1 / 26

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.

istas
Download Presentation

Operational Amplifiers - Introduction

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related