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Re: Really? You like that?

Posted by corerosin (A) on February 14, 2006 at 21:34:46

In Reply to: Really? You like that? posted by Allen Wright on February 14, 2006 at 13:02:19:

I am having a problem with the ECL 86 circuit. It may just be my problem, but I think it has to do with mating a pentode with a triode in a mu follower ciruit.

The whole point... the reason why the circuit is called a mu follower is because the entire voltage output of the lower triode is across the lower triode's plate resistance and the upper triode's plate resistance times the mu of the upper triode. That makes the upper triode appear to the lower triode as if it were an enormous resistor from a very high voltage source. Because the lower triode's plate resistance is so much smaller than that mu x upper triode plate resistance, all the voltage gain goes across to the upper triode. The output of the circuit follows the mu of the lower triode. Simple, right? Yeah, I know, not the way I tell it, and I apologise for that, but the ECL 86 circuit posted in the above thread fails as a mu follower because, although the lower triode is AC coupled to the grid of the upper pentode and the plate resistance of the pentode would be enormous compared to the 62K plate resistance of the triode, unfortunately the pentode's plate resistance is in parallel with the 15K/2 watt cathode resistor to ground. The 15K resistor effectively murders the mu of the lower triode and the structure of the gain is the pentode resistance in parallel with the 15K resistor, which is effectively 15K, plus the 220K plate resistor at the plate of the lower triode. So, the net circuit is a common cathode lower triode with a 235K resistor connected to a V+ which is half the voltage potential of the pentode portion of the circuit. Like I said above, one would achieve greater voltage gain by simply connecting the 220K resistor to the 200V V+ directly.

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