Chapter 8. O.C., Tristate AND TRISTATE GATES

Transcription

1 Chapter 8 O.C., Tristate AND TRISTATE GATES

2 Lesson 3 Tristate Gate

3 Outline Tristate gate circuit Tristate gate features Tristate gate applications

4 Definition Tristate means a state of logic other than 1 and 0 in which there is a high impedance state and there is no i or i at the output stage source sink transistor (or MOSFET). A gate capable of being in 1, 0 and tristate is known as tristate gate

5 An element of Tristate gate circuit Analog/Digital Switch as A MOSFET analog/digital switch A CMOS analog switch is made from a pair of n-channel and p-channel MOSFETs. Two features: (i) Bi-directional current flow and (ii) Symmetrical input and output

6 Drain D F V + DD Gate I DS V + Y D A G S V V control X F S CMOS p-channel and n channel pair as Bidirectional current flow symmetrical switch

7 Switch Working When X is at either state '1' or '0, we shall get state '1 or '0' another end, Y. It when the switch is closed by applying a control Voltage of about V + (by making the control pin at CMOS logic state '1')

8 Switch Working No logic output appears at other end if the switch is opened by V - at the control gate (by logic state 0 ) When the switch is opened by applying control Voltage of about V - i.e. control input logic state is '0', we will get no current flow from one end X to another Y and currents for both the states are nil. Neither state 1 nor state 0 can be transmitted now from one end to another end

9 Digital and analog A digital circuit is also actually an analog circuit but is one where we consider only signals in terms of 1 s and '0's. Where 1 s and '0's correspond to two discrete regions of Voltages or currents or frequencies

10 Analog switch as digital switch An analog switch also works as digital switch provided (i) Voltage levels corresponding to the digital logic '1's and '0's are fed and are usable at X end called input/output or at Y end called output/input, and (ii) Control Voltage can be fed from a digital logic output at both 1 and '0' to set switch in its either ON (close) or OFF (open)

11 Tristate gate It is possible to have a switching circuit which will neither permit state '1' or state '0' from one end to another unless appropriate control input (enable input) is activated [say, by applying V + (logic 1 ) at it]

12 Tristate gate. Enable input if not active then it will give an high impedance state which will neither let a current source nor a current sink between X and Y ends (Enable input inactive) then previous stage whether at 1 or at 0 does not transmit onward

13 Outline Tristate gate circuit Tristate gate features Tristate gate applications

14 Tristate Gate Buffer Symbols Tristate Buffer with output Enable when E = 1 E E Tristate Buffer with output Enable when E = 0

15 Tristate Gate NOT Symbols Tristate NOT with output Enable when E = 1 E E Tristate NOT with output Enable when E = 0

16 Tristate Gate Features A tristate gate output 0 or 1 state enables either when E = 1(left side circuit) or when E = 0

17 Tristate Gate from Multi-emitter npn Circuit of TTL NAND gate having a multi-emitter transistor can be used as tristate circuit Two p-n junctions at a control input E. One of the input of multi-emitter transistor at the input stage is used a control input E.

18 Tristate Gate from Multi-emitter npn E input also connects the n-end of a p-n junction diode, the p-end of which connects the next stage transistor T collector. Switching of the TTL data input(s) at the gate by the use of two p-n junctions in place of MOSFETs based digital switches Fast switching at the outputs. Output pull up circuit consists of a pair of transistors T and T iv

19 Two multi-emitter junctions When we use two multi-emitter junctions of T, one for control input and others for the input A, the circuit works as tristate input NOT. When we use more than two multiemitter junctions of T, one for control input and others for the inputs A, B,.., the circuit works as tristate input NAND

20 Outline Tristate gate circuit Tristate gate features Tristate gate applications

21 Tristate gate Application To a single wire number of gates can be at interconnected. Only input gate and output gate circuits are enabled through E pins. Rest of the gates even though connected remain in tristate.

22 Summary

23 Analog/digital switch made from MOSFETs is symmetrical and bidirectional

24 Tristate gate is made from digital/analog switch consisting of p-channel and n- channel MOSFETS. Tristate gate is made from Multi emitter junction also Tristate gate permits transmission of input 1 or 0 to output provided enable pin is activated.

25 End of Lesson 3on Tristate Gate

26 Thank you