# Transmission Gates Notes | EduRev

## : Transmission Gates Notes | EduRev

``` Page 1

1
Transmission Gates
• Use of  transistors as
switches are called
transmission gates because
switches can transmit
information from one circuit
to another.
NMOS transmission gate as an
open switch.
• The figure shows NMOS
transmission gate. The
transistor in the gate can
conduct current in either
direction. The bias
applied to the
transistor determines
which terminal acts as the
drain and which terminal
acts as the source.
When gate voltage f=0
The n-channel transistor is
cut off and the transistor
acts as an open switch
Characteristics of NMOS transmission gate (at high
input)
If f=V
DD
, V
I
=V
DD
, and initially, the output V
0
is 0 and
capacitance C
L
is fully discharged.
Under these conditions, the terminal ‘a ‘acts as the
drain because its bias is VDD, and terminal ‘b’
acts as the source because its bias is 0.
The gate to source voltage can be written as
V
GS
=f-V
O
or
V
GS
= V
DD
-V
O
As C
L
charges up and Vo increases, the gate to
source voltage decreases. When the gate to
source voltage V
GS
become equal to threshold
voltage V
TN
, the capacitance stop charging and
current goes to zero.
This implies that the
V
O
=V
O
(max) when V
GS
=V
TN
Or
V
O
(max) = V
DD
-V
TN
d
S
G
This implies that output voltage never will be equal to V
DD
. ; rather it will be lower by V
TN
.
This is one of the disadvantage of an NMOS transmission gate when VI=high
Characteristics of NMOS
transmission gate (at low input)
•When V
I
=0 and f=V
DD
and V
O
=V
DD
-V
TN
at t=o (initially).
It is to be noted that in the present case
terminal b acts as the drain and terminal a
acts as the source.
Under these conditions the gate to source
voltage is,
V
GS
=f-V
I
V
GS
=V
DD
-o
v
GS=
v
DD
This implies that value of V
GS
is constant.
In this case the capacitor  is  fully discharge
to zero as the drain current goes to zero.
V
O
=0
This implies that the NMOS transistor
provide a “good” logic 0 when V
I
=low
V
DD
-V
t
G
S
D
source
drain
gate
Page 2

1
Transmission Gates
• Use of  transistors as
switches are called
transmission gates because
switches can transmit
information from one circuit
to another.
NMOS transmission gate as an
open switch.
• The figure shows NMOS
transmission gate. The
transistor in the gate can
conduct current in either
direction. The bias
applied to the
transistor determines
which terminal acts as the
drain and which terminal
acts as the source.
When gate voltage f=0
The n-channel transistor is
cut off and the transistor
acts as an open switch
Characteristics of NMOS transmission gate (at high
input)
If f=V
DD
, V
I
=V
DD
, and initially, the output V
0
is 0 and
capacitance C
L
is fully discharged.
Under these conditions, the terminal ‘a ‘acts as the
drain because its bias is VDD, and terminal ‘b’
acts as the source because its bias is 0.
The gate to source voltage can be written as
V
GS
=f-V
O
or
V
GS
= V
DD
-V
O
As C
L
charges up and Vo increases, the gate to
source voltage decreases. When the gate to
source voltage V
GS
become equal to threshold
voltage V
TN
, the capacitance stop charging and
current goes to zero.
This implies that the
V
O
=V
O
(max) when V
GS
=V
TN
Or
V
O
(max) = V
DD
-V
TN
d
S
G
This implies that output voltage never will be equal to V
DD
. ; rather it will be lower by V
TN
.
This is one of the disadvantage of an NMOS transmission gate when VI=high
Characteristics of NMOS
transmission gate (at low input)
•When V
I
=0 and f=V
DD
and V
O
=V
DD
-V
TN
at t=o (initially).
It is to be noted that in the present case
terminal b acts as the drain and terminal a
acts as the source.
Under these conditions the gate to source
voltage is,
V
GS
=f-V
I
V
GS
=V
DD
-o
v
GS=
v
DD
This implies that value of V
GS
is constant.
In this case the capacitor  is  fully discharge
to zero as the drain current goes to zero.
V
O
=0
This implies that the NMOS transistor
provide a “good” logic 0 when V
I
=low
V
DD
-V
t
G
S
D
source
drain
gate
2
Why NMOS transmission gate does not remain
in a static condition?
• The reverse leakage
current due to reverse
bias between terminal
b and ground begins to
discharge the
capacitor, and the
circuit does not remain
in a static condition.
V
DD
-V
t
source
drain
gate
Page 3

1
Transmission Gates
• Use of  transistors as
switches are called
transmission gates because
switches can transmit
information from one circuit
to another.
NMOS transmission gate as an
open switch.
• The figure shows NMOS
transmission gate. The
transistor in the gate can
conduct current in either
direction. The bias
applied to the
transistor determines
which terminal acts as the
drain and which terminal
acts as the source.
When gate voltage f=0
The n-channel transistor is
cut off and the transistor
acts as an open switch
Characteristics of NMOS transmission gate (at high
input)
If f=V
DD
, V
I
=V
DD
, and initially, the output V
0
is 0 and
capacitance C
L
is fully discharged.
Under these conditions, the terminal ‘a ‘acts as the
drain because its bias is VDD, and terminal ‘b’
acts as the source because its bias is 0.
The gate to source voltage can be written as
V
GS
=f-V
O
or
V
GS
= V
DD
-V
O
As C
L
charges up and Vo increases, the gate to
source voltage decreases. When the gate to
source voltage V
GS
become equal to threshold
voltage V
TN
, the capacitance stop charging and
current goes to zero.
This implies that the
V
O
=V
O
(max) when V
GS
=V
TN
Or
V
O
(max) = V
DD
-V
TN
d
S
G
This implies that output voltage never will be equal to V
DD
. ; rather it will be lower by V
TN
.
This is one of the disadvantage of an NMOS transmission gate when VI=high
Characteristics of NMOS
transmission gate (at low input)
•When V
I
=0 and f=V
DD
and V
O
=V
DD
-V
TN
at t=o (initially).
It is to be noted that in the present case
terminal b acts as the drain and terminal a
acts as the source.
Under these conditions the gate to source
voltage is,
V
GS
=f-V
I
V
GS
=V
DD
-o
v
GS=
v
DD
This implies that value of V
GS
is constant.
In this case the capacitor  is  fully discharge
to zero as the drain current goes to zero.
V
O
=0
This implies that the NMOS transistor
provide a “good” logic 0 when V
I
=low
V
DD
-V
t
G
S
D
source
drain
gate
2
Why NMOS transmission gate does not remain
in a static condition?
• The reverse leakage
current due to reverse
bias between terminal
b and ground begins to
discharge the
capacitor, and the
circuit does not remain
in a static condition.
V
DD
-V
t
source
drain
gate
3
V
O
(max) = V
DD
-V
TN
```
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