At 17:32 29/01/2002 +0100, you wrote:
I am not familiar with that totem pole scheme, but, isn't it dangerous to
drive a totem pole with 0 degrees ? Wouldn't both the Fet (transistors,
whatever) be brought in conduction state at the same time ? Or is there
a current limiting device ? Just curious...
73 Alberto I2PHD
Dear Alberto, LF Group,
"Totem pole" = 2 transistors in series with the top one connected to +ve
supply rail, the bottom transistor connected to the -ve rail, and the
output taken from the connection between the two transistors; also called
complementary push-pull and similar things. Examples are TTL and CMOS
outputs, most transistor audio PAs, and of course the Decca PAs
The discussion so far relates to a "full bridge" output stage, 2 totem
poles with the load connected between the two outputs. As Alberto points
out, both transistors in the same totem pole must not be switched on at the
same time, or they will short out the supply, called cross-conduction or
"shoot through". But provided in one totem pole, transistor 1 is always off
while transistor 2 is on, and vice-versa, or both transistors are off, no
shoot-through will occur. In the Decca, the two totem poles are always
driven 180degrees out of phase from each other. However, the two totem
poles could be driven with any relative phase because the impedance of the
load limits the possible current flow between the two outputs, and this
leaves several possible combinations of duty cycle and relative phase which
could be used to vary the power going into the load.
Cheers, Jim Moritz
73 de M0BMU
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