Diode Transistor Logic : Circuit, Working, Truth Table & Its Applications

There are different types of logic families available that are used in designing digital logic circuits; Resistor Transistor Logic (RTL), Emitter Coupled Logic (ECL), Diode Transistor Logic (DTL), Complementary Metal Oxide Semiconductor Logic (CMOS), and Transistor-Transistor Logic (TTL). Out of these logic families, the DTL logic family was used commonly before the 1960s & 1970s to replace more advanced logic families like CMOS and TTL. Diode-transistor logic is a class of digital circuits that is designed with diodes & transistors. So the combination of diodes and transistors allows for making complex logic functions with quite small components. This article provides brief information on DTL or diode transistor logic and its applications.

What is Diode Transistor Logic?

Diode transistor logic is a logic circuit that belongs to the digital logic family which is used to create digital circuits. This circuit can be designed with diodes and transistors where diodes are used at the input side and transistors are used at the output side, thus it is known as DTL. DTL is a specific class of circuit that is used in current digital electronics for processing electrical signals.

In this logic circuit, diodes are useful in performing logic functions, while transistors are used to perform the amplification functions. The DTL has many benefits as compared to resistor transistor logic like; the higher fan-out values & high noise margin thus, DTL is replaced RTL family. The characteristics of Diode Transistor Logic mainly include; digital cultureless, digital strategist, digital architect, organizational agilest, customer centrist, data advocate, digital workplace landscaper & business process optimizer.

Diode Transistor Logic Circuit

The diode transistor logic circuit is shown below. This is a two-input diode transistor logic NAND gate circuit. This circuit is designed with two diodes & a transistor where two diodes are indicated with D1, and D2 & the resistor is indicated with R1 which forms the input side of the logic circuit. The Q1 transistor CE configuration & R2 resistor form the output side. The ‘C1’ capacitor in this circuit is used to give an overdrive current throughout the switching time and this decreases the switching time to some level.

Diode Transistor Logic Working

Whenever both the inputs of the circuits A & B are LOW, then both D1 & D2 diodes will become forward biased, thus these diodes will conduct within the forward direction. Thus the current supply because of the voltage supply (+VCC = 5V) will supply to the GND throughout the R1 resistor & the two diodes. The voltage supply gets reduced within the R1 resistor & it will not be enough to turn ON the Q1 transistor, thus the Q1 transistor will be in the cut-off mode. So, the o/p at the ‘Y’ terminal will be Logic 1 or HIGH value.

When any one of the inputs is LOW, then the corresponding diode will be forward-biased so, a similar operation will happen. As any one of these diodes is forward biased, then current will be supplied to the ground throughout the forward-biased diode, thus the ‘Q1’ transistor will be within cut-off mode, so the output at the ‘Y’ terminal will be high or logic 1.

Whenever both the A & B inputs are HIGH then both the diodes will be reverse biased, thus both the diodes will not conduct. So in this condition, the voltage from the +VCC supply will be sufficient to drive the Q1 transistor into conduction mode.

Therefore the transistor conducts throughout emitter & collector terminals. The whole voltage gets reduced within the ‘R2’ resistor & the output at the ‘Y’ terminal will have LOW o/p and is considered as low or logic 0.

Truth Table

The DTL truth table is shown below.

The diode transistor logic propagation delay is quite large. Whenever all inputs are logic high then the transistor will go into saturation and charge build-ups within the base region. Whenever one input is low then this charge should be removed, changing propagation time. To speed up the diode transistor logic in one way technique is by adding a capacitor across resistor R3. Here, this capacitor assists in turning off the transistor by eliminating the accumulated charge at the base terminal. The capacitor in this circuit also assists in turning on the transistor through enhancing the first base drive.

Modified Diode Transistor Logic

The modified DTL NAND gate is shown below. The resistors & capacitors components’ large values are very difficult to economically fabricate on an IC. So the following DTL NAND gate circuit can be modified for implementation of IC by simply eliminating the C1 capacitor, decreasing the values of the resistor & using transistors & diodes wherever achievable. This modified circuit simply uses a single positive supply and this circuit includes an input stage with D1, and D2 diodes, an R3 resistor, and an AND gate which is followed through a transistorized inverter.

Working

The working of this circuit is, this circuit has two input terminals A and B, and input voltages like A & B can be either HIGH or LOW.

If both the inputs A & B are low or logic 0, then both diodes will get forward biased, thus the potential at ‘M’ is the voltage drop of one diode that is 0.7 V. Although to drive the ‘Q’ transistor into conduction, then we need 2.1 V to forward bias the diodes D3, D4 & the BE junction of the ‘Q’ transistor, thus this transistor is the cutoff & provides output Y = 1

Y = Vcc = Logic 1 and for A = B = 0, the Y = 1 or High.

If any one of the inputs either A or B is low, then any one of the inputs can be connected to GND with any terminal connected to +Vcc, the equivalent diode will conduct, and VM ≅ 0.7 V & Q transistor will be cut off, and provide output ‘Y’ = 1 or logic High.

If A = 0 & B =1 (or) if A = 1 & B = 0, then output Y = 1 or HIGH.

If two inputs like both A & B are HIGH and both A & B are connected simply to + Vcc, then both D1 & D2 diodes will be reverse-based & they do not conduct. The D3 & D4 Diodes are forward biased & current at the base terminal is supplied simply to the Q transistor through Rd, D3, & D4. The transistor can be driven into saturation & the o/p voltage will be a low voltage.

For A = B = 1, the output Y = 0 or LOW.

The applications of modified DTL include the following.

Greater fan out is possible due to subsequent gates having high impedance with the logic HIGH condition. This circuit has superior noise immunity. The use of multiple diodes instead of resistors and capacitors will make this circuit very economical within the integrated circuit form.

Diode Transistor Logic NOR Gate

The diode transistor logic NOR gate is designed similarly to the DTL NAND gate with a DRL OR gate with a transistor inverter. DTL NOR circuits can be designed more elegantly by simply combining various DTL inverters through a common output. In this manner, several inverters can be united to let the necessary inputs for the NOR gate.

This circuit can be designed with the components of the DTL Inverter circuit apart from the power supply & two 4.7 K resistors, 1N914 or 1N4148 silicon diodes. Connect the circuit as per the circuit shown below.

Working

Once the connections are made, need to provide the power supply to the circuit. After that, apply four possible input combinations at A & B from the power supply with a dip switch. Now for every input combination, need to note down the logic condition of the output ‘Q’ as represented with the LED & record that output. Compare the results with the NOR gate operation. Once you have finished your observations, then turn off the power supply.

Diode Transistor Logic AND Gate

The diode transistor logic AND gate is shown below. In this circuit, the logic states like; 1 & 0 are taken as +5V positive logic & 0V correspondingly.

Whenever any input from A1, A2 (or) A3 is at a low logic state then the diode that is connected to that input will be in forward bias after that, the transistor will come into cut off & the output will be LOW or logic 0. Similarly, if all the three inputs are at logic 1 then none of the diodes conduct & transistor heavily conducts. After that, the transistor saturates & the output will be HIGH or logic 1.

The truth table of diode transistor logic and gate is shown below.

Comparison between DTL, TTL & RTL

The differences between DTL, TTL, and RTL are discussed below.

Advantages

The advantages of a diode transistor logic circuit include the following.

• The switching speed of DTL is faster as compared to RTL.
• The use of diodes within DTL circuits makes them cheaper because the fabrication of diodes on ICs is simpler as compared to resistors & capacitors.
• Power loss within DTL circuits is very low.
• DTL circuits have faster switching speeds.
• The DTL has greater fan-out & improved noise margin.

The disadvantages of diode transistor logic circuits include the following.

• DTL has a low operating speed as compared to TTL.
• It has an extremely large gate propagation delay.
• For high input, the output of DTL goes into saturation.
• It generates heat throughout the operation.

Applications

The applications of diode transistor logic include the following.

• Diode- Transistor Logic is used to design & fabricate digital circuits where logic gates use diodes within the input stage & BJTs at the output stage.
• DTL is a specific type of circuit that is used in current digital electronics for processing electrical signals.
• DTL is used to make simple logic circuits.

Thus, this is an overview of diode transistor logic, circuit, working, advantages, disadvantages, and applications. DTL circuits are more complex as compared to RTL circuits, but this logic has changed RTL because of its superior FAN OUT capability & enhanced noise margin but the DTL has a slow speed. Here is a question for you, what is RTL?