Pass Transistor Logic

This presentation discusses the basics of Pass Transistor Logic, its advantages, limitation and finally implementation of Boolean functions/Combinational Logic circuits using Pass Transistor Logic.

1. Lecture: October 9 th , 2017 Pass Transistor Logic

2. PTL uses a NMOS or PMOS transistor to transfer charge from input node to the output node, under the control of gate voltage. The output remains in High impedance state when gate voltage is zero. Pass Transistor circuits are widely used in design of ROMs, PLAs, multiplexers etc

3. Defining STRONG and WEAK logic Strong ‘1’: An output very close to the positive supply rail (V DD ) Weak ‘1’: An output voltage that is above V IH but lower than a Strong ‘1’ Strong ‘0’: An output very close to the negative supply rail ( V ss )or Gnd Weak ‘0’: An output that is below V IL but higher than a Strong ‘0’ . Signal Range (in Volts) Strong ‘1’ 4.5-5 Weal ‘1’ 3.5-4.5 Weak ‘0’ 0.5-1.5 Strong ‘0’ 0.0- 0.5

4. NMOS pass transistor passes Strong ‘0’ but weak ‘1’ An NMOS pass-transistor can pull down to the negative rail, but it can pull-up to a threshold voltage below the positive rail. => It can output a strong zero, but a weak one. Why???

5. NMOS pass transistor passes Strong ‘0’ but weak ‘1’ For an NMOS to pass Logic ‘1’, the node Vs gets gradually charged from 0 towards V DD . When Vs reaches V DD - V t,n then V GS =(V G - V S ) = V DD -(V DD - V t,n ) = V t,n which is the minimum voltage required for the NMOS to be ON state for a current to flow. So node out reaching to a potential more than V DD - V t,n turns off the NMOS.

6. PMOS pass transistor passes Strong ‘1’ but weak ‘0’ An NMOS pass-transistor can pull down to the positive supply rail, but it can only pull-down to a threshold voltage above the negative rail. => It can output a strong zero, but a weak one. Why???

7. Similarly for a PMOS to pass logic ‘0’, gate should be logic 0 and the output node should be gradually discharged from its previous value to zero potential. In this process when output node reaches | V t,p | then |V SG | reaches (| V t,p |-0) which is the minimum voltage required for the PMOS to be ON state for a current to flow. So node out reaching to a potential less than V t,p turns off the PMOS. So the maximum voltage level that the output node can be discharged to is | V t,p |

8. Strong and Weak Logic Summary nMOS passes a strong Logic ‘0’ but a degraded Logic ‘1’. The opposite is true for pMOS

9. The source voltage is always the lower of voltages V D and V G -V T

10. Three pass-transistors driving an inverter are shown. Let the threshold voltage of each transistor be 1.5V. Then the node voltages are as shown. With the gate and drain of 1 st pass-transistor at V DD, its source rises to 3.5V. And the device is at onset of pinching-off. The 2 nd pass-transistor has gate at 5V and drain at 3.5V, its source rises to drain potential of 3.5V. This is repeated along the chain. This is called charge steering.

11. Circuit Limitation: The voltage presented to the inverter input is only 3.5V. This must be sufficient to drive the inverter output low.

12. General function Block One application of pass-transistor logic is the universal Logic Module, also called General Function Block. Various functions can be obtained from the same circuit by changing Control and Logic Inputs

13. 2-input NAND gate

14. 2-input NOR gate

15. 2 X 1 Multiplexer

16. Advantages of Pass-Transistor Logic They are not ratioed devices and can be minimum geometry They do not have a path from VDD to ground and do not dissipate stand-by power

17. Limitations A sneak path is created when two pass-transistors are both ON at the same time and one is connected to VDD and the other is connected to ground.

18. Design Constraints Issue with discharge path Charge sharing

19. Exercise 1 Design a pass-transistor circuit for a three-input majority gate. The output of a 3-input majority gate is true if atleast two-inputs are true. The controls are A and B. Show the Karnaugh map and a circuit diagram.

20. Exercise 2 Draw a pass-transistor circuit for the priority encoder whose truth table is given below. The controls are A and B. Show the Karnaugh Map and a circuit diagram. Realize Y1 and Y2 simultaneously. Which form of pass-transistor is better suited for this realization, and why? . Input Output A B C Y1 Y0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1