Transistor-Transistor TTL Logic

Purpose of Transistor-Transistor TTL Logic is to do digital signal processing according

to logic rule of Boolean algebra. The scheme of standard TTL Logic NAND with TTL

Totem pole output is given in figure bellow.

ttl-nand-gate

Input stage of TTL Logic NAND circuit is realized with multiple-emitter transistor Q1. It has functionality of LOGICAL AND circuit. Parameters values are as follows: RB=4k7, RE2=1k, RC2=1k4, RC4=100Ω. Voltage supply for TTL logic family is 5V only. Transistor Q2 is logic inverter. It means that when input voltage is low, Q2 is cut-off and voltage on it’s collector is high. If input voltage is high, Q2 is saturated and voltage on it’s collector is 0.2V, i.e. low. in order to increase the speed of TTL Logic NAND, output stage is realized as an TTL Totem pole output shown in figure bellow.

ttl-totem-pole-output

This structure reminds a bit on totem pole, so that’s how it obtained name from. Output transistor is Q3 and it is in the same “logic phase” as Q2, as it operates as emitter follower. Point is how to make that this transistor operates faster and has higher logic level margin. For this reason, resistor RC4, transistor Q4 and diode D1 are added. If output is low, both Q3 and Q2 are saturated. Low voltage on Q2 collector will cut-off transistor Q4, making impedance in collector Q3 very high. This high impedance will separate VCC from the output that is low now. On the other hand, if output is “high”, then both Q2 and Q3 are high. Further, this mean that collector of Q3 has to be connected with VCC in order to provide high output. For that reason, RC4 is 100Ω only. Q4 has to be saturated. Base voltage of Q4 is “high”, since collector of Q2 is “high”. In order to make VBE4 voltage as high as possible, we need to reduce potential of Q4 emitter, even though output suppose to be “high”. For this reason, we use diode D1 as an passive voltage source, that will reduce Q4 emitter potential for amount of 0.6V – 0.7V. Transfer function of TTL Logic NAND circuit is given in figure bellow.

ttl-output-characteristic

Depending on technologies, there are following TTL circuit families:

  • Low-power TTL(L)
  • High-speed TTL(H)
  • Schottky TTL(S)
  • Low-power Schottky TTL(LS)
  • Fast TTL(F)
  • Advanced-Schottky TTL(AS)
  • Low-voltage TTL(LVTTL)

External links:

Transistor-Transistor, TTL Logic on Wikipedia
Transistor-Transistor, TTL Logic on St-andrews
TTL Totem pole output on Wiki.analog.com

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