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The 741 Operational-Amplifier

The 741 Operational-Amplifier. 1. Reference Bias Current : The 741 op-amp circuit. . Reference Bias Current. V CC. -V EE. DC Analysis of the 741 Reference Bias Current. Bias for input stage The 741 op-amp circuit. . Bias for input stage. I C10. Input Stage Bias. +. +. V BE11. V BE10.

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The 741 Operational-Amplifier

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  1. The 741 Operational-Amplifier 1

  2. Reference Bias Current :The 741 op-amp circuit. Reference Bias Current

  3. VCC -VEE DC Analysis of the 741Reference Bias Current

  4. Bias for input stageThe 741 op-amp circuit. Bias for input stage IC10

  5. Input Stage Bias + + VBE11 VBE10 - - + + - IC10 IC10can be determined by knowing IREF and R4

  6. Biasing Input Stage : The 741 op-amp circuit. IC1 IC2 IC3 IC4

  7. The dc analysis of the 741 input stage. npnβ very high RelationshipIREF & IO Base current IE/1+βP Base currents add together

  8. A Simple BJT Current Source

  9. Negative Feed-back Loop : 741 input stage. Causes current pulled from Q8 to increase For some reason I in Q1 & Q2 increases Output current of Q8 Q9 correspondingly increase Negative Feed back Loop Since Ic10 remains constant, it forces combined current of Q3 & Q4 to decrease

  10. Input Stage : The 741 op-amp circuit. IC7

  11. 2 5 1 3 6 7 4 The dc analysis of the 741 input stage, continued. Ic5=I Bias Current of Q7

  12. The dc analysis of the 741 input stage, continued. 50 KΩ

  13. The 741 op-amp circuit : SECOND STAGE IC16 IC17

  14. DC Analysis : Second Stage Q13 is lateral pnp transistor Q13B has a scale of 0.75 times that of Q12 IC13B=0.75 IREF IREF = 0.73 mA & βP>>1 IC13B = 550 µA & IC17 = 550 µA Neglect Base Current of Q23 IC17=IC13B

  15. Output Stage Bias : The 741 op-amp circuit.

  16. The 741 output stage without the short-circuit protection devices. Q13 is lateral pnp transistor IS of Q13A is 0.25 times IS of Q12 Neglect Base current of Q14 & Q20 Base Current of Q23 is 180/50=36 μ A Negligible as assumed

  17. + VBB - 14 20 Output Stage Bias Voltage VBE18 ≈ 0.6V Current Thru R10=0.6/40k=15 µ A

  18. Summary Collector Currents : 741 Op Amp

  19. Small-signal analysis of the 741 input stage. Collectors Q1 & Q2 connected to dc voltage so are grounded Q3 & Q3 are biased by constant current source so are open cct

  20. Small-signal analysis of the 741 input stage. Input appears across four input resistors

  21. The 741 op-amp circuit.

  22. Small signal model : The load circuit of the input stage.

  23. The load circuit of the input stage Q5 & Q6 are identical and their bases are tied together So their collector currents are equal

  24. The load circuit of the input stage

  25. Output Resistance : 741 Op Amp Ro1 is parallel equivalent of Ro4 & Ro6 Assume that common bases of Q3 & Q4 are at virtual ground Output Resistance Ro1

  26. Output Resistance : 741 Op Amp Assume that the base of Q6 is at virtual ground Because signal is very small Output Resistance Ro1

  27. Output Resistance : 741 Op Amp Ro1 is parallel equivalent of Ro4 & Ro6 Ro1=Ro4||Ro6 Ro1=6.7 MΩ Output Resistance Ro1

  28. Figure 9.22 Small-signal equivalent circuit for the input stage of the 741 op amp.

  29. Second Stage :The 741 op-amp circuit.

  30. Figure 9.24 The 741 second stage prepared for small-signal analysis.

  31. Input Resistance : Second Stage

  32. Transconductance : Second Stage Thus current through the output resistance of Q13B is zero

  33. Output Resistance R02 : Second Stage R02

  34. Output Resistance R02 : Second Stage Since the resistance between the base of Q17 and ground is relatively small, The base is grounded and circuit is CB R02

  35. Output Resistance R02 : Second Stage R02

  36. Output Resistance R017 Since the resistance between the base of Q17 and ground is relatively small, The base is grounded and circuit is CB

  37. Figure 9.25 Small-signal equivalent circuit model of the second stage.

  38. Figure 9.27 Thévenin form of the small-signal model of the second stage. Open Circuit Voltage Gain =

  39. Output Stage :The 741 op-amp circuit.

  40. The 741 output stage.

  41. The 741 output stage. • Input from second stage Q17 • Loaded with 2 kΩ resistor • Q18 & Q19 and R10 provide Class AB bias to output stage. • Q14 & Q20are output transistors • Output stage is driven by emitter follower Q23 acts as buffer

  42. Output Voltage Limits Maximum positive output voltage vomax is limited by input circuit Saturation of Q13A Minimum negative output voltage vomin is limited by input circuit Saturation of Q17

  43. Small Signal Model for the 741 output stage. vo2=-Gm2R02vi2 Gm2 = 6.5mA/V & RO2 = 81kΩ Rin3is input resistance of the output stage with load RL

  44. Rin Rin3 Input resistance Rin3of output stage

  45. Rout18 Rin Rin20 Input resistance Rin Rin=Rin20||Rout18 Suppose Q20 is conducting and Q14 is cut-off

  46. Rin20 Input resistance Rin20

  47. Rout18 Input resistance Rout18 Rout18 is ro13A in series with output resistance of Q18 & Q19 Output resistance of Q18 & Q19 = 163 Ω ro13A >>Output resistance of Q18 & Q19

  48. Rout18 Rin3 Rin Rin20 Input resistance Rin3 of the output stage Rin=Rin20||Rout18 β20 = β23 = 50 , RL= 2kΩ, ro13A= 280kΩ Rin3 = 3.7 MΩ

  49. Small Signal Model for the 741 output stage. Rin3 = 3.7 MΩ Ro2 = 81 kΩ Rin3 >> Ro2 So Rin3 will have little effect On the performance of the op amp = -515 V/V

  50. Open Circuit Overall Voltage Gain Gvo Small Signal Model for the 741 output stage.

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