|
 If the emitters of Tr1
are open circuit as
shown, then its
base/collector junction
is forward biased.
Tr2 will be turned on,
with its collector low
and its emitter high.
Tr3 will be off and Tr4
on.
The output will be low,
about 0.6 volts.
If all emitters are
taken high, it is the
same as them being open
circuit, and the output
stays low.
If one input is taken
low then Tr1 behave like
a normal transistor with
a reverse biased
base/collector junction.
The base of Tr2 is no
longer forward biased by
this junction, and is
turned off.
Tr2 collector goes high
and its emitter low.
Tr3 comes on and Tr4
goes off.
The output goes high to
about 3.5 volts.
A and B and C must all
be high to make the
output go low (a nand
gate).
Tr3 and Tr4 are in a
"totem pole"
configuration.
They can be considered
as two switches, when
one is on the other is
off. |
