High Threshold Logic

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Schematic of basic unbuffered three-input HTL NAND gate.

High Threshold Logic (HTL) is a variant of Diode–transistor logic which is used in such environments where noise is very high.

FZH251, four two-input HTL AND


The threshold values at the input to a logic gate determine whether a particular input is interpreted as a logic 0 or a logic 1.(e.g. anything less than 1 V is a logic 0 and anything above 3 V is a logic 1. In this example, the threshold values are 1V and 3V). HTL incorporates Zener diodes to create a large offset between logic 1 and logic 0 voltage levels. These devices usually ran off a 15 V power supply and were found in industrial control, where the high differential was intended to minimize the effect of noise.

Schematic of a real buffered two-input HTL NAND gate FZH101A; PV = 180 mW; tpd = 175 ns.


  • Increased Noise Margin
  • Spike Control
  • High Noise Threshold Value


  • Slow speed due to increased supply voltage resulting in use of high value resistors.
  • High power drawn


It is used extensively in industrial environments. e.g.

  • Logic Controllers with heavy noise
  • Heavy Process Machinery

Circuital peculiarity[edit]

The buffer in this device is exactly the same of what was used in RGB video output stages in TV circuits in this way the upper NPN rises quickly a cathode capacitance with a relatively high load resistor on lower NPN, while lower NPN turn-on controls carefully K voltage

This shows the same improvement in charge-discharge parasitic capacitances in such high voltage logic circuit

See also[edit]