>
>How does one determine / calculate the input impedance in a cathode driven
>GG circuit? A pointer to a book / article would be great.
>
>THX, Bill
? Murphy said that things are usually more complicated than they appear,
and this is seemingly just such a case. When the sinusoidal driving
signal is swinging positive, the grid looks relitively negative with
respect to the cathode, so the tube is cut-off. With no current flowing
in the cathode, the input R is virtually infinite. However, when the
sinusoidal driving signal swings negative, the grid looks relatively
positive, so cathode current flows. When the driving signal reaches its
peak negative value, cathode current is maximal, and input R is minimal.
Thus, input R is not a constant in a cathode-driven amplifier. To
smooth out the wild varations in cathode R, an electronic flywheel is
needed. This is usually a low-pass pi-network. So how do tube
manufacturers arrive at a figure for average input R? Probably by
adjusting C1, C2 and L until the match is optimal - and then calculating
output R using the C & L values that were arrived at experimentally.
cheers
- R. L. Measures, 805.386.3734, www.vcnet.com/measures.
end
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