TTL (Transistor-Transistor Logic)

(TTL) A common semiconductor technology for building discrete digital logic integrated circuits. It originated from Texas Instruments in 1965.

+Today, TTL more commonly refers to a signal level where the "on" voltage is close to +5 volts as opposed to RS232 where the signals are +/- 12 volts or low voltage logic which may be "on" at 3.3, 3.0 or as little as 2 volts.

As integrated circuits (ICs) grew "real estate" became more important. Since it takes as much room on the IC chip to construct a diode as it does to construct a transistor, the designers looked for a way to avoid large numbers of input diodes.

     +Vcc
      /   +Vcc
      \    /
      /    \
      \    /
A -|<-+    +--- Output = not A * B * C * D
B -|<-+  |/ 
C -|<-+--|  
D -|<-+  |\
           Y
           /
           \
           /
           \
           |
          GND

They realized that the opposed diodes are the same as two junctions of a transistor. We can think of a bipolar transistor as two diodes placed very close together, with the point between the diodes being the transistor base. Thus, we can use transistors in place of diodes to obtain logic gates that can be implemented with transistors and resistors only. In fact, an inverter would have a single input diode, and we could replace the two opposed diodes with an NPN transistor to do the same job.

          +Vcc     +Vcc
           /        /
           \        \
           /        /
           \        \
         __|__      +--- Output1 = not Input
         /   \    |/
Input --'     '---|
                  |\
                    Y
                    +--- Output2 = Input
                    /
                    \
                    /
                    \
                    |
                   GND

A NAND gate could be constructed from a two-emitter transistor connected to a transistor inverter. The multi-emitter transistor is the critical piece of technology that makes TTL logic gates possible. Multiple emitters can be added to the input transistor with little increase in real estate so a multiple-input gate takes about the same space as an inverter. Less space, less cost. Many different combinations of input and output can be constructed without change to the basic gate.

This configuration has another significant advantage over the diode implementation. Besides being voltage--controlled switches, transistors also act as amplifiers. When the transistor base is undergoing a change in voltage, the transistor can amplify this change, thus speeding up the rate at which the transistor turns on or off. The result is faster gate switching.

Of course, with only a resistor to pull the output high, the speed and drive of the gate is limited. The designers of commercial TTL IC gates reduced that problem by modifying the output circuit by adding another set of transistors and diode. This is the "totem pole" output circuit of the first TTL series: The 7400/5400 TTL ICs.

           +Vcc
             /
             \
             /
             \
             +
           |/
Output1 ---|
           |\
             Y
            _|_
            \ /
           __V__
             |
             +------- Output TTL
           |/
Output2 ---|
           |\
             Y
              +
              |
              |
             GND

A major achievement of TTL logic is the ease with which different circuits can be interfaced and cascaded to form more complex logic functions. In part, this is due to the concepts of guaranteed voltage levels and noise margins. A guaranteed voltage is one at which circuits always detect the correct voltage level, within a specified temperature range (0-70\xa1 C), voltage range (5 V ± 5%), loading, and the parametric variance of the semiconductor devices themselves. TTL circuits are characterized by four voltage specifications: Voh, Vol, Vih, and Vil. Voh (output high voltage) is the minimum voltage at which the circuit delivers a logic 1. Vol (output low voltage) is the maximum voltage at which the circuit can produce a logic 0.

Similarly, Vih (input high voltage) is the minimum voltage at which a circuit detects a logic 1. Vil (input low voltage) is the maximum voltage at which it recognizes a logic 0.

For TTL circuits, Voh = 2.4 V, Vol = 0.4 V, Vih = 2 V, and Vil = 0.8 V. The input and output voltages differ by 0.4 V. This permits the output signals to be degraded by the wires between circuits but still be recognized as good logic values. The difference between Voh and Vih is called the high-state DC noise margin. The difference between Vol and Voh is called the low-state DC noise margin.

+ (up) +

David A Cary of Motorguide Pinpoint Says:

Figure 3 is not yet accurate.
nit: the bottom (emitter) of the lower transistor connects directly to ground, there is no resistor there

+

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