2.1 Transistors and Logic Gates

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Look at the diagram. Describe the logic that would make Q true?

Q is true when (A AND NOT B) OR (NOT A AND B) is true

What kind of logic gate does this circuit express? What happens when there is a 1 input in A, or a 1 input in B? What happens when there is a 0 input in either A or B?

This is a manual NOR gate. When there is a 1 input in A, the circuit is closed. This means that the current can flow to the ground. The same will happen if the there is a 1 input in B. There will therefore be a 0 output because there is less resistance going to the ground. (A OR B = 1, OUTPUT = O) When there is a 0 input in A, the current can still flow through B to the ground. The same is the case if there is a 0 input in B. When both inputs are 0, then the current can only flow to the output, so the output will be 1. (A OR B = 0, OUTPUT = 1)

What kind of logic gate does this circuit express? What happens when there is a 1 input? What happens when there is a 0 input?

This is a manual not gate (an inverter) When there is a 1 input, the circuit is closed. The current will take the path of least resistance, so will go to the ground. This means there will be a 0 output. 1 input = 0 output. When there is a 0 input, the circuit is closed. The current only has one path, so will go to the output. This means that there will be a 1 output. 0 input = 1 output.

What kind of logic gate does this circuit express? What happens when there is a 1 input in A transistor, or a 1 input in B transistor? What happens when there is a 0 input in either A or B?

This is a transistor NOR gate. When there is a 1 input in transistor A, there is no resistance. This means that the current can flow to the ground. The same will happen if the there is a 1 input in B. There will therefore be a 0 output because there is less resistance going to the ground. (A OR B = 1, OUTPUT = O) When there is a 0 input in A, the current can still flow through B to the ground. The same is the case if there is a 0 input in B. When both inputs are 0, then the current can only flow to the output because there is an infinite resistance to the ground, so the output will be 1. (A OR B = 0, OUTPUT = 1)

What kind of logic gate does this circuit express? What happens when there is a 1 input at the transistor? What happens when there is a 0 input at the transitor?

This is a transistor not gate (an inverter) When there is a 1 input, the transistor has no resistance. The current will take the path of least resistance, so will go through the transistor to the ground. This means that there will be a 0 output. 1 input = 0 output. When there is a 0 input, the transistor has infinite resistance. The current cannot flow through the transistor, so can only flow through the output. This means that there will be a 1 output. 0 input = 1 output.


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