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CHAPTER ELEVEN

CHAPTER ELEVEN. Basic Emitter-Coupled Logic [ ECL ]. Digital Electronics. Introduction. E mitter- C oupled L ogic (ECL). Analogous to the analog difference amplifier. The BJTs in ECL circuits do not operate in saturation mode, but either in cut-off or forward-active modes.

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CHAPTER ELEVEN

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  1. CHAPTER ELEVEN Basic Emitter-Coupled Logic [ ECL ] Digital Electronics.

  2. Introduction Emitter-Coupled Logic (ECL) Analogous to the analog difference amplifier The BJTs in ECL circuits do not operate in saturation mode, but either in cut-off or forward-active modes The ECL circuits are the fastest switching time of commercially digital circuits.  Typical propagation delay times are on the order of 1ns, allowing for clock frequencies up to 1GHz. However, ECL circuits have the highest power dissipation of all logic families, typically 25mW per gate. 

  3. BJT Current Switch This figure shows an ideal BJT current switch Basic ECL current switch The input is at the base of QI , and VBB is a constant reference voltage Non-Inverting The coupled emitters are ideally connected to a constant current source IEE. Inverting

  4. BJT Current Switch Resistor ECL current switch This figure shows an early ECL implementation Non-Inverting Outputs are taken at the collectors of QI and QR. Inverting and

  5. Voltage Transfer Characteristics of ECL Current Switch Resistor ECL current switch Output High Voltage Non-Inverting For VI<VBB, QI is off, QR is in the forward-active mode Inverting Proof: with QR F.A.: and <0 For VI<VBB (Low), QI is off, i.e. IC,I=0

  6. Voltage Transfer Characteristics of ECL Current Switch Resistor ECL current switch Threshold Voltage Non-Inverting For VI=VBB, both QI and QR are in the forward-active mode (VBE,I=VBE,R) Inverting assuming and

  7. Voltage Transfer Characteristics of ECL Current Switch Input high and low Voltages Resistor ECL current switch For VI is slightly less than VBB, QI is forward-active mode BUT not conducting as heavily as QR. Non-Inverting Inverting For VI is slightly greater than VBB, QR is forward-active mode BUT not conducting as heavily as QI. Experimentally, the transition width is found to be about VTW=0.1V and centered around VI=VBB

  8. Voltage Transfer Characteristics of ECL Current Switch Resistor ECL current switch Output low Voltage For VI > VBB, QI begins to conduct. Non-Inverting Inverting As VI increases, VE also increases, while VB,R=VBB (fixed) Thus, raisingVI by 0.05V beyond VBB, VBE,R sufficiently decreases to cut-off QR. The input voltage which turns QR off is VI=VIH resulting in VO=VOL.

  9. Voltage Transfer Characteristics of ECL Current Switch Resistor ECL current switch Output low Voltage Non-Inverting Inverting

  10. Voltage Transfer Characteristics of ECL Current Switch VTC beyond VIH Resistor ECL current switch As VI increases beyond VIH Non-Inverting The output voltage VO decreases linearly with VI Inverting QI will eventually saturate with further increase of VI if: Saturation voltage This region of saturation is avoided

  11. Voltage Transfer Characteristics of ECL Current Switch QR( DOES NOT SATURATE) QI(sat)

  12. Voltage Transfer Characteristics of ECL Current Switch • Example Calculate the critical values of the VTC of VI for ECL current switch shown previously assuming: VEE=0V, VBB=2.6V, VBE(ECL)=0.75V, VBC(sat)=0.6V VCC=5V,RCI=RCR=RE=1kΩ, • Solution

  13. Basic ECL NOR/OR Gate Adding additional input transistors with coupled collectors and coupled emitters to the ECL current switch:VINV becomes NOR output and VNINV becomes OR output. For any high-state input, the corresponding transistor is forward-active and then the corresponding collector current flows through RCI and If all inputs are low, then all the corresponding transistors are cut-off and then

  14. MECL I NOR/OR Gate with Output Buffer First ECL gate in its (NOR/OR form) for Motorola using output buffers is called MECL I • QB,N and QB,O are output buffers and provide large sourcing current {note: VC>VB} to the load (larger fan-out). • QB,N and QB,O also provide voltage level shift by VBE(ECL) resulting in an output voltage swing in a range compatible with the input voltage swing

  15. VTC of MECL I NOR/OR Gate with Output Buffer Output High Voltage When all inputs are low; VI‘s are low, QI‘s are cut-off

  16. VTC of MECL I NOR/OR Gate with Output Buffer Input Low and High Voltages

  17. VTC of MECL I NOR/OR Gate with Output Buffer Output Low Voltage If one input is high; VI is high, QI is forward-active

  18. VTC of MECL I NOR/OR Gate with Output Buffer VNOR Saturation Region As VI increases beyond VIH Refer to slide 10 of the last lecture

  19. VTC of MECL I NOR/OR Gate with Output Buffer • Example Calculate the critical values of the VTC of VI for MECL I circuit shown previously assuming: VEE=-5.2V, VBB=-1.175 V, VCC=0V, βF=49 VBC(sat)=0.6V, VBE(FA)=0.75V, VBE(sat)=0.8V RCI=0.27kΩ, RCR=0.3kΩ , RE=1.24kΩ , and RD,O=RD,N=2kΩ • Solution

  20. VTC of MECL I NOR/OR Gate with Output Buffer • Example Calculate the critical values of the VTC of VI for MECL I circuit shown previously assuming: VEE=-5.2V, VBB=-1.175 V, VCC=0V, βF=49 VBC(sat)=0.6V, VBE(FA)=0.75V, VBE(sat)=0.8V RCI=0.27kΩ, RCR=0.3kΩ , RE=1.24kΩ , and RD,O=RD,N=2kΩ • Solution

  21. VTC of MECL I NOR/OR Gate with Output Buffer • Example Calculate the critical values of the VTC of VI for MECL I circuit shown previously assuming: VEE=-5.2V, VBB=-1.175 V, VCC=0V, βF=49 VBC(sat)=0.6V, VBE(FA)=0.75V, VBE(sat)=0.8V RCI=0.27kΩ, RCR=0.3kΩ , RE=1.24kΩ , and RD,O=RD,N=2kΩ • Solution QI(sat)

  22. Fan-Out of MECL I NOR/OR Gate with Output Buffer When QI1 is cut- off, VC1 is high and therefore VNOR is also high state The maximum fan-out of an ECL is therefore dependent on the output high state of the driving gate M-Fan-Out M- Load gates

  23. Fan-Out of MECL I NOR/OR Gate with Output Buffer M-Fan-Out M- Load gates

  24. Fan-Out of MECL I NOR/OR Gate with Output Buffer • Example Calculate the maximum fan-out in the last example, assuming that the load gates have reduced VOH of the driving gate from -0.76V to -0.79V: • Solution

  25. Power-Dissipation in MECL I circuits Output high current supplied (ICC(H))+(IEE(H))+(IBB(H)) For High output,Input is low Very small compared to IEE(OH)

  26. TTL Power-Dissipation in MECL I circuits Output low current supplied (ICC(L))+(IEE(L)) For Low output,Input is high Zero if VCC=0

  27. Power-Dissipation in MECL I circuits • Example Calculate the dissipated power in the driver gate for the last example • Solution

  28. HW #9:Solve Problems: 11.1-3, 11.08-10, and 11.13-19

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