Chapter 8 - Devi Ahilya · PDF filelogic state ‘0’) ... • To a single wire number of gates can be at interconnected. Only input gate and output gate circuits are enabled through

Chapter 8

O.C., Tristate AND TRISTATE GATES

Lesson 3

Tristate Gate

Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006

Outline

• Tristate gate circuit• Tristate gate features• Tristate gate applications


Definition

• Tristate means a state of logic other than ‘1’ and ‘0’ in which there is a high impedance state and there is no isource or isink at the output stage transistor (or MOSFET). A gate capable of being in ‘1’, ‘0’ and tristate is known as tristate gate


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


CMOS p-channel and n channel pair as Bi-directional current flow symmetrical switch

V–

S

V+DD

G

DrainGate IDS

V+

S

D

Vcontrol

A

F

D

X

YF’


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')


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


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


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)


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]


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


Outline



Tristate Gate Buffer Symbols

Tristate Buffer with output Enable when E = 1

E

Tristate Buffer with output Enable when E = 0

E


Tristate Gate NOT Symbols

Tristate NOT with output Enable when E = 1

E

Tristate NOT with output Enable when E = 0

E


Tristate Gate Features

• A tristate gate output ‘0’ or ‘1’ state enables either when E = 1(left side circuit) or when E = 0


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.


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 Tiv


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 multi-emitter junctions of T, one for control input and others for the inputs A, B,.., the circuit works as tristate input NAND


Outline



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.


Summary


• Analog/digital switch made from MOSFETs is symmetrical and bi-directional


• 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.


End of Lesson 3on

Tristate Gate


Thank you

Documents

Chapter 8 - Devi Ahilya · PDF filelogic state ‘0’) ... • To a single wire number of gates can be at interconnected. Only input gate and output gate circuits are enabled through