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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