24-07-2017, 10:48 PM
(24-07-2017, 11:59 AM)ppppenguin Wrote: There may be a misunderstanding of what is meant by "grid bias". My understanding, which I think is correct, is that it is the DC potential, usually in the absence of signal**. As Terry has said, it is measured between the grid and cathode. This is never positive in any design I've ever seen. In some designs, notably high power output stages, the +ve peaks of the signal may drive the grid positive with respect to the cathode and hence cause grid current to flow for short periods.
That's what I would say, too. Bias is the steady voltage on which the signal is superimposed.
I've never seen any circuits which operate with a positive control-grid bias, but I have seen examples where the bias is zero (battery Class B double triodes). The signal can and does take the grid positive. Yes, grid current does flow and yes it is a nuisance, but to say it's physically impossible to take the grid positive, is wrong.
In Class AB2, and RF Class C, the grid is biased negatively but the signal is greater than the negative bias so again, the grid goes positive on signal peaks (and sometimes positive by several volts, not just fractionally).
