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Op Amp Parameters

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Op Amp Parameters
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  P H Y S 3 7 5 - L A B O R A T O R Y E L E C T R O N I C S I 1 2 . 2 - F O R T N E R 1 of 10 Op-Amp Parameters Input Impedanceã  Impedance is calculated on one input when the other is ground. ã  Equal to transistor base/gate impedance with feedback. ã  Bipolar input (741) - 2 M Ω . ã  JFET input (TL071) - 10 12 Ω . v in V  out  −+ v + v - i in Z  in v in i in -------=  P H Y S 3 7 5 - L A B O R A T O R Y E L E C T R O N I C S I 1 2 . 2 - F O R T N E R 2 of 10 Input Currentã  Bias current = common-mode input current,  I   B  = (  I  +  +  I  - )/2. ã  The bias current causes a voltage drop across the feedback resistor. ã  Typical values: Bipolar 10-100 nA, FET 10-100 pA. ã  With small feedback resistors, bias current can be neglected. ã  Offset current = differential-mode input current,  I   B  = (  I  +  -  I  - )/2. ã  Typical values:  I  io  = 0.1  I   B  to 0.5  I   B . A 0 v in  v out  i in  R i  R  f   i in - I   B i -  v out  −+ v + v - i +  P H Y S 3 7 5 - L A B O R A T O R Y E L E C T R O N I C S I 1 2 . 2 - F O R T N E R 3 of 10 Output Impedanceã  Intrinsic output impedance is measured with no feedback. ã  Operational voltage feedback reduces the output impedance. ã  The output impedance is:  Z’ out   =  v th  /  i  s . ã  The thevenin output voltage is:  v th  = - i in  R  f   . ã  With  v out   shorted to ground,  v  A  =  i in  R  f   . ã  Internally the voltage drop is  i  s  Z  out   =  A 0  v  A  =  A 0  i in  R  f   . V  CC  v 1 v out   =  A 0 ( v 1  -  v 2 ) − V   EE  v 2 Z  in Z  out  A 0 v in  v out  i in  R i  R  f   i in v  A Z  out  Z  ' out  i in  –   R  f    A 0 i in  R  f    Z  out   ⁄  --------------------------------- Z  out   –   A 0 -------------= =  P H Y S 3 7 5 - L A B O R A T O R Y E L E C T R O N I C S I 1 2 . 2 - F O R T N E R 4 of 10 Voltage Gainã  Typical  A 0  = 10 4 to 10 6 , = 80 to 120 dB. ã  Gain is a function of frequency. ã  Open-loop gain is measured with no feedback. Closed-loop includesthe feedback , so the gain is limited by the feedback resistor. ã  Op-amps act like low-pass RC filters, including a phase shift at the break frequency  f    B . ã  Typical maximum frequency is  f   T   = 0.1 to 10 MHz. ã  For a “compensated” op-amp,  f    B  =  f   T   /  A . log A 0 log f   open-loopclosed-loop BODE PLOT

Afiche

Jul 23, 2017
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