On Saturday, 23 March 2013 15:37:46 UTC+11, Alex Pogossov wrote:
Hello, I uploaded another article. http://www.valveradio.net/en/home/audio/darlington.html Though it is not about vacuum tubes, I will be referring to it later when describing hybrid amplifiers.
What must we hold our breath for with regard to hybrid amps?
Something or other I guess.
But the principles of the Sziclai pair hasn't much to to with a darlington pair.
Darlington pairs, or triples are 2 or 3 transistors of the same polarity ie, both 2 or all 3 are either PNP or NPN.
Meanwhile, a Sziclai pair are made of one NPN and one PNP arranged as fairly simple direct coupled 2 stage cascaded amp. I think it was first thought of about 1 day after N and P devices became available, and possibilities were considered. One could use an NPN darlington pair for the input transistor and a PNP darlington pair for the output transistor. This would maximise the open loop gain because the Rin for PNP base is high, so output pnp darlo has high voltage gain. So the usual series voltage NFB then becomes most effective, and very low THD is achieved if the total open loop gain of say 4,000 is reduced to say 20 for a preamp means 4% THD is reduced to 0.02%, not a bad result, although maybe 10 times more than an op-amp.
The input darlo pair would have very high input resistance, just what's wanted.
Patrick Turner.