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Operational Amplifiers - Introduction

Operational Amplifiers - Introduction. Alan Murray. Agenda. Op-Amps - Physical Appearance Allocation of Signals to IC Pins Op-Amps - A peek inside (again!) (IDEAL) Op-Amp behaviour Open-Loop (Comparator) circuit Unity-Gain Buffer. 1. 8. 2. 7. -. 3. 6. +. 4. 5.

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Operational Amplifiers - Introduction

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  1. Operational Amplifiers -Introduction Alan Murray

  2. Agenda • Op-Amps - Physical Appearance • Allocation of Signals to IC Pins • Op-Amps - A peek inside (again!) • (IDEAL) Op-Amp behaviour • Open-Loop (Comparator) circuit • Unity-Gain Buffer Alan Murray – University of Edinburgh

  3. 1 8 2 7 - 3 6 + 4 5 Op-Amps (Real Ones) Texas Instruments 741 Op Amp Offset? No connection Inverting Input (-) Positivesupply, V+ (+15V?) Noninverting Input (+) Output Negative supply, V- (-15V?) Offset? Alan Murray – University of Edinburgh

  4. 741 Circuit diagram Alan Murray – University of Edinburgh

  5. Op-Amps (Real Ones) Texas Instruments 747 Op Amp(s) 1 Inverting Input A(-) Offset A? 14 2 Noninverting Input A(+) Positive supply A, V+ (+15V?) 13 3 Offset A? Output A 12 4 Negative supply, V- (-15V?) No Connection 11 5 Offset B? Output B 10 6 Positive supply B, V+ (+15V?) 9 Noninverting Input B(+) 7 Offset B? Inverting Input B(-) 8 Alan Murray – University of Edinburgh

  6. Op-Amps (Real Ones) Nat. Semi. LM348N Op Amp(s) Output A 1 14 Output D Inverting Input A(-) 2 13 Inverting Input D(-) Noninverting Input A(+) 3 12 Noninverting Input D(+) Positive supply V+ (+15V?) 4 Negative supply, V- (-15V?) 11 5 Noninverting Input C(+) Noninverting Input B(+) 10 Inverting Input C(-) 6 Inverting Input B(-) 9 7 Output B 8 Output C Alan Murray – University of Edinburgh

  7. + - An Op-Amp Positive Supply V+ Non-Inverting Input Vnoninv Output Vout Inverting Input Vinv Negative Supply V- Alan Murray – University of Edinburgh

  8. + - An Op-Amp Vnoninv Vout = A(Vnoninv-Vinv) Vinv And A is huge … ≈105 for a 741 at DC Alan Murray – University of Edinburgh

  9. IDEAL Op-Amp Characteristics • Differential Amplifier • Vout = A × ( Vnoninv - Vinv ) • Vinv = Vnoninvshould give Vout = 0 • Input impedance = ∞ • no current enters the input terminals • Output impedance = 0 • large currents can, if necessary,flow out of, or into the output terminal,without losing some or all of Vout • Gain A = ∞ Alan Murray – University of Edinburgh

  10. +15V +15V + Vout 10kΩ - 5kΩ -15V 0V Clickers out … Gain A = 1000 (not quite ∞!) 4.9V Solution Alan Murray – University of Edinburgh

  11. VOLTAGE Vinv = 3V 15 +15V (d) 10 + 5 Vnon 0 (b) -5 - -10 (a) -15V -15 TIME A Useful Op-Amp Circuit?Sweep Vnoninv from -12V → +12V Vout = 100(Vnon-Vinv) here Vnon (c) Vout 3V Vout Gain = 100 (large, not ∞) This is effectively a COMPARATOR (a) Vout = 100×(-12 - 3) = -1500V → -15V (b) Vout = 100×(2.85 - 3) = -15V (c) Vout = 100×(3.15 - 3) = +15V (d) Vout = 100×(+12 - 3) = +900V → +15V Alan Murray – University of Edinburgh

  12. Vmax Vsignal This is useful ... Audio warning lights Vsignal +15V + Warning lights Overload? = Vmax - -15V -15V Light flashes when Vsignal>Vmax Alan Murray – University of Edinburgh

  13. Vempty Vgauge This is useful ... Fuel gauge Vempty +15V + Warning, fuel low? = Vgauge - -15V -15V Light on when Vgauge<Vempty Alan Murray – University of Edinburgh

  14. Reminder – I/O Impedance Sensor Amplifier Alan Murray – University of Edinburgh

  15. Vnon Rout Rin A(Vnon- Vinv) Vinv Formally… Vnon + = Vout Vinv - Vout This the EQUIVALENT CIRCUIT For an ideal Op-Amp, Rin= ∞, Rout = 0, A= ∞ Alan Murray – University of Edinburgh

  16. +15V + Vout - -15V Is This a Silly Circuit? • Vout = A(Vnon-Vinv) • Vout = A(Vinput-Vinv) • Vinv=Vout • Vout = A(Vinput-Vout) • ÷A • Vout/A= Vinput-Vout • A →∞, -15V<Vout<+15V • 0 = Vinput-Vout • Vout=Vinput Vinput = Vnon Vinv Note = we have found that Vnon = Vinv Not for the last time … Alan Murray – University of Edinburgh

  17. 15 15 Vinput = Vnon 10 10 5 5 0 0 Vinv -5 -5 -10 -10 -15 -15 +15V + Vout - -15V Is This a Silly Circuit? VOLTAGE Vin TIME VOLTAGE Vout =Vinv “Animation” TIME Alan Murray – University of Edinburgh

  18. And this does exactly the same … • Vout = A(Vnon-Vinv) • Vout = A(Vinput-Vinv) • No current flows into Vinv • So no current in R • V=RI = 0V across R • So Vinv=Vout • Vout = A(Vinput-Vout) • ÷A • Vout/A= Vinput-Vout • A →∞ • Vinput-Vout = 0 • Vout=Vinput Vinput = Vnon + Vout Vinv - I=0 R Alan Murray – University of Edinburgh

  19. Vin Vout Gain=1 0V 0V Unity Gain Buffer Vin • Vout = Vin • Gain = 1 • Rin = ∞ • Rout = 0 • With an ideal Op-Amp • Draws no current from source • Delivers current to load "with ease" Vout + - Alan Murray – University of Edinburgh

  20. Vin Vout Vdrop > 0 I >> 0 Vin Vout < Vin + No Unity Gain Buffer • Rout for source > 0, Rin for Rload < ∞ Rin < ∞ Rout > 0 Source Load Alan Murray – University of Edinburgh

  21. Vdrop = 0 I > 0 I = 0 Vout = Vin + Unity Gain Buffer • Rout for source > 0, Rin for Rload < ∞ 0 ∞ Rin < ∞ Vout = Vin Rout > 0 Vin Source Load Alan Murray – University of Edinburgh

  22. Vdrop = 0 I > 0 + - I = 0 Vout = Vin Unity Gain Buffer • Rout for source > 0, Rin for Rload < ∞ Rin < ∞ Rout > 0 Vin Source Load Alan Murray – University of Edinburgh

  23. For example … Microphone Input stage Buffer Stage Volume Control Alan Murray – University of Edinburgh

  24. For example … Microphone Input stage 300Ω We want ALL of the 0.2mV and ALL of Vout to be passed ... • maximum voltage transfer • minimum current drawn • maximise input impedance for "input stage" and "buffer stage" ~ 0.2mV Volume control ... Buffer Stage Vout Vin Alan Murray – University of Edinburgh

  25. For example … Microphone + - 300Ω ALL of the 0.2mV and ALL of Vout is passed ... • maximum voltage transfer • no current drawn • infinite input impedance for "input stage" and "buffer stage" from Unity-Gain Buffer ~ 0.2mV Volume control ... + Vout Vin - Alan Murray – University of Edinburgh

  26. Summary • You should now know ... • What an Op-Amp looks like • What an Op-Amp does • What signals it expects and produces • Why it is of limited use as it stands • How a Unity Gain Buffer works • How to "buffer" system components with high-impedance outputs Alan Murray – University of Edinburgh

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