High Threshold Logic Gate

In this article, we are going to learn about one important logic family gate called the High Threshold Logic Gate. We will discuss the HTL gate in very detail. We will see the whole HLT gate using the NAND Gate example. So let’s start from the basics of it.

Table of Contents

When the industrial environment is noisy due to the presence of motors, high-volume switches, etc. the above logic families will not work satisfactorily. Hence we require a logic family having very high noise immunity, like the HTL family.

The HTL family uses higher supply voltage like 15V instead of 5V. An HTL circuit working as s NAND gate is shown in the below figure.

The use of higher supply voltage and a zener diode in place of diodes as in a DTL circuit, makes the HTL circuit more immune to noise.

To obtain approximately the same currents as in the DTL gate, higher-value resistances are used in the HTL circuit.

Also Read: Diode Transistor Logic – DTL Gates

Operation of HTL NAND Gate

A and B are the input terminals. The input voltages A and B can be either LOW or HIGH.

A and B both are LOW:

If A and B both are connected to the ground, i..e A = B = 0, then diodes D₁ and D₂ are forward biased, hence the potential at M with respect to ground is V_M ≅ 0.7 V. But this voltage is insufficient to switch on transistor Q₂ and the zener diode Z, hence Q₁ is OFF.

∴ Y = 1 (HIGH) … For A = B = 0 (LOW)

Either A or B is LOW:

If any one of the inputs, A or B is connected to the ground with the other terminal connected to + V_cc. Then the corresponding diode conducts. Again V_M ≅ 0.7 V and Q₂ will still be OFF giving output Y = logic 1.

∴ Y = 1 (HIGH) … If A = 0 and B =1 or if A = 1 and B = 0

The voltage at point M required to forward biasedQ₁, Q₂, and the zener diode is,

V_M = V_BE2 + V_Z + V_BE1 = 0.7 + 6.8 + 0.7 = 8.2 V

Therefore minimum voltage at A and B input required is,

V_in = V_M – V_{F D1/ D2} = 8.2 – 0.7 = 7.5 V

Hence V_in required is much higher than the noise levels present in the environment, which makes the HTL gate more noise-immune.

A and B both are HIGH:

If A and B both are connected to + V_cc, then both diodes D₁ and D₂ get reverse biases and do not conduct. Now Q₂ gates are switched ON, conduct the Zener diode and switching ON Q₁, and output voltage Y is pulled down to a low voltage.

∴ Y = 0 (LOW) … For A = B = 1 (HIGH)

Temperature Sensitivity of HTL Gate

The temperature sensitivity of HTL is considerably better than TTL which means HTL is less dependent on temperature.
This is due to the high value of the resistors used for HTL.
With change in temperature, the value of each resistor will change. However since the original value of resistors in HTL is high, the change in resistance is a small percentage of the original value.
Hence the input-output characteristics are independent of temperature variations.

Advantages of High Threshold Logic Gate

Higher noise immunity.
Input-output characteristics are independent of the temperature variable.

Disadvantages of High Threshold Logic Gate

Slow speed due to increased supply voltage resulting in the use of high-value resistors.

High Threshold Logic Gate – HTL Gate