TTL(Transistor Transistor Logic)


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TTL(Transistor Transistor Logic)

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TTL(Transistor Transistor Logic)

  1. 1. Diode-Transistor Logic (DTL)University of Connecticut 56
  2. 2. Diode Logic VA VOUT AND VCC VB GATE VA VOUT OR GATE VB n Diode Logic suffers from voltage degradation from one stage to the next. n Diode Logic only permits the OR and AND functions. n Diode Logic is used extensively but not in integrated circuits!University of Connecticut 57
  3. 3. Level-Shifted Diode Logic VCC=5V With either input at 0V, Vx=0.7V, AND DL is just cut off, and VOUT=0V. GATE RH With both inputs at 1V, VX=1.7V and VA VOUT VOUT=1V. x DL RL VB With VA=VB=5V, both input diodes are cut off. Then  VCC − 0.7V  VOUT = RL    RH + RL  n Level shifting eliminates the voltage degradation from the input to the ouput. However, n the logic swing falls short of rail-to-rail, and n the inverting function still is not available without using a transistor!University of Connecticut 58
  4. 4. Diode-Transistor Logic (DTL) VCC VA RC VB VOUT Primitive Precursor to DTL VC Q1 n If any input goes high, the transistor saturates and VOUT goes low. n If all inputs are low, the transistor cuts off and VOUT goes high. n This is a NOR gate. n “Current Hogging” is a problem because the bipolar transistors can not be matched precisely.University of Connecticut 59
  5. 5. Diode-Transistor Logic (DTL) VCC RB RC VOUT Improved gate with VA reversed diodes. VB Q1 VC n If all inputs are high, the transistor saturates and VOUT goes low. n If any input goes low, the base current is diverted out through the input diode. The transistor cuts off and VOUT goes high. n This is a NAND gate. n The gate works marginally because VD = VBEA = 0.7V.University of Connecticut 60
  6. 6. Diode-Transistor Logic (DTL) VCC RB RC Basic DTL VA x VOUT NAND gate. VB Q1 VC RD n If all inputs are high, Vx = 2.2V and the transistor is saturated. n If any input goes low (0.2V), Vx=0.9V, and the transistor cuts off. n The added resistor RD provides a discharge path for stored base charge in the BJT, to provide a reasonable tPLH.University of Connecticut 61
  7. 7. DTL VTC VCC 5 VOUT 4 RB RC 3 VA x VOUT 2 VB Q1 1 VC RD 0 0 1 2 3 4 5 VIN VOH = VIL = VOL = VIH = n The noise margins are more symmetric than in the RTL case.University of Connecticut 62
  8. 8. DTL Power Dissipation RB=3.4kΩ VCC RC=4.8kΩ PL = RD=1.6kΩ RB RC VA x VOUT VB Q1 VC RD PH = n Scaling RB and RC involves a direct tradeoff between speed and power. P=University of Connecticut 63
  9. 9. DTL Fanout RB=3.4kΩ RC=4.8kΩ VCC I CS = RD=1.6kΩ βF=50 RB RC VA x VOUT VB Q1 VC RD I BS = I CS = n Good fanout requires high β F, large RD /RB. N max =University of Connecticut 64
  10. 10. 930 Series DTL (ca 1964 A.D.) βF=50 VCC n One of the series diodes is replaced by Q1, providing 1.75kΩ 6kΩ more base drive for Q2 and improving the fanout (Nmax 2kΩ = 45). VA Q1 n Does Q1 saturate? VOUT VB Q2 VC 930 DTL Characteristics 5kΩ VOH / VOL 5.0V / 0.2V VIH / VIL 1.5V / 1.4V Fanout 45 Dissipation 10mW tP 75nsUniversity of Connecticut 65
  11. 11. 930 DTL Propagation Delays βF =50 CL=5pF VCC n tPLH >> tPHL 1.75kΩ 6kΩ n tPLH = tS+tr /2 2kΩ ts = VA Q1 VOUT VB Q2 VC 5kΩ tr ≈University of Connecticut 66
  12. 12. Transistor-Transistor Logic (TTL)University of Connecticut 67
  13. 13. Why TTL? n The DTL input uses a VA number of diodes which take VB DTL up considerable chip area. VC n In TTL, a single multi-emitter BJT replaces the input diodes, resulting in a more VA area-efficient design. VB TTL VC n DTL was ousted by faster TTL gates by 1974.University of Connecticut 68
  14. 14. Basic TTL NAND Gate. VCC=5V n ALL INPUTS HIGH. • QI is reverse active. RB RCP • QO is saturated. VOUT D1 • VOL = VCES VA QI QO VB RD n ANY INPUT LOW. VC • QI is saturated. • QO is cut off. • VOH = VCC Multi-emitter transistor. Forward-biased emitter base junctions override reverse-biased junctions in determining the base and collector currents.University of Connecticut 69
  15. 15. TTL Switching Speed: tPLH VCC=5V n The depletion capacitance of the QI EB junction must discharge; RB RC RCP n Base charge must be removed from the saturated VOUT QS; VA QI QS QO n Ditto for QO; and VB CL n The capactive load must be VC RD charged to VCC. Multi-emitter transistor. Forward-biased emitter base junctions override reverse-biased junctions in determining the base and collector currents.University of Connecticut 70
  16. 16. TTL Switching Speed: tPLH VCC=5V n The time required to discharge the QI depletion layers is << 1ns. RB RC RCP n The time required to extract the QS base charge is also VOUT << 1ns: VA QI QS VB QO • QI becomes forward CL active; VC RD • |IBR| becomes large for QS n Removal of base charge from QO is similar to the DTL case. With RD = 1 kΩ, ts = 10ns (these are typical values for 7400 series TTL).University of Connecticut 71
  17. 17. TTL Switching Speed: tPLH VOUT VCC=5V 5 4 RB RC RCP 3 VOUT 2 VA QI QS VB QO 1 CL VC RD 0 t (ns) 0 100 200 300 n Charging of the capacitive load can be slow with “passive pullup.” e.g., with a 5kΩ pull-up resistor and a 15 pF load (ten TTL gates) RC = 75 ns and tr = 2.3RC = 173 ns!University of Connecticut 72
  18. 18. TTL with Active Pullup n In the previous example, the dominant switching speed limitation was the charging of capacitive loads through the pullup resistor. n A small pullup resistance will improve the switching speed but will also increase the power and reduce the fanout. VCC With active pullup, we can achieve the best of both worlds: RC n When the output is low, QP is cutoff, QP minimizing the power and maximizing the VOUT fanout; QO n when the output goes high, QP becomes forward active to provide maximum drive RD current for a quick rise time.University of Connecticut 73
  19. 19. TTL with Active Pullup n With a high output, VCC=5V n QS is cutoff RB RC n QP is forward active n With a low output, QP VOUT n QS is saturated VA QI QS n QP should be cutoff VB QO The low output case is unsatisfactory VC RD with this circuit: VBP = VEP = VBEP = The “Totem Pole Output” solves this problem.University of Connecticut 74
  20. 20. TTL with “Totem Pole Output” VCC=5V n During turn-off, QS switches off before QO. RB RC RCP n QP begins to conduct when VCS = VCESO+VD+VBEAP QP = 1.6V n Initially, VA QI QS DL VB VOUT I BP = VC QO RD RCP limits the collector current to a safe value.University of Connecticut 75
  21. 21. Typical 74xx Series TTL VCC=5V 74xx TTL Characteristics tPLH 12 ns 4kΩ 1.6kΩ 130Ω tPHL 8 ns Fanout 10 QP Dissipation 10 mW VA QI QS PDP 100 pJ DL VB VOUT VC QO The anti-ringing diodes at the 1kΩ input are normally cut off. During switching transients, 1/3 T. I. 7410 they turn on if an input goes triple 3-input NAND more negative than -0.7V.University of Connecticut 76
  22. 22. Standard TTL: VTC β F =70 VCC=5V n VIN=0. QI is saturated; QS, QO β R = 0.1 are cutoff; QP is forward active. 4kΩ RC 130Ω 1.6kΩ VOH = QP VIN QI QS DL (the drop in the base resistor is VOUT small) QO n First Breakpoint. QS turns on. 1kΩ VIL = 1/6 NSC 7404 Hex Inverter (at the edge of conduction, IC=0)University of Connecticut 77
  23. 23. Standard TTL: VTC β F =70 VCC=5V n Second Breakpoint. QO turns on. β R = 0.1 RC VIN = 4kΩ 130Ω 1.6kΩ VOUT = QP VIN QI QS DL VOUT n Third Breakpoint. QO saturates. QO VIH = 1kΩ 1/6 NSC 7404 Hex InverterUniversity of Connecticut 78
  24. 24. Standard TTL: VTC 1/6 NSC 7404 VCC=5V VOUT Hex Inverter 5 4kΩ RC 130Ω 4 1.6kΩ 3 QP QI QS 2 VIN DL VOUT 1 QO β F =70 β R = 0.1 1kΩ 0 VIN 1 2 3 4 5 VNML = VNMH =University of Connecticut 79
  25. 25. Standard TTL: Low State ROUT 100 2.4 mA 2 mA For the saturated BJT 1.6 mA with IB = 2.4 mA, the collector current (mA) 80 output impedance is 0.08V 1.2 mA 60 0.8 mA ROL = 21 mA 40 The very low output IB = 0.4mA impedance means that 20 noise currents are translated into tiny Characteristics for a 0 noise voltages. Thus Texas Instruments only a small noise junction isolated 0 0.1 0.2 0.3 0.4 0.5 digital npn BJTat 25 oC. margin is neccesary. collector-emitter voltage (V)University of Connecticut 80
  26. 26. Standard TTL: Input Current 1/4 TI 7400 Quad VCC=5V n IIH (QI is reverse active) 2-input NAND RC I BI = 4kΩ 130Ω 1.6kΩ QP VA QI QS DL VB VOUT I IH = QO β F =70 β R = 0.1 1kΩ n IIL (QI is saturated) I IL =University of Connecticut 81
  27. 27. Standard TTL: DC Fanout 1/4 TI 7400 Quad VCC=5V n With high inputs, 2-input NAND I CI = 4kΩ RC 130Ω 1.6kΩ I CS = QP VA QI QS DL I BO = VB VOUT QO n To keep QO saturated, β F =70 β R = 0.1 1kΩ N max = AC considerations usually limit the fanout to a much lower number.University of Connecticut 82
  28. 28. Standard TTL: DC Dissipation 1/4 TI 7400 Quad VCC=5V PH = 2-input NAND 4kΩ RC 130Ω 1.6kΩ QP VA QI PL = QS DL VB VOUT QO β F =70 β R = 0.1 1kΩ N = 10 P=University of Connecticut 83
  29. 29. Advanced TTL Designs n Schottky Clamping. QS and QO may be Schottky clamped, preventing saturation. This greatly improves tPLH. n Darlington Pullup. The Darlington pullup arrangement increases the average output drive current for charging a capacitive load. Although RCP limits the maximum output current, this maximum drive is maintained over a wider range of VOUT than with a single pullup transistor. n Squaring Circuit. Active pull-down for the base of the output transistor squares the VTC, improving the low noise margin. An added benefit is faster charge removal for the output transistor. n Improved Fabrication. Smaller devices, and oxide isolation, have steadily reduced parasitic capacitances and reduced RC time constants.University of Connecticut 84
  30. 30. Darlington Pullup n QP2 is added, forming a Darlington VCC=5V pair with QP. RC n The EB junction of QP2 introduces RCP 900Ω 50Ω a 0.7V level shift, so DL can be QP eliminated. QP2 n QP2 can not saturate, so Schottky clamping is not neccessary. REP 3.5kΩ VOUT n REP is needed to provide a discharge path for QP2 base charge. The Darlington emitter follower provides a very low output impedance, approaching RC /β 2. This greatly reduces the rise time.University of Connecticut 85
  31. 31. Squaring Circuit VOUT 5 VIL 7400 VOUT QS 74S00 QO VOH 4 BP2 RBD 3 RCD 500Ω 2 250Ω QD 1 BP3 VOL 0 VIN 1 2 3 4 5 VIH n There is no path for QS emitter current until QD and QO turn on. n QS and QO begin to conduct simultaneously. n BP1 is eliminated from the VTC; in other words, the VTC is “squared.” n VIL is increased, improving the low noise margin.University of Connecticut 86
  32. 32. Schottky TTL (74S / 54S Series) 1/4 74S00 quad 2-input NAND VCC=5V Features: RB RC RCP n Schottky clamping 2.8kΩ 900Ω 50Ω n Schottky anti-ringing diodes QP n Darlington pullup circuit QP2 n Squaring circuit QI QS REP VA 3.5kΩ n Scaled resistors VB VOUT QO Performance: RBD RCD 500Ω n P = 20 mW 250Ω QD n tP = 3 ns (15 pF) n PDP = 60 pJUniversity of Connecticut 87