Transistor Odds and Ends

1. Transistor Odds and Ends

2. RTL NOR

3. NOT from NOR

4. OR from NOR

5. AND from NOR

6. Other Logic Families • As one has an increasing number of logic gates one has to be concerned with their power performance and stability. • The logic gates can be made out of various combinations of resistors, diodes, and transistors. They differ in power and stability. • Let us examine an inverter from the TTL (transistor-transistor logic) family.

7. TTL Inverter

8. TTL Inverter

9. On-Off • Recall that a transistor can be thought of a switch. • When the switch is off, the transistor has very high resistance. • When the switch is on, the transistor has relatively low resistance. • It “transfers resistances.”

10. Transistor etymology

11. Totem Pole • The right-hand side of the TTL inverter is an arrangement of transistors known as a totem pole. • The transistors are arranged to that one is on and one is off. • The inverter output is just above the lower transistor in the totem pole. • If the lower transistor is on, there is little voltage drop across the lower transistor and so the output voltage is close to 0 (ground).

12. Totem Pole (Cont.) • If the lower transistor is off, then there is a large voltage drop across the lower transistor and so the output voltage is high. • One is almost directly connected to the high or the ground giving this arrangement good power/stability characteristics.

13. Similar arrangement/Opposite idea • In the totem pole arrangement, one guarantees that one of the two transistors is on and the other is off – giving a low-resistance connection to high or low as the case may be. • If we arrange for a third possibility that both transistors are off, then there is a high resistance between the output and both the high and the low. • This high-resistance or high-impedance state is neither high nor low, but effectively disconnected.

14. Poor Man’s TriState (Enabled Data Low)

15. Poor Man’s TriState (Enabled Data High)

16. Poor Man’s TriState (Not Enabled)

17. Poor Man’s TriState (Not Enabled)

18. Same idea as the tri-state buffer • This circuit has the essential ingredients to make a tri-state buffer. • Recall that tri-state buffers are used in conjunction with buses. • When one has several devices that could place their information on the bus (“drive the bus”) only one of them should. • If two devices attempt to drive the bus to opposite voltage levels, there will be a short.

19. Three State Logic

20. Tri-state buffer Compare the Electronics Workbench tri-state buffer to the previous circuit made of transistors and logic gates.

21. In the high impedance state

22. In the high impedance state

23. In the “enabled” state

24. In the “enabled” state

25. Sequential Logic • Whereas combinatorial logic depends only on the current inputs, sequential logic can also depend on the previous “state” of the system. • Circuitry designed to hold a high or low state is known as a flip-flop. • A flip-flop is the smallest unit of RAM – random access memory. • Recall there are two basic categories of RAM: dynamic RAM (DRAM) and static RAM (SRAM).

26. Flip Flops • Flip-flops serve as the elementary units for memory in digital systems. Two features are needed: • 1. The circuit must be able to “hold” either state (a high or low output) and not simply reflect the input at any given time. • 2. But in some circumstances, we must be able to change (to “set” and “reset”) the values.

27. Remembrance of states past • The way in which the previous state information is held is different for different types of memory • In DRAM (dynamic random access memory), the state (1 or 0) is held by a charge (or lack thereof) remaining on a capacitor • Charges tend to leak off of capacitors, which is why DRAM must be periodically refreshed

28. Simple DRAM (Reset)

29. Simple DRAM (Set)

30. Simple DRAM (Hold)

31. Simple DRAM (Hold)

32. Simple DRAM (Reset)

33. Simple DRAM (Hold)

34. Simple DRAM (Hold)

35. Analog-to-Digital Converter ADC

36. Simple Digital to Analog Converter .111 corresponds to 7/8 7/8 of 5 is 4.375

37. Simple Digital to Analog Converter .100 corresponds to 1/2 1/2 of 5 is 2.5

38. Analog-to-Digital • We have seen a simple digital-to-analog converter, now we consider the reverse process • For this purpose we introduce a new circuit element — the comparator • A digital comparator would compare two binary inputs A and B and determine if A is larger than B (as well as if A = B). • An analog comparator would determine whether voltage A is larger than voltage B

40. Comparator (analog) + Input higher than – input, output is high

41. Comparator (analog) + Input lower than – input, output is low

42. 1-bit analog-digital converter Input voltage is less than half of reference voltage, result is low. Reference Voltage Input voltage

43. 1-bit analog-digital converter Input voltage is more than half of reference voltage, result is high. Reference Voltage Input voltage

44. Toward a 2-bit analog-digital converter

45. Toward a 2-bit analog-digital converter

46. Toward a 2-bit analog-digital converter

47. Toward a 2-bit analog-digital converter

48. Finish this truth table Doesn’t occur

49. Integrated circuit version Warning: may need to flip switch back and forth.

50. 3.7 / 5 (in Scientific Mode)