Chapter 8 - Devi Ahilya · PDF filelogic state ‘0’) ... • To a single wire...
Transcript of Chapter 8 - Devi Ahilya · PDF filelogic state ‘0’) ... • To a single wire...
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
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
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
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
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
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
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’
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
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')
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
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
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
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
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
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)
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
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]
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
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
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
Outline
• Tristate gate circuit• Tristate gate features• Tristate gate applications
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
Tristate Gate Buffer Symbols
Tristate Buffer with output Enable when E = 1
E
Tristate Buffer with output Enable when E = 0
E
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
Tristate Gate NOT Symbols
Tristate NOT with output Enable when E = 1
E
Tristate NOT with output Enable when E = 0
E
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
Tristate Gate Features
• A tristate gate output ‘0’ or ‘1’ state enables either when E = 1(left side circuit) or when E = 0
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
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.
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
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
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
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
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
Outline
• Tristate gate circuit• Tristate gate features• Tristate gate applications
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
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.
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
Summary
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
• Analog/digital switch made from MOSFETs is symmetrical and bi-directional
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
• 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.
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
End of Lesson 3on
Tristate Gate
Ch08L3-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006
Thank you