Whilst reading up on long tail pair design recently, I came across some good info about the transistor version. The gain of the circuit is dependant on the collector resistor value, which is determined by DC bias considerations. To increase the gain, Rc was replaced by a current source (infinite resistance), which neatly explains all those OP amp schematics I've looked at over the years.
Question: Has anyone tried replacing Rp with a current source in their tube long tail pair design?
According to what I understand, replacing Rp with a current source makes the gain of the circuit go to infinity, this means you can go without a preamp altogether and control the gain of the whole circuit with the feedback loop... Wait a minute, a tube OP amp?
I wonder what Bob Pease would say. (see the October 18 issue of Electronic Design for an article on Philbrick K2-W tube OP amp http://www.elecdesign.com/Articles/Arti ... /8880.html )
Again I would be delighted to hear your comments.
Don