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Consider the circuit in Figure P2.62. The output of a diode AND logic gate is connected to the input of a second diode AND logic gate. Assume Vγ=0.6V for each diode. Determine the outputs VO1 and VO2 for: (a) V1=V2=5V ; (b) V1=0 , V2=5V ; and (c) V1=V2=0 . What can be said about the relative values of VO1 and VO2 in their “low” state?
Figure P2.62
(a)

Values of output voltages VO1 and VO2 for given voltages in all parts.
Answer to Problem 2.62P
The values of output voltages are VO1=5 V and VO2=5 V .
The values of output voltages are VO1=0.6 V and VO2=1.2 V
The values of output voltages are VO1=0.6 V and VO2=1.2 V
Explanation of Solution
Given:
The given circuit is shown below.
Voltages given:
V1=V2=5 V
V1=0,V2=5 V
V1=V2=0
Calculation:
(i) Assume voltage Vy=0.6 V across each diode in the circuit.
The entire circuit can be divided into two parts.
Now considering the first part, V1 & V2 are the inputs.
On considering the connection of Diodes it is known that diodes are connected in reverse bias.
So, taking considerations from the first part, it is concluded that the output, VO1 is LOW if any one of the inputs, V1 and V2 is LOW
Hence, if the inputs of circuit are V1=V2=0 then the output is in LOW state, which means that the diodes are ON.
Similarly, if the inputs of circuit are V1=0,V2=5 V then the output is in LOW state, which means that D1 is ON.
Similarly, if the inputs of circuit are V1=5 V,V2=0 then the output is in LOW state, which means that D2 is ON.
Similarly, if the inputs of circuit are V1=V2=5 V then the output is in HIGH state, which means that diodes are in OFF.
Let’s make the truth table for VO1 from all the above combinations.
Table 1
V1 | V2 | VO1 |
0 0 5 5 | 0 5 0 | 0 0 0 5 |
On considering, the Boolean expression for VO1 is
VO1=V1 AND V2
Now, taking considerations from the second part, as diodes are connected in reverse bias it is concluded that if any one of the inputs, VO1 and Vx values is LOW then the output, VO2 is LOW. Also, the voltage Vx is always HIGH. So, the output VO2 depends on VO1 .Since, if VO1 is HIGH the output voltage VO2 is also HIGH.
So the expression for the output voltage VO2 for ideal diode can be,
VO2=VO1
Now for the given inputs V1=V2=5 V .
The diodes D1 and D2 are in the forward biased or ON state.
Hence, from the expression for ideal diode output voltages are VO1=5 V and VO2=5 V .
(ii) Let’s assume voltage Vy=0.6 V across each diode in the circuit.
The entire circuit can be divided into two parts.
Now considering the first part, V1 & V2 are the inputs.
On considering the connection of Diodes it is known that diodes are connected in reverse bias.
So, taking considerations from the first part, it is concluded that the output, VO1 is LOW if any one of the inputs, V1 and V2 is LOW
Hence, if the inputs of circuit are V1=V2=0 then the output is in LOW state, which means that the diodes are ON.
Similarly, if the inputs of circuit are V1=0,V2=5 V then the output is in LOW state, which means that D1 is ON.
Similarly, if the inputs of circuit are V1=5 V,V2=0 then the output is in LOW state, which means that D2 is ON.
Similarly, if the inputs of circuit are V1=V2=5 V then the output is in HIGH state, which means that diodes are in OFF.
Let’s make the truth table for VO1 from all the above combinations.
Table 1
V1 | V2 | VO1 |
0 0 5 | 0 5 0 5 | 0 0 0 5 |
On considering, the Boolean expression for VO1 is,
VO1=V1 AND V2
Now, taking considerations from the second part, as diodes are connected in reverse bias it is concluded that if any one of the inputs, VO1 and Vx values is LOW then the output, VO2 is LOW. Also, the voltage Vx is always HIGH. So, the output VO2 depends on VO1 .Since, if VO1 is HIGH the output voltage VO2 is also HIGH.
So the expression for the output voltage VO2 for ideal diode can be,
VO2=VO1
Now for the given inputs, V1=0,V2=5 V .
The diodes D1 is in the reverse biased while diode D2 is in forward biased.
Calculating the value of output voltage, VO1 is,
VO1=V1+Vy =0+0.6 =0.6 V
Calculating the value of output voltage, VO2 is,
VO2=VO1+Vy =0.6+0.6 =1.2 V
Hence, output voltages are VO1=0.6 V and VO2=1.2 V .
(iii) Let’s assume voltage Vy=0.6 V across each diode in the circuit.
The entire circuit can be divided into two parts.
Now considering the first part, V1 & V2 are the inputs.
On considering the connection of Diodes it is known that diodes are connected in reverse bias.
So, taking considerations from the first part, it is concluded that the output, VO1 is LOW if any one of the inputs, V1 and V2 is LOW
Hence, if the inputs of circuit are V1=V2=0 then the output is in LOW state, which means that the diodes are ON.
Similarly, if the inputs of circuit are V1=0,V2=5 V then the output is in LOW state, which means that D1 is ON.
Similarly, if the inputs of circuit are V1=5 V,V2=0 then the output is in LOW state, which means that D2 is ON.
Similarly, if the inputs of circuit are V1=V2=5 V then the output is in HIGH state, which means that diodes are in OFF.
Let’s make the truth table for VO1 from all the above combinations.
Table 1
V1 | V2 | VO1 |
0 0 5 5 | 0 5 0 5 | 0 0 0 5 |
On considering, the Boolean expression for VO1 is,
VO1=V1 AND V2
Now, taking considerations from the second part, as diodes are connected in reverse bias it is concluded that if any one of the inputs, VO1 and Vx values is LOW then the output, VO2 is LOW. Also, the voltage Vx is always HIGH. So, the output VO2 depends on VO1 .Since, if VO1 is HIGH the output voltage VO2 is also HIGH.
So the expression for the output voltage VO2 for ideal diode can be,
VO2=VO1
Now for the given inputs V1=V2=0 .
The diodes D1 and D2 are in the reversed biased or OFF state.
Calculating the value of output voltage, VO1 is
VO1=V2+Vy =0+0.6 =0.6 V
Calculating the value of output voltage, VO2 is
VO2=VO1+Vy =0.6+0.6 =1.2 V .
Hence, output voltages are VO1=0.6 V and VO2=1.2 V
Therefore, in their LOW states the value of VO2 is 0.6 V greater than VO1
(b)

Relative values of VO1 and VO2 in their “low” state
Answer to Problem 2.62P
Logic “0” signal degrades as it goes through additional logic gates.
Explanation of Solution
Given:
The given circuit is shown below.
Circuit diagram:
Logic “0” signal degrades as it goes through additional logic gates.
The diodes D1 and D2 are in the reversed biased or OFF state.
Calculating the value of output voltage, VO1 is,
VO1=V2+Vy =0+0.6 =0.6 V
Calculating the value of output voltage, VO2 is,
VO2=VO1+Vy =0.6+0.6 =1.2 V .
Hence, output voltages are VO1=0.6 V and VO2=1.2 V .
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Chapter 2 Solutions
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