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Chapter 30

Chapter 30. Operational Amplifiers. Introduction. Characteristics High input impedance Low output impedance High open-loop gain Two inputs One output Usually + and – dc power supplies. Introduction. Ideal Characteristics z in (inverting) ≈ ∞ z in (non-inverting) ≈ ∞ z out ≈ 0

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Chapter 30

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  1. Chapter 30 Operational Amplifiers

  2. Introduction • Characteristics • High input impedance • Low output impedance • High open-loop gain • Two inputs • One output • Usually + and – dc power supplies

  3. Introduction • Ideal Characteristics • zin (inverting)≈ ∞ • zin (non-inverting)≈ ∞ • zout≈ 0 • Av≈ ∞ V+ Inverting Input - Output + Non-Inverting Input V-

  4. Introduction • Uses • Comparators • Voltage amplifiers • Oscillators • Active filters • Instrumentation amplifiers V+ Inverting Input - Output + Non-Inverting Input V-

  5. Introduction • Single-ended amplifier • One input grounded • Signal at other input • Double-ended amplifier/Differential amplifier • Signals at both inputs V+ Inverting Input - Output + Non-Inverting Input V-

  6. Differential Amplifier and Common-Mode Signals • Basic differential amplifier • Q1 identical to Q2 • RC1 = RC2 • IC1 = IC2 and emitter currents equal • Also, IC≈ IE for high β • and VBE ≈ 0.7 V • Similar calculation of Bias

  7. ◦ VCC IC2 IC1 RC1 RC2 Q1 Q2 __ _ - __ _ - IE RE ◦ –VEE Differential Amplifier and Common-Mode Signals

  8. ◦ VCC IC2 IC1 RC1 RC2 Q1 Q2 __ _ - __ _ - IE RE ◦ –VEE Differential Amplifier and Common-Mode Signals • Apply same signal to both Bases Vout = Vout1 – Vout2≈ 0 • Eliminates common-mode signals • 60 Hz • Noise

  9. ◦ VCC IC2 IC1 RC1 RC2 Q1 Q2 __ _ - __ _ - IE RE ◦ –VEE Differential Amplifier and Common-Mode Signals • Apply sinusoids to both bases: • Same amplitude, 180° difference in phase, if Vin1 = –Vin2 Vout = 2Vin

  10. Differential Amplifier and Common-Mode Signals • Common-mode signals • Differential voltage gain also called open-loop voltage gain 20,000≤Av≤200,000

  11. Differential Amplifier and Common-Mode Signals • Common-mode signals • Common-mode voltage gain

  12. Differential Amplifier and Common-Mode Signals • Common-mode rejection ratio (CMRR) • Equations • Values

  13. Differential Amplifier and Common-Mode Signals • Noise • Static in audio signal • Increases as signal is amplified • Common mode signal • Significantly reduced by differential amplifier vnoise - vin + __ _ - __ _ -

  14. Negative Feedback • Op-amp • Large differential, open-loop voltage gain • Avol≈ 100,000 • Small input yields saturated output (VCC or VEE)

  15. Negative Feedback • Negative feedback • Returns a portion of output signal to the input • Open-loop voltage gain decreased

  16. Negative Feedback • Input impedance still high • Output impedance low • Circuit voltage gain, Av • Adjustable • Stable Negative Feedback - + vout vin __ _ -

  17. Inverting Amplifier • Basic circuit

  18. Inverting Amplifier • Output 180° out of phase with input • Significant decrease in gain • Gain now called closed-loop voltage gain • Output impedance ≈ 0 • vd ≈ 0

  19. Inverting Amplifier • Inverting input at virtual ground, vin(-)≈ 0 • iin to op-amp ≈ 0 • Input current only dependent on vinand R1 • Avclonly dependent on input resistor and feedback resistor

  20. Inverting Amplifier • Model • vd≈ 0 • Rin ≈ ∞ • iin = if • zin ≈ R1 RF R1 i = 0 if + - + iin vin vd Rin - + - Rout vout(OC) - + __ _ - Avolvd internal __ _ -

  21. Inverting Amplifier • Low output impedance RF R1 i = 0 if + - + iin vin vd Rin - i1 Rout iout(sc) - + __ _ - __ _ - Avolvd internal __ _ -

  22. Non-Inverting Amplifier • Circuit

  23. The Non-Inverting Amplifier • Very high input impedance • Voltage gain related to the two resistors • Very low output impedance • Excellent buffer

  24. Non-Inverting Amplifier • Differential voltage • vd≈ 0 • Input current to op-amp • i = 0 • Closed-loop voltage gain (Avcl) is a resistor ratio

  25. Non-Inverting Amplifier

  26. Non-Inverting Amplifier • Model • Input impedance vin + iin vd Rin zin vout - Rout - + + - Avolvd internal R1 __ _ - - + __ _ - RF if

  27. Non-Inverting Amplifier • Model • Output impedance i2 + iin vd Rin - Rout - + + - Avolvd internal R1 __ _ - - + __ _ - __ _ - RF if iout(sc)

  28. Non-Inverting Amplifier • Very high zin • Very low zout • Good buffer circuit • Also called voltage follower (gain = 1) • Or adjustable gain > 1

  29. Non-Inverting Amplifier • Voltage Follower Buffer Circuit • Gain = 1 • High impedance source drives low impedance load

  30. Op-Amp Specifications • LM 741 series • Inexpensive • Widely used • Good general specifications • Characteristic of all op-amp specifications • Provide Minimum, Typical, and Maximum ratings

  31. Op-Amp Specifications • Input Offset Voltage, Vio • LM741C, Vio typical is 2 millivolts • Model is voltage source with value, Vio in series with + input

  32. Op-Amp Specifications • Input Offset Voltage, Vio • Without feedback this would saturate output with no input • With negative feedback, output due to Vio is closed-loop gain times Vio

  33. Op-Amp Specifications • Input Offset Current, Ios • Ios= Difference between bias currents at + and – inputs of op-amp • 741C typical Ios is 20 nanoamps • Multiplying resistor used to measure Ios

  34. Op-Amp Specifications • Input Resistance • 741C: minimum = .3 MΩ, typical = 2 MΩ • Open-Loop Voltage gain (Avol) • 741C: Avol = Large Signal Voltage Gain • minimum = 20,000, typical = 200,000 • Closely related to Bandwidth, BW

  35. Op-Amp Specifications • Gain-bandwidth product • 741C = 1,000,000 = 106 MHz

  36. Op-Amp Specifications • Gain versus frequency curve for op-amp

  37. Op-Amp Specifications • Slew rate • Maximum rate of change of output voltage • 741C maximum slew rate = 0.5 V/μsec

  38. Op-Amp Specifications • Fastest time for output to go from 0 to 10 volts is 20 μsec • Can distort waveforms that have too fast a rise time

  39. Op-Amp Specifications • Slew rate required for Sinusoid with frequency f and amplitude A • Maximum amplitude of a sine wave with frequency f for a given slew rate

  40. Op-Amp Specifications • Bias Compensation: use RC = R1||RF

  41. Troubleshooting an Op-Amp Circuit • Problems occur when circuit is first built • Most important • Correct connection of dual power supply • Connecting a – supply to a + input (or vice versa) can burn out an op-amp • Single earth ground • Short connecting wires

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