bit 011: TTL circuit design
bit 101: TTL circuit design
using transistors and logic gates work together to create a working circuit that can be evaluated using a truth table
transistors as circuits
As we have discussed in bit 01, transistors are the basic building blocks of the CPU. They are made up of three parts - a source, a gate, and a sink. Since they are used to process information, the larger number of transistors on the CPU, the faster it will be able to analyze binary data. The operation of a transistor can be explained using a simple faucet analogy. Much like a water faucet, all transistors are connected to a source, typically the CPU. The flow of electricity is controlled by a gate, which transmits the signal allowing it to flow from the source to the sink. Similar to a faucet, the electricity will only flow if the gate is switched on. The visual below depicts the difference between an on/off transistor:
Transistors work as part of a TTL circuit which stands for transistor-transistor logic. What is unique about logic gate circuits is they are designed to input only two types of signals: ‘high’ (1) and ‘low’ (0) which are represented by the value of the voltage. TTL gates operate on a power supply of 5 volts, which is a perfect high signal, represented by the binary digit 1. However, due to stray voltage drops in the transistor circuitry, logic signals rarely operate at these extreme voltage limits, and must interpret signals which have deviated from these values. Take a look at the image below of an input and output TTL gate and the range of voltage signals it can interpret. You will notice that the output range is narrower to ensure that the output signal transmits a voltage that can be interpreted by the receiving gate. This is known as an OR gate.
In order to operate as a switch, the voltage at the gate terminal of the transistor determines whether a connection will be made between the source and the sink depending on the value. For the circuit above, the value must be between 2V and 5V to be interpreted as high. If the value is anywhere below 2V, it will be interpreted as low, and there will be no flow of electricity. However, it is important to keep in mind that there are multiple types of transistors. A complementary transistor has the same structural function, yet the on/off gate voltages are opposite. This means that a ‘high’ signal would not allow electricity to flow, while a ‘low’ signal between 0V and 2V would produce a current. This is known as an inverter circuit, or a NOT gate.
understanding truth tables
When evaluating TTL circuits, truth tables are used as a diagram of the outputs from all possible combinations of input. In most circuits, there are more than one input variable, but all combinations of inputs do not equate to a ‘high’ signal which is required for the voltage to flow through the circuit of an OR gate. The image below shows the evaluation of two input values, A and B, corresponding to the Q output value. Since this is a simple OR gate, it can be interpreted as either an A or B input produces Q output. The Boolean expression of this equation would be Q = A+B.
For more information on truth tables and how they function using more than 2 inputs, this website is a great resource which details the function of transistors in relation to truth tables. If you find yourself confused evaluating truth tables, think back to the structure of a transistor. It can only produce an electrical current if the input is a ‘high’ signal for an OR gate, or a ‘low’ signal for a NOT gate, much like how a faucet can only pour water into the sink if the valve is switched in the proper direction.
More information on our digital world coming soon…