- TTL Logic Definition: Transistor Transistor Logic (TTL) is a digital logic circuit made using NPN transistors, PN junction diodes, and resistors.
- Standard TTL: Standard TTL uses NAND gates as the basic building block, featuring quad two-input circuits with NPN transistors.
- Low Power TTL: This subfamily reduces power consumption by using higher resistance values, though it operates at a slightly slower speed.
- High Power TTL: High power TTL focuses on high-speed operation with higher power dissipation, using additional transistors and resistors.
- Schottky TTL: Schottky TTL doubles the speed of high power TTL without extra power use, employing Schottky diodes for faster switching.
TTL means Transistor Transistor Logic, a bipolar digital family built from NPN transistors, PN junction diodes and diffused resistors. The usual gate is a NAND gate. Subfamilies include standard 74, low-power 74L, high-power 74H, Schottky 74S plus later LS, ALS and 74F parts. The sections below cover the inner circuit of the main groups.
Standard TTL
The figure shows the inner circuit of a standard TTL NAND gate, a quad two-input device. The 5400/7400 package holds four such gates. Input is through Q1, a two-emitter NPN transistor.
That multi-emitter input acts like two transistors with their bases and collectors in common. Clamp diodes D2 and D3 limit negative input voltages.
Low Power TTL
Low Power TTL (74L) cuts dissipation by raising resistor values, so the gate is slower. The figure shows a low power TTL using AND gates. The commercial 74L00 or 54L00 is a quad two-input NAND, like standard TTL, but with higher resistance values so less power is drawn.
High Power TTL
High-power TTL (74H) is the faster, hungrier version of standard 74. The diagram is a high-power NAND, type 74H00 or 54H00, a quad two-input gate. The circuit follows standard TTL except the Q3 transistor and D1 diode pair, which is replaced by Q3, Q5 and R5. Delay falls and dissipation rises.
Schottky TTL
Schottky TTL clamps each bipolar transistor with a schottky diode from base to collector so the device does not saturate deeply. That cuts storage delay. 74S is faster than 74H; it still dissipates more than standard 74. The figure is a Schottky NAND, close to the 74H circuit. Low-power Schottky and advanced Schottky are later members of the same idea.
