Transistor Transistor Logic ( TTL )
- The TTL is so named because of its independence from transistors alone to perform basic logic operations.
- Moreover, The Transistor Transistor Logic uses transistors operating in a saturated mode.
- It is the fastest of saturated logic families.
- Also, The basic Transistor Transistor Logic logic circuit is the NAND gate.
- Good speed, low manufacturing cost, a wide range of circuits and the availability in SSI and MSI are its advantages.
- So, Tight VCC tolerance, relatively high power consumption, moderate packing density, generation of noise spikes and susceptibility to power transients are its disadvantages.
- Transistor Transistor Logic logic family consists of several subfamilies such as:
- Standard TTL
- High-Speed TTL
- Low Power TTL
- Schottky TTL
- Low Power Schottky Transistor Transistor Logic
- Advanced Schottky TTL
- Advanced Low Power Schottky TTL
- F (Fast) Transistor Transistor Logic
For standard Transistor Transistor Logic,
- Propagation Delay time = 9 ns
- Power dissipation per = 10 mW
- Noise Margin = 0.4 mV
- Fan-in = 8
- Fan-out = 10
- Logic ‘0’ = 0 V to 0.8 V
- Logic ‘1’ = 2 V to 5 V
- Indeterminate Range = 0.8 V to 2 V
Emitter-Coupled Logic (ECL)
- This logic family is also called Current Mode Logic or Current Steering Logic.
- It is the fastest of all logic families.
- Moreover, ECL operates on the principle of current switching, whereby a fixed bias current less than IC switched from one transistor’s collector to another.
- Because of this mode operation, this login form is also referred to as Current Mode Logic (CML).
- It is also called Current Steering Logic (CSL) because a current is steered from one device to another.
- Also, The ECL family is used in very high-frequency applications where its speed is superior.
The important characteristics of ECL are: Transistor Transistor Logic
- Transistors never saturate. So, speed is high o Logic Levels are negative, -0.9 V for Logic 1 and -1.7 V for Logic 0.
- Noise Margin is less, about 250 mV. This makes ECL, unreliable for use in the heavy industrial environment.
- ECL circuits produce the output and its complement, and therefore, eliminate the need for inverters.
- Moreover, Fan-out is large because the output impedance is low. It is about 25.
- Power dissipation per gate is large.
- So, The total current flow in ECL is more or less constant. So, no noise spikes will be internally generated.
- MOSFET family are simpler & inexpensive to fabricate, require much less power, have better noise margin, a greater supply voltage range, a higher fan-out and require much less chip area as compared to other bipolar logic families.
- For MOS logic,
- Propagation Delay, tpd = 50 ns.
- Noise Margin, NM = 1.5 V.
- Power Dissipation, PD = 0.1 Mw.
- Fan out = 50 for frequencies greater than 100 Hz and it is virtually unlimited for dc or low frequencies.
- So, The propagation delay associated with MOS gates is large (50 ns) because of their high output resistance (100 k) and capacitive loading presented by the driven gates.
There are three types of MOSFET:
- P-channel MOSFET (PMOS)
- Enhancement Type PMOS
- Depletion Type PMOS
- N-channel MOSFET (NMOS)
- Enhancement Type NMOS
- Depletion Type NMOS
- Complementary MOSFET (CMOS)