Introduction • Uses bipolar technology including NPN transistors, diodes and resistors. • The NAND gate is the basic building block • Contains different subfamilies with varying characteristics such as speed and power consumption
NAND Output Current Sinking & Sourcing Sinking Sourcing
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Totem pole in TTL ? • What is totem pole? • addition of an active pull up circuit in the output of a gate is called totem pole. Using Q3 and Q4 to achieve this purpose • Why totem pole? • To increase the switching speed of the gate which is limited due to the parasitic capacitance at the output.
Low Power TTL (74L) Same as standard TTL Except that the value of the resistance is increased to lower power dissipation. However, this is at the expense of reduced speed.
High Speed High Power TTL (74H) Reduced Resistors increase power dissipation. Removed output diode and added a Darlington pair transistor to reduce output resistance
Standard TTL Transistor Switching Problem • Trs are driven into deep saturation to fully conduct, or cutoff to switch off. • The result of deep saturation is that the two junctions are now forward biased. • The forward biasing of the BC junction forces a large number of minority carriers to the collector region. • When the Tr switches off, these minority carriers needs to be removed. This takes a finite amount of time called the storage time (major component of the propagation delay) and thus increases the switch off time.
Solution 1 • Prevent the Tr from going deep in saturation. This accomplished by preventing the BC junction from becoming forward biased. • The Shottky diode is used to do the above by placing it across the BC junction. Because of its lower barrier potential, it will conduct current from the base directly to the collector before the BC is forward biased. Thus less carriers are stored in the collector area and the switching becomes much faster.
SOLUTION 2 Normal Transistor in saturation Transistor with Shottky diode in saturation
Advanced Low Power Shottky 74ALS • All saturating transistors are clamped by using Schottky diodes. • Inputs and outputs are clamped by Schottkydiodes • New manufacturing and materials reduce stray capacitance to increase speed. • active turn-off of the LOW-level output transistor, producing a better HIGH-level output voltage and thus a higher HIGH-level noise immunity.