Project # 2 – Resistor Transistor Logic



Project # 2 – Resistor Transistor Logic

Logic gates using Resistor-Transistor-Logic (RTL) are faster, more powerful and more accurate than gates made with diodes. These RTL gates used to be very popular and still have some use. Integrated circuits (I.C.’s), or chips today use special types of transistors called Field Effect Transistors (FET’s) that are even better. These chips are a lot faster, more accurate and use very little power, all great advantages, but when you need to drive loads requiring more current, regular transistors like in the circuits shown below are still used (at least for the output stage). Study the circuits below before building any circuit.

|Circuit # 1 | |

|[pic] | This circuit should look familiar. It looks like a regular amplifier, but with the resistors |

| |shown it actually acts like a switch. When the input, A, is HIGH (connected to +9V) it turns the |

| |transistor on. In fact, the transistor becomes saturated, that is with the maximum current flowing |

| |through the collector and emitter. When this happens Q, the output becomes LOW. Why? Since Q is |

| |like VOUT for an amplifier, Q = VCC – ICRC, so when a lot of current (IC) flows, Q becomes lower and |

| |lower. This circuit therefore behaves like an INVERTER, since when the input, A, is HIGH, the |

| |output, Q, is LOW. |

|Circuit # 2 | |

|[pic] | This circuit behaves exactly like the one above, except the output, Q, is taken from the |

| |emitter, which contains the resistor. When A goes HIGH, it turns the transistor on; current flows |

| |and raises the voltage at the emitter making the output, Q, also HIGH. Here, Q = 0 + ICRE, which |

| |comes from Ohm’s Law. This circuit therefore is a BUFFER. |

|Circuit # 3 | |

|[pic] | Can you figure out what logic gate this is? Well, you should so don’t ask me, after all I gave |

| |you enough information! Sorry. |

Procedure:

1. Do experiments 2 and 3 (pages 12 – 15) from the lab manual. You can use 2N3904 transistors instead of the MPSA20. The book basically asks you to build circuits # 1 and # 2 shown above (the INVERTER and BUFFER). It also shows you how to use the logic probe.

2. Build circuit # 3 on the LARGE breadboard. Connect two long (3”) wires to inputs A and B and leave the other ends unconnected. Do the same for the output, Q. Use 2N3904 NPN transistors.

3. Connect the inputs, A and B, to either HIGH or LOW just as you did in the last project. Do all 4 combinations of inputs. For the output, use the logic probe you built onto the breadboard at the beginning of the term. Simply touch the probe to Q and see if it registers a HIGH (red) or a LOW (green).

4. Identify the gate. Is it a NAND, NOR, OR, or an AND?

5. DO NOT TAKE THE CIRCUIT APART.

In case you can’t the next circuit to work you can get partial credit for showing me this one. Once you get the next circuit working properly, take this one apart, and so on.

6. Using your knowledge of transistor switches, figure out how to make the three other logic gates (NAND, NOR, OR, AND) using exactly the same parts used in circuit 3, only the connections are different.

7. Test each circuit to see if it is indeed the logic gate you intended.

8. You need to obtain teacher’s initials for ONE circuit only; however the amount of credit you will receive depends on which circuit you show me (circuit # 3 shown on the front gets the least amount of credit, while the OR gets more, and the NOR gates get the most credit).

Conclusion:

For each of the four basic logic gates (AND, NAND, OR, NOR) do the following:

1. Draw a schematic diagram and label it (AND, NAND, etc).

2. Explain how each circuit works based on transistor switch operation.

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