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Operational Amplifier (OpAmp)


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Operational Amplifier (OpAmp)

  1. 1. Opamp
  2. 2. The 741 Op-Amp Circuit • Schematic diagram of OP-AMP consists: • The input stage • The intermediate stage • The output stage • The biasing circuits
  3. 3. Schematic diagram of lm741
  4. 4. • 24 transistors, few resistors and only one capacitor • Two power supplies • Short-circuit protection General Description
  5. 5. • The input stage consists of transistors Q1 through Q7. • Q1-Q4 is the differential version of CC and CB configuration. • High input resistance. • Current source (Q5-Q7) is the active load of input stage. It not only provides a high- resistance load but also converts the signal from differential to single-ended form with no loss in gain or common-mode rejection. The Input Stage
  6. 6. • The intermediate stage is composed of Q16, Q17 and Q13B. • Common-collector configuration for Q16 gives this stage a high input resistance as well as reduces the load effect on the input stage. • Common-emitter configuration for Q17 provides high voltage gain because of the active load Q13B. The Intermediate Stage
  7. 7. • The output stage is the efficient circuit called class AB output stage. • Voltage source composed of Q18 and Q19 supplies the DC voltage for Q14 and Q20 in order to reduce the cross-over distortion. • Q23 is the CC configuration to reduce the load effect on intermediate stage. The Output Stage
  8. 8. (a) The emitter follower is a class A output stage. (b) (b) Class B output stage. The Output Stage
  9. 9. Wave of a class B output stage fed with an input sinusoid. Positive and negative cycles are unable to connect perfectly due to the turn-on voltage of the transistors. This wave form has the nonlinear distortion called crossover distortion. To reduce the crossover distortion can be implemented by supplying the constant DC voltage at the base terminals. The Output Stage
  10. 10. QN and QP provides the voltage drop which equals to the turn-on voltages of QN and QP. This circuit is call Class AB output stage. The Output Stage
  11. 11. • Short-circuit protection circuitry Forward protection is implemented by R6 and Q15. Reverse protection is implemented by R7, Q21, current source(Q24, Q22) and intermediate stage. Short-circuit protection
  12. 12. • Reference current is generated by Q12, Q11 and R5. • Wilder current provides biasing current in the order of μA. • Q13B provides biasing current for intermediate stage, Q13A for output stage. • Q5, Q6 and Q7 is composed of the current source to be an active load for input stage. The Biasing Circuits
  13. 13. Ideal Opamp
  14. 14. Equivalent Circuit of the Ideal Op Amp
  15. 15. Characteristics of the Ideal Op Amplifier The ideal OPAMP has the following characteristic : Differential Input resistance Ri= ∞ Output resistance Ro = 0 Differential voltage gain Av=- ∞ Bandwidth = ∞ Offset voltage and current is zero. a) No difference voltage between inverting and noninvertying terminals. b) No input currents. A Vo = (A V -A V ) = A (V - V ) + + - - OP AMP is a direct coupled high gain amplifier to which feedback is added to control its overall response characteristic
  16. 16. Operational Amplifier (OP AMP) Basic and most common circuit building device. Ideally, 1. No current can enter terminals V+ or V-. Called infinite input impedance. 2. Vout=A(V+ - V-) with A ∞→ 3. In a circuit V+ is forced equal to V-. This is the virtual ground property 4. An opamp needs two voltages to power it Vcc and -Vee. These are called the rails. A Vo = (A V -A V ) = A (V - V ) + + - -
  17. 17. OPAMP: COMPARATOR Vout=A(Vin – Vref) If Vin>Vref, Vout = +∞ but practically hits +ve power supply = Vcc If Vin<Vref, Vout = -∞ but practically hits –ve power supply = -Vee Compare the voltage of one input with the voltage with other input Two types: inverting comparator when the reference voltage apply to the inverting terminal non inverting comparator when the reference voltage apply to the non inverting terminal A (gain) very high
  18. 18. 24 (a) The unity-gain buffer or follower amplifier. (b) Its equivalent circuit model. V+ = VIN. By virtual ground, V- = V+ Thus Vout = V- = V+ = VIN !!!! OPAMP: VOLTAGE FOLLOWER
  19. 19. SJTU Zhou Lingling 25 The inverting closed-loop configuration. Virtual ground. OPAMP: The Inverting Configuration
  20. 20. 26 OPAMP: The Inverting Configuration
  21. 21. SJTU Zhou Lingling 27 OPAMP: The Inverting Configuration
  22. 22. OPAMP: INVERTING AMPLIFIER 1. V- = V+ 2. As V+ = 0, V- = 0 3. As no current can enter V- and from Kirchoff’s Ist law, I1=I2. 4. I1 = (VIN - V-)/R1 = VIN/R1 5. I2 = (V- - VOUT)/R2 = -VOUT/R2 => VOUT = -I2R2 6. From 3 and 5, VOUT = -I2R2 = -I1R2 = -VINR2/R1 7. Therefore VOUT = (-R2/R1)VIN 8. Gain = V / V = - R / R
  23. 23. SJTU Zhou Lingling 29 The noninverting configuration. Series-shunt negative feedback. OPAMP: The Non Inverting Configuration
  24. 24. 30 OPAMP: The Non Inverting Configuration
  25. 25. OPAMP: NON – INVERTING AMPLIFIER 1. V- = V+ 2. As V+ = VIN, V- = VIN 3. As no current can enter V- and from Kirchoff’s Ist law, I1=I2. 4. I1 = VIN/R1 5. I2 = (VOUT - VIN)/R2 => VOUT = VIN + I2R2 6. VOUT = I1R1 + I2R2 = (R1+R2)I1 = (R1+R2)VIN/R1 7. Therefore VOUT = (1 + R2/R1)VIN
  26. 26. SUMMING AMPLIFIER VOUT = -Rf (V1/R1 + V2/R2 + … + Vn/Rn) If Recall inverting amplifier and If = I1 + I2 + … + In Summing amplifier is a good example of analog circuits serving as analog computing amplifiers (analog computers)! Note: analog circuits can add, subtract, multiply/divide (using logarithmic components, differentiate and integrate – in real time and continuously.
  28. 28. 34 )()())(())(( 4 4 3 3 2 2 1 1 R R v R R v R R R R v R R R R vv cc b ca b ca o −−+= SUMMING AMPLIFIER
  29. 29. Difference AMPLIFIER •This type is of the same characteristic of the inverting and non inverting OPAMP. •Vo is the differences between the two inputs • Rin in both inputs must be equal, and equal to Rf Vo = Rf (V1 –V2)/ Rin Rf Rin Rin V2 V1 Rf Vo
  30. 30. 37 Linear amplifier. Theorem of linear Superposition. Difference AMPLIFIER
  31. 31. 38 Application of superposition Inverting configuration 1 1 2 1 Io v R R v −= Difference AMPLIFIER
  32. 32. 39 Application of superposition. Non inverting configuration. 2 34 4 1 2 2 )(1( Io v RR R R R v ) + += Difference AMPLIFIER
  33. 33. 40 The inverting configuration with general impedances in the feedback and the feed-in paths. Integrators AMPLIFIER
  34. 34. SJTU Zhou Lingling 41 The Miller or inverting integrator. The Inverting Integrators AMPLIFIER