalance/change the level of NFB you've designed for. It's not a prob with SS op-amps since these already use buffered output stages that have very low impedances.
 
Thanks for all the info! I'll look into the Circlotron topology. I understand the necessity for balanced (buffered) sources, but for some reason, the workings of global NFB to the LTP elude me...
To make sure we're all talking about the same thing, I sketched up a schematic of what's in my head, as Jehan suggested. Is it still viable??
NFBtoLTPconcept.jpg
 

VaNarn

Member
Paid Member
2010-12-11 12:39 am
Ivanhoe
If you can find,beg or borrow or just wait for someone who still has an ''RCA Receiving Tube Manual" to post the circuit;then the Class AB1,50 Watt circuit may be your answer.Feedback was applied from the O/P valve anodes to the anodes of the differential drivers and to the cathodes.The result of this feedback enabled the O/P valves to present a low source impedance to the transformer as well as improved linearity.This type of feedback was used in many early ccts that RCA published and gradually disappeared with the growing popularity of the ultra linear topology.
 

FLT

Member
2009-11-03 9:51 pm