pass-transistors.ppt working of transistors

to understand pass transistor

1. 1
Basic Principles of Pass Transistor Circuits
Logic “1” Transfer
Logic “1” Transfer:VX(t=0)=0V, Vin=VOH=VDD, CK=0VDD
•VGS= VDD-VX, VDS= VDD–V0= VGS.
•Therefore, VDS> VGS–VT,MPMPis in saturation.
•Note that the VT,MPis subject to substrate bias effect and therefore,
depends on the voltage level VX. We will neglect the substrate
bias effect for simplicity. VVV
k
I
TDD
n
D 
0
2
2

2. 2
Basic Principles of Pass Transistor Circuits
Logic “1” Transfer (Cont.)
•V0rises from 0V and approaches a limit value Vmax= = VDD-VTbut it
can not exceed this value, since the pass transistor will turn off at this
point (VGS=VT,MP). Therefore, it transfers a “weak logic 1”.
•The actual Vmaxby taking the body effect into account is,  22
,0 FmaxFMPTDDmax VVVV 
VX
Vmax
Vmax=VDD-VT,MP
t
0
VDD/2
t
PLH

3. 3
Basic Principles of Pass Transistor Circuits
Logic “0” Transfer
Logic “0” Transfer:VX(t=0)=Vmax= VDD–VT,MP, Vin=VOL=0V,
•VGS= VDD, VDS= Vmax= VDDtherefore saturation.
•once VDSVGS–VT,MPMPis in linear region.
•Note that the VSB=0. Hence, there is no body effect for MP
(VT= VT0,).   22
2
, VoVoVV
k
I
MPTDD
n
D   







2
2
VVk
I
TDDn
D

4. 4
Basic Principles of Pass Transistor Circuits
Logic “0” Transfer (Cont.)
•VXdrops from Vmax= to 0V. Hence, unlike the charge-up case, it
transfers a “strong logic 0”.
Vo
VDD
Vmax=VDD
t
0

5. Transmission Gates
The transmission gate utilizes a pair of
complementary transistors connected in parallel.
It acts as an excellent switch, providing
bidirectional current flow, and it exhibits an
on-resistance that remains almost constant for
wide ranges of input voltage.
Transmission gate not only an excellent switch in
digital applications but also an excellent analog
switch in such applications as data converters and
switched-capacitor filters
5

6. TGs……..
Figure a. shows the transmission-gate switch in the "on" position with
the input,Vi, rising to VDD at t = 0.
Assuming, as before, that initially the output voltage is zero, we see that
Q
Nwill be operating in saturation and providing a charging current of
6
Transistor Q
Nwill conduct a diminishing current that reduces to zero at
V0=VDD-Vtn. Observe, however, that Qp operates with VSG=VDD and is
initially in saturation,
Qp will enter the triode region at VO= | Vtp |, but will continue to conduct
until C is fully charged and VO=VOH=VDD
,

7. TGs
7

8. TGs
Q
N and Q
Pinterchange roles. Analysis of the circuit
in Fig.b.will indicate that Q
Pwill cease conduction
when Vo falls to |Vtp|, where |Vtp|is given by
Transistor Q
N, however, continues to conduct until C
is fully discharged and Vo= VOL=0V,a "good 0."
8

9. 9
Dynamic CMOS Precharge-Evaluate Logic
Reduced Transistor Count
•=0Cprecharges to
VDD(output is not available
during precharge)
•=1Cselectively
discharges to 0 (output is
only available after
discharge is complete)

VDD
nMOS
Logic
inputs
C
Vout
Me
Mp
Internal
capacitance

t
t
Vout
precharge precharge
evaluate

10. 10
Dynamic CMOS Precharge-Evaluate Logic
An Example

VDD
Vout
Me
Mp
A1
A2
A3
B1
B2
Z is high when=0
Z=(A1 A2A3+B1B2)

11. 11
Dynamic CMOS Precharge-Evaluate Logic
Advantages/Disadvantages
Advantages
•Need only N+2transistors to implement a N-input gate.
•Low static power dissipation
•No DC current paths to place constraints on device sizing
•Input capacitance is same as pseudo nMOS gate.
•Pull-up time is improved by active switch to VDD.
Disadvantages
•The available time of output is less than 50 % of the time.
•Pull-down time is degraded due to series active switch to 0.
•Logic output value can be degraded due to charge sharing with other gate
capacitances connected to the output.
•Minimum clock rate determined by leakage on C.
•Maximum clock rate determined by circuit delays.
•Input can only change during the precharge phase. Inputs must be stable
during evaluation; otherwise an incorrect value on an input could
erroneously discharge the output node. (single phase P-E logic gates can
not be cascaded)
•Outputs must be stored during precharge, if they are required during the
next evaluate phase.