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- 1. High Speed Amplifiers Part 1 <ul><li>Source: Analog Devices </li></ul>
- 2. Introduction <ul><li>Purpose </li></ul><ul><ul><li>This training module introduces the basic knowledge of high speed operational amplifiers and some characteristics. </li></ul></ul><ul><li>Outline </li></ul><ul><ul><li>High Speed Amplifiers Defined </li></ul></ul><ul><ul><li>Architectures </li></ul></ul><ul><ul><li>Noise Gain vs. Signal Gain </li></ul></ul><ul><ul><li>Noise Analysis </li></ul></ul><ul><ul><li>Voltage Offset </li></ul></ul><ul><ul><li>Driving Capacitive Loads </li></ul></ul><ul><ul><li>Compensating for Input Capacitance </li></ul></ul><ul><li>Content </li></ul><ul><ul><li>33 pages </li></ul></ul>
- 3. How to Classify High Speed Amplifiers? <ul><li>High Speed Amplifiers </li></ul><ul><ul><li>Have Bandwidths > 50MHz </li></ul></ul><ul><ul><li>AC specification driven </li></ul></ul><ul><ul><ul><li>Wide bandwidths </li></ul></ul></ul><ul><ul><ul><li>High slew rates </li></ul></ul></ul><ul><ul><ul><li>Fast settling time </li></ul></ul></ul><ul><li>Precision Amplifiers </li></ul><ul><ul><li>BW < 50MHz </li></ul></ul><ul><ul><li>Offset voltage < 1mV </li></ul></ul><ul><ul><li>Low drift </li></ul></ul><ul><li>HS AMPs Applications: Communications, video, medical imaging, commercial imaging, consumer electronics, industrial and data acquisition </li></ul>
- 4. Typical High Speed Amplifier Specifications <ul><li>Bandwidth: 50MHz to 2.1GHz </li></ul><ul><li>Slew Rate: 30V/ µ sec to 5500V/ µ sec </li></ul><ul><li>Settling Time: 1 µ sec to 4nsec </li></ul>
- 5. Voltage Feedback (VFB) OP-AMP Model ~ + – v – A(s) v A(s) = OPEN LOOP GAIN R2 R1 V OUT V IN V OUT V IN 1 + R2 R1 1 + 1 A(s) 1 + R2 R1 = 1 + R2 R1 1 + 1 A(s) β =
- 6. Gain-Bandwidth Product for Voltage Feedback OP AMPS GAIN dB OPEN LOOP GAIN, A(s) IF GAIN BANDWIDTH PRODUCT = X THEN Y · f CL = X f CL = X Y WHERE f CL = CLOSED-LOOP BANDWIDTH LOG f f CL NOISE GAIN = Y Y = 1 + R2 R1
- 7. Current Feedback (CFB) OP AMP Model + – i – T(s) i T(s) = TRANSIMPEDANCE OPEN LOOP GAIN T(s) R O i ×1 R2 R1 V IN V OUT V OUT V IN 1 + R2 R1 1 + R2 T(s) 1 + R O = 1 + R2 R1 1 + R2 T(s) R1 + R O R2 ASSUME R O << R1, AND R1 R2, THEN V OUT V IN ×1
- 8. Frequency Response for Current Feedback OP AMPS <ul><li>Feedback resistor fixed for optimum performance. Larger values reduce bandwidth, smaller values may cause instability. </li></ul><ul><li>For fixed feedback resistor, changing gain has little effect on bandwidth. </li></ul><ul><li>Current feedback op amps do not have a fixed gain-bandwidth product. </li></ul>GAIN dB G1 G2 G1 · f 1 G2 · f 2 f 1 f 2 <ul><ul><li>LOG f </li></ul></ul>
- 9. Simplified Current Feedback (CFB) OP AMP
- 10. CFB OP AMP Model and Bode Plot
- 11. Differential Amplifier with Folded Cascode Simplified Circuit
- 12. Model and Bode Plot for a VFB OP AMP
- 13. ADI’s Amplifier Architectures Differential Pair “ H” Bridge Common Mode Linearized Low Noise Medium Noise Low Noise Low Slew Rate High Slew Rate High Slew Rate AD8021 AD8057 AD8099 ADA4899
- 14. Differential Pair, Noise *add RSS for total noise Vn Vcc Vee 2*Ic Rd Rd Ru Ru Rb Rb Q1 Q2 Q4 Q3 Qm Qm Vp 1 Vo C Vb + - Vb + - Vu + - Note: Main Noise Terms* Q1&2 => Ru => Rb => Rd =>
- 15. "QUAD-CORE” or H-BRIDGE VFB gm Stage or CURRENT-ON-DEMAND
- 16. H-Bridge Noise Main Noise Contributors Qi&Qo4 => Rh’s => Rc => Rm’s => Much Higher Slew Rate +1 Qo’s Qi Rh Rh Rh Rh Rc Qo’s Rm Rm Rm Rm Qi Qi Qi
- 17. Common Mode Linearized Input Stage Vo +1 A A Ib It It Vp Vn Vb1 Vb2 Rmt Rmt Rmb Rmb Main Noise Terms Qi’s=> R’s=> Ic AIc Ic Note: Greatly Improves Slew Without Degrading Noise! Qi_n Qi_p Qi_n Qi_p
- 18. Noise Gain and Signal Gain Comparison
- 19. Raising Noise Gain (DC or AC) for Follower (A) or Inverter (B) Stability
- 20. Noise Gain Stability Analysis for VFB and CFB Op Amps with Feedback Capacitor
- 21. Current “Noise Gain” Definition for CFB OP AMP for Use in Stability Analysis
- 22. OP AMP Noise Model for A First-Order Circuit with Resistive Feedback
- 23. Referring All Noise Sources to the Output
- 24. AD8011 Output Noise Analysis / H z 4 n V / H z 5 p A / H z 4 n V / H z AD 80 11 5 p A / H z 0 . 9 n V / H z 2 n V / H z R1 1 k R2 1 k (G • R S ) (G ) ( 1 ) (R 2) (- R 2/ R 1) G = 1 + R2 R1 (G ) + - R S 5 0 f CL = 180MHz 1 . 8 n V / H z 0 . 5 n V / H z 4 n V / H z 4 n V / H z 5 n V / H z 4 n V OUT PUT NO ISE SP ECTRAL DENS ITY = 8.7nV/ Hz TOTAL NOISE = 8.7 1.57 X 180 X 10 6 = 146 V rms
- 25. AD8011 Noise Figure for Unterminated and Terminated Input Conditions Unterminated 50 1k 1k + – R S V no(total) = 8.7nV / Hz, from previous slide V no(Rs) = G = 2 4kTR G 4kTR 0.9nV/ Hz = 1.8nV/ Hz NF = 20 log 8.7 1.8 = 13.7 dB 50 1k 1k + – R S V no(total) 8.7nV / Hz (See Note) V no(Rs) = G = 2 4kTR G kTR 0.9nV/ Hz = 0.9nV/ Hz NF = 20 log 8.7 0.9 = 19.7 dB 50 Note: Input noise current (I n+ ) flows through 50 (unterminated case) or 25 (terminated case), but the overall effect of this is negligible. I n+ I n+ Terminated AD8011 AD8011
- 26. High Speed OP AMP Noise Summary <ul><li>Voltage Feedback Op Amps: </li></ul><ul><ul><li>Voltage Noise: 2 to 20nV/ Hz </li></ul></ul><ul><ul><li>Current Noise: 0.5 to 5pA/ Hz </li></ul></ul><ul><li>Current Feedback Op Amps: </li></ul><ul><ul><li>Voltage Noise: 1 to 5nV/ Hz </li></ul></ul><ul><ul><li>Current Noise: 5 to 40pA/ Hz </li></ul></ul><ul><li>Noise Contribution from Source Negligible if < 100 </li></ul><ul><li>Voltage Noise Usually Dominates at High Gains </li></ul><ul><li>Reflect Noise Sources to Output and Combine Root Sum Squared (RSS) </li></ul><ul><li>Errors Will Result if there is Significant High Frequency Peaking </li></ul>
- 27. Model for Calculating Total Op Amp Output Voltage Offset V O = ±V OS 1 + + I b+ R3 1 + - I b- R2 IF I b+ = I b- AND R3 = R1||R2 V O = ±V OS 1 + I b - V OS + - R3 R2 R1 R2 R1 I b+ R2 R1 R2 R1 V O
- 28. Output Offset Voltage Summary <ul><li>High Speed Bipolar Op Amp Input Offset Voltage: </li></ul><ul><ul><li>Ranges from 0.1mV to 3mV for VFB and CFB </li></ul></ul><ul><ul><li>Offset TC Ranges from 5 to 15µV/°C </li></ul></ul><ul><li>High Speed Bipolar Op Amp Input Bias Current: </li></ul><ul><ul><li>For VFB Ranges from 0.1 to 5µA </li></ul></ul><ul><ul><li>For CFB Ranges from 5 to 15µA </li></ul></ul><ul><li>Bias Current Cancellation Doesn't Work for: </li></ul><ul><ul><li>Bias Current Compensated Op Amps </li></ul></ul><ul><ul><li>Current Feedback Op Amps </li></ul></ul>
- 29. External Load Capacitance to An OP AMP Output Capacitive loading on op amp generally reduces phase margin and may cause instability, but increasing the noise gain of the circuit improves stability.
- 30. Open-loop Series Resistance Isolates Capacitive Load for AD811 Current Feedback Op Amp 6 7 4 3 2 - + A D 8 1 1 R I N 30. 9k R F 750 R X 1 2 R L 500 V OUT C L 1n F 0.1 F +1 2V -1 2V 100 F/ 2 5V 100 F/ 2 5V 0.1 F R B 1 k V IN
- 31. Compensating For Input Capacitance in a Current-to-voltage Converter Using VFB OP AMP
- 32. Generalized Model for High Speed Photodiode Preamp C2 R2 C1 I + _ 1 f 2 f 1 f u f NOISE GAIN OPEN LOOP GAIN UNCOMPENSATED COMPENSATED f 1 = f 2 = f 2 = f 1 • f u C2 = 1 2 R2 C1 1 2 R2 C2 C1 2 R2 f u FOR 45° PHASE MARGIN GAIN f u = OP AMP UNITY GAIN BW PRODUCT f 2 = SIGNAL BW f u Total Input Capacitance f 2 = f u 2 R2 C1 – V B
- 33. Additional Resource <ul><li>For ordering high speed operational amplifiers, please click the part list or </li></ul><ul><li>Call our sales hotline </li></ul><ul><li>For additional inquires contact our technical service hotline </li></ul><ul><li>For more product information go to </li></ul><ul><ul><li>http://www.analog.com/en/amplifiers-and-comparators/operational-amplifiers-op-amps/products/index.html </li></ul></ul>Newark Farnell

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