logic gates different types of - devi ahilya · pdf fileother next stage(s) logic gates (htl...
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Chapter 6Chapter 6
DIFFERENT TYPES OF LOGIC GATES
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Ch06L7-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 2
Lesson 7
HTL gate
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Ch06L7-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 3
Outline
• High Threshold Logic (HTL) • HTL Features
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Ch06L7-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 4
Three Input HTL Circuit with one NPN output stage Transistor
V+CC
T
3kRd
T’
12k V+CC
B5k
ViA
Logic NAND Circuit
AViB
V-EE
Next stage
RB
Next stage
C
ViC
R’d
Rd
R’d Rd
R’d
6.9V
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Ch06L7-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 5
HTL Circuit Supply and Inputs VCC supply is of 15V in place of 5V Input Stage: It is based on the application
of two or more inputs to two or more n-ends of the p-n junction diodes in place of passive 450-Ohm resistance in RTL circuit. The p-ends are common and connect to two 12kΩΩΩΩ and 3kΩΩΩΩ resistances RD and R’D, which connects to supply voltage of 15V.
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Ch06L7-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 6
HTL Circuit Input StageThe common point of RD and R’D, also
connects to collector of a transistor T. The common point of p-ends also connects to the transistor-base
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Ch06L7-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 7
HTL Input Stage CircuitThe common point of RD and R’D, also
connects to collector of a transistor T. The common point of p-ends also connects to the transistor-base
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Ch06L7-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 8
Output Stage For the next Input Stage
• A common output from the transistor T’ at F is given to other diodes at the other next stage(s) logic gates (HTL gates), which will get the input from the transistor, T
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Ch06L7-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 9
Cut-off region inputIf input A or B or C is low ~0.2V(near
supply ground), the diode in the path of A or B, respectively will start conducting. Current through the diode will be approximately equal to (15V- 0.7V –0.2V)/(15000) Ohm = ~0.9 mA. [Voltage drop across a conducting p-n junction diode is threshold voltage = ~0.7V.]
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Ch06L7-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 10
Cut-off region input Voltage drop of 0.9V connects to the base of
T’ through 2 –diodes, one formed by base-emitter of T and other formed by Zener needing threshold voltage of 6.9V and total 7.6 V to turn ON and conduct.
Hence the transistor base is at ~0V, below cutoff voltage. When T’ is not conducting the output F = 1, high (~15V). T’ does not conduct when any (or both of the inputs) is low because voltage at the base drops below the cut-off voltage needed at the base
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Ch06L7-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 11
Saturation Region Case ‘1’ at both inputs A, B and C
If all the inputs A, B and C are high (>0.7V + 7.6V), the voltage at common p-ends will start exceeding 7.6V and the Zener diode circuit to the base will start conducting. When input A, B and C exceeds 8.3V and the voltage at the common p-ends exceeds (8.3V + VBE (ON))= 9V, the base-emitter junction of T’starts conducting
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Ch06L7-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 12
Saturation Region Case at input A or B
When the A and B inputs exceeds 7.6 V, the diode stops conduction and when exceeds 8.3V, becomes reverse biased. VBE (ON)remains at 0.7V. If transistor T’ base-emitter current exceeds a limit, the T’goes in saturation mode and it will start conducting current IC through R and VCE= ~ 0.2V. Therefore, F = 0 when A and B both ‘1’
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Ch06L7-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 13
Output at F Property of a NAND is that its output is 0 when all the inputs are 1
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Ch06L7-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 14
Outline
• HTL• HTL Features
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Ch06L7-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 15
Propagation delay
Let base-emitter capacitance = C nF [nF means nanoFarad.] If m = 4 stages, which connects to F, the total capacitance being all T next stages in parallel = 4C. Resistance is very small between base and emitter in logic ‘1’ state. Therefore, transistor turn-on delay is small.
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Ch06L7-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 16
Propagation delayResistance in logic ‘1’ state is 15000 Ohm, therefore turn-off propagation delay = (15000) mC ns. [nF * Ohm = ns.] Typically, the turn-On delay is 90 ns and turn-off delay is 240 ns. Circuit temperature sensitivity is small compared to DTL as the Zener has less temperature coefficient. The high threshold voltages give higher noise margin
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Industrial Environment
Since the threshold potential for 1 is high, the HTL suits the industrial environment.
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Power Dissipated per Gate
50 mW
10
Fan out
100 MHz
Maximum Operation frequency
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Summary
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• HTL gate has each input connection to n-p-n through a transistor followed by a Zener of 6.9 V in place of 1.4 V (diode pair in DTL). It functions as NAND.
• HTL gate has high threshold for logic 1 so that it functions in Industrial environment
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End of Lesson 7
HTL Gates
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THANK YOU