|
|
Demo: CMOS Transmission Gate |
Any single N-type or P-type transistor may be used as a contact.
However, detailed analysis reveals that a voltage drop is found
from drain to source of a conducting N-type transistor when the
source voltage is near VDD.
Similarly, a voltage drop is found across a conducting P-type
transistor when its source voltage is near GND.
This poses no problem in the static CMOS gates, where all source
contacts of N-type transistors are connected to GND and all source
contacts of P-type transistors are connected to VDD.
But using a single N-type or P-type transistor as a contact introduces
a voltage drop across the conducting transistor, which may or may not
be critical.
The voltage drop can be avoided using a combination of N-type and
P-type transistors allows to realize the contact. The circuit consists
of one N-type and one P-type transistor connected in parallel and
controlled by inverted gate voltages, the transmission gate:
If the source voltage is near VDD, there is a voltage
drop across the N-type transistor but (almost) no voltage drop across
the P-type transistor. If the source voltage is near GND, the N-type
transistor has (almost) no voltage drop.
Because of the symmetry of standard MOS transistors, there is no
reason to differantiate between source and drain in a transmission gate.
The contacts are therefore usually called 'L' (left) and 'R' (right).
If the gate voltage of the N-type transistor is '0', the P-type
transistor has a gate voltage of '1' and both transistors are
nonconducting.
On the other hand, if the gate voltage of the N-type transistor is '1'
and the gate voltage of the P-type transistor is '0', both transistors
are conducting.
