Resistor Transistor Logic : Circuit, Working, Differences, Characteristics & Its Applications

Resistor Transistor Logic or RTL was invented by Fairchild In 1961 after the discovery of ICs which has become the base technology for semiconductor development. This is the first IC composed of resistors & bipolar transistors. It became the primary digital logic family to be created as a monolithic IC. RTL was the first logic family with bipolar transistors and later it was completely replaced with the later DTL (diode-transistor logic). These ICs were used within the Apollo Guidance Computer. This article provides brief information on resistor transistor logic or RTL.

What is Resistor Transistor Logic (RTL)?

The first integrated circuit composed of resistors & bipolar transistors is known as resistor transistor logic. The name of RTL comes from the truth that the logic functions were achieved by resistor networks whereas the signal amplification was achieved by a transistor. The basic RTL configuration has a single input resistor & a single transistor where the resistor is used as a current limiter and the transistor is used as a switch. It has an inverter logic function that inverts an input signal logically & outputs it. Resistor-transistor logic is used to design and fabricate digital circuits that use logic gates including resistors & transistors.

Resistor Transistor Logic Circuit

The basic logic circuit used most frequently in digital logic families is the resistor transistor logic circuit which is a bipolar saturated device. The resistor transistor logic circuit is shown below. Here the circuit used is a 2-input RTL NOR gate which is designed with resistors and transistors. The resistors (R1 and R2) in the circuit are connected at the input side & transistors (Q1 and Q2) are connected at the output side.

In this circuit, emitter terminals of transistors are connected simply to the ground terminal. The collector terminals of two transistors are coupled jointly & given to the voltage supply throughout the ‘RC’ resistor. In this circuit, the collector resistor is also called a passive pull-up resistor.

How Does Resistor-Transistor Logic Work?

The 2-input RTL NOR gate works as; whenever both the inputs of the circuit like A & B are at logic 0, then it is not sufficient to activate the gates of two transistors. Thus, the two transistors will not perform, so the +VCC voltage will appear at the ‘Y’ output. Therefore the output of this circuit is logic HIGH or logic 1 at the ‘Y’ terminal.

Whenever any one of the two inputs is given as logic 1 or HIGH voltage, then the HIGH gate input transistor will be activated. So this will make a lane for the voltage supply to go to the GND throughout the RC resistor & transistor. Therefore the output of this circuit is logic LOW or logic 0 at the ‘Y’ terminal.

Whenever both the inputs of the circuit are HIGH, then it drives both the transistors in this circuit to activate. Thus, it will make a lane for the voltage supply to supply to the GND throughout the RC resistor & transistor. Therefore the output of this circuit is logic LOW or logic 0 at the ‘Y’ terminal. The truth table of the NOR gate is shown below.

Characteristics

The resistor transistor logic characteristics include the following.

• The RTL fan-out – 5.
• Its Propagation delay – 25 ns
• RTL Power dissipation – 12 MW.
• Noise margin for low signal input – 0.4 v.
• Its noise immunity is poor.
• It has less speed.

Difference between RTL, DTL, and TTL

The differences between RTL, DTL, and TTL include the following.

Advantages & Disadvantages

The resistor transistor logic advantages include the following.

• The RTL circuit uses a least amount of transistors for combining various input signals, which helps in amplifying & inverting the combined resulting signal
• RTL gates are simple & inexpensive.
• These are handy due to both normal as well as inverted signals are frequently available.
• RTL is simple to design & less component count which makes it popular in digital electronics.
• Resistor transistor logic is replaced with very advanced logic families like TTL & CMOS because of their enhanced performance & efficiency.
• It reduces the usage of several semiconductor components.

The resistor transistor logic disadvantages include the following.

• The resistor transistor logic has high current dissipation whenever the transistor behaves to overdrive the o/p biasing resistor.
• It has high power dissipation whenever the transistor is turned on by supplying current within the base & collector resistors.
• It has limited fan-in.
• These circuits’ speed is quite slow as compared to other types of logic families because of the utilization of transistors & resistors.
• RTL circuits are complex.
• These circuits have poor noise immunity that makes them vulnerable to interference & degradation of signal.
• RTL circuits need fairly high voltage levels mainly for proper operation, which limits their compatibility with other systems.

Applications

The applications of resistor transistor logic include the following.

• RTL ICs were utilized in the Apollo Guidance Computer,
• These are the fundamental logic circuits used in digital logic families.

Thus, this is an overview of resistor–transistor logic which is a class of digital circuits, designed with resistors & BJTs. RTL is one of the main logic circuits used in digital logic families and is considered to be the primary logic family introduced for ICs. Logic gates with RTL technology are mainly designed by using resistors and NPN transistors where resistors are used as current limiters and NPN transistors are used as switches. Here is a question for you, what is DTL?