RTL | What is RTL in logic families

Logic Families

Digital integrated circuits (ICs) are composed based on the characteristics of different types of semiconductor devices. Many complex digital functions were developed in different forms and each form was named a logic family. In this article we will learn about RTL which is a member of bipolar logic families. Click here to read this article in Hindi

RTL- Resistor Transistor Logic Families

RTL was widely used before the development of ICs. RTL circuits are made of Resistor Transistor and are the first logic families to be integrated. Although RTL-Resistor Transistor Logic is not used much now, but studying its principles is beneficial in understanding the working of other gates. Inverters, NOR and NAND logic functions can be realized in this system.

RTL consist of | Parts of RTL

RTL circuits are made up of Resistor, Transistors, RTL gate is basically a NOR gate. A 2-input NOR gate is shown in photo (a). It is driving a similar n gates(b). By increasing the number of input NOR gates in the circuit, a much larger number of input terminals can be obtained. The number of input terminals is called FAN-IN.


Photo(a) 2-input NOR (RTL) circuit
Photo(b) Driving n identical gates with a 2-input NOR (RTL) gate

How does RTL function?

Input voltage is given at terminal A and B. Suppose input A is HIGH then transistor T1 will conduct and output will be LOW (as output P will be grounded). Similarly if B input is HIGH then transistor T2 will conduct and again the output will be at ground level ie LOW.

If either A or B input is LOW (Input HIGH means its level should be high enough to saturate the transistor and Input LOW means the input voltage level should be low enough to saturate the transistor cut off) then there will be no conduction in transistor T1 or T2 and the output will be HIGH.

If both the inputs are LOW then both transistors T1 and T2 will be on cut off and the output will be HIGH. When one of the inputs is HIGH, the transistor related to it will drive on saturation and the output will be LOW. The value of output voltage of LOW level (i.e. 0 level) will be equal to VCE(sat)(=0.2V ) of the transistor and the value of output voltage of HIGH(1) level will depend upon the number of gates connected to the output.

Parameters of RTL

The parameters of the RTL logic circuit shown in the photo are as follows-

  • Fan Out-4
  • Noise Margin – 0.2 Volts
  • Propagation Delay-12ns
  • Power Dissipation 30-100mW
  • Power Supply Voltage(Vcc)-3.8 Volts

RTL advantages & Disadvantages

Due to the use of resistance in Resistor Transistor Logic (RTL), the switching speed of the circuit becomes slow. This is the reason that the operation speed of RTL is less. Since in this circuit only resistances are used to separate one signal from the other, there is a possibility of cross talk between the inputs. That is, when a signal is given to input A, the voltage of input B may also change. For this reason, noise immunity of RTL circuits is not good. The main advantage of RTL Logic Families is its low cost power.

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