BJT\'s can be used as switches to create gates that implement logical operations

Question

BJT's can be used as switches to create gates that implement logical operations on digital

signals. In this case, the gates are called Resistor-Transistor Logic (RTL)

because they depend on using transistors as switches with "pull-up" or "pull-down"

resistors to make the logic functions work properly.

Configure the "Emitter Follower" circuit shown in Figure 2 using one of the transistors

(2N3904 in parts drawer) supplied. Use a +6V supply to the collector of the transistor. Is

the output high or low when the input is 6 volts (high, a logical 1)? What is it when the

input is 0 volts (low, a logical 0)? Reproduce the input/output table shown in your

notebook and fill in the experimentally determined values (1 or 0). A circuit that

performs a logic function like this is called a "gate". The input/output table you just

produced is called a "truth table". Its purpose is to show the output under all possible

input conditions. Here you have only one input, which can have only two states (high or

low, 1 or 0).

Q1: If you had two inputs, how many rows would you need for your table?

Three inputs? N inputs?

Q2: Why is the "pull down" resistor needed between the emitter

and ground? (Hint: Use the equivalent circuit models (the 'On' and 'Off' circuits above) to

try to understand the answer. Consider whether it is possible to determine what Vout

would be when the transistor is off if there were nothing connected to the emitter erminal? What would happen if we were to replace the resistor with a length of wire to

accomplish pull-down?)

The value of the resistor is chosen to set the amount of current

that flows from the power supply, through the collector-emitter terminals, and then to

ground when the transistor is on).

Configure the "Emitter Follower" circuit shown in Figure 2 using one of the transistors (2N3904 in parts drawer) supplied. Use a +6V supply to the collector of the transistor. Is the output high or low when the input is 6 volts (high, a logical 1)? What is it when the input is 0 volts (low, a logical 0)? Reproduce the input/output table shown in your notebook and fill in the experimentally determined values (1 or 0). A circuit that performs a logic function like this is called a "gate". The input/output table you just produced is called a "truth table". Its purpose is to show the output under all possible input conditions. Here you have only one input, which can have only two states (high or low, 1 or 0). If you had two inputs, how many rows would you need for your table? Three inputs? N inputs? Why is the "pull down" resistor needed between the emitter and ground? (Hint: Use the equivalent circuit models (the 'On' and 'Off' circuits above) to try to understand the answer. Consider whether it is possible to determine what Vout would be when the transistor is off if there were nothing connected to the emitter erminal? What would happen if we were to replace the resistor with a length of wire to accomplish pull-down?) The value of the resistor is chosen to set the amount of current that flows from the power supply, through the collector-emitter terminals, and then to ground when the transistor is on).

Explanation / Answer

sol)

2)

when we relace with ON

we replace the transistor with short circuit

now current Ic flow through the transistor then there will be voltage developed across the transistor. which we treat as

logic high

when we relace with OFF switch

we replace the transistor with open circuit

now no current flow through the transistor then there the voltage drop across the resistor well be zero in other words as resistor is passive element it can not store the vlotage it brings the voltage down to zero i.e pull down

the output will be treated as logic low

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