CHAPTER 10: Op-Amp Limitations pptx

CHAPTER 10 Op-Amp Limitations OBJECTIVES Describe and Analyze: • Input bias currents • Input offset current • Input offset voltage • Frequency response • Slew rate limitations • Troubleshooting Introduction Ideal op-amps have no limitations, but real ones do: • Small bias currents flow into (or out of) both inputs • The two bias currents are not exactly the same • The differential-pair transistors at the input are not perfectly matched, causing an input offset voltage • The bandwidth is not infinite Input Bias Current: IIB Op-amps made with BJTs have a very small base current that flows into the input pins (for NPNs) or out of the inputs (for PNPs) on the order of nA • Op-amps made with JFET front-ends have bias currents on the order of pA • Input bias current must be considered when there are large resistor values (RS) in series with the inputs: VDROP = RS  IIB • That VDROP looks like an input signal if it occurs on one input but not the other Input Bias Current Equal resistance in series with both inputs cancels almost all the VDROP caused by IIB Input Offset Current: IIO Let IIB+ be the bias current for the (+) input and let IIB- be the bias current for the (-) input. Then the input offset current is: IIO = | IIB+ | - | IIB- | Even if the resistances (RS) in series with the inputs are perfectly matched, there will be a voltage, call it VIS, that will look like an input signal: VIS = RS  IIO IIO for a modern bipolar op-amps can be picoAmps To minimize VIS, use an op-amp with low IIO Input Offset Voltage: VIO Input offset voltage comes from the slight mismatch between transistors in the differential pair at the front end of an op-amp. JFET op-amps can have more input offset than bipolar op-amps. Input offset voltage for bipolar op-amps can be in the range of milliVolts for old clunkers like the 741, to microVolts, even down to nanoVolts for newer ones like the LT1112. Offset Compensation In production, it’s cheaper to buy a better op-amp than to buy a 10-turn pot and pay someone to adjust it Output Voltage Swing: VO Once VO hits the voltage rails, it clips Output Voltage Swing Clipping also occurs if output current is too high [...]... “slew-rate” of an op-amp is the maximum rate of change of VOUT: SR = VOUT / t • To keep bias currents low, the internal currents in an op-amp are limited Also inside the op-amp is a a few picoFarads of capacitance • The time it takes to charge or discharge a capacitor depends on the current: t = (C/I) V • So with C and I fixed, the slew rate is also fixed It’s a parameter on the op-amp s data sheet... low Troubleshooting • As always, check to see if DC voltages are within the correct range • If an op-amp needs to be replaced, use the same part number if possible • An op-amp with better specs can usually replace a unit with worse specs (almost anything is better than a 741) But be careful: if the new op-amp has a much higher bandwidth than the original, the circuit might oscillate • If bread-boarding... fast the gain drops as frequency increases A typical roll-off is 20dB per decade • The frequency where the op-amp s open-loop gain (AOL) falls to 0dB (AV = 1) is called the “unity-gain bandwidth” (UGB) or the “gain-bandwidth product” (GBW) Bandwidth: ACL & AOL • The closed-loop gain (ACL) of an op-amp circuit is typically much less than the open-loop gain (AOL) • On a Bode plot, ACL is a horizontal... C and I fixed, the slew rate is also fixed It’s a parameter on the op-amp s data sheet in volts per microsecond (V / s) Slew-Rate a slew-rate of 1V / s (not high, but better than a 741) Slew-Rate An op-amp s slew-rate (SR) limits the range of sinewave signals it will amplify: fMAX = SR  106 / 2  VP) where SR is in Volts/s, and VP is the peak amplitude of the sinewave on the output The equation...Bandwidth Above a certain frequency, the op-amp s gain drops Bode Plots: Gain vs Frequency Bode plots show gain vs frequency characteristics Bode Plots • The vertical axis (Y-axis) shows gain in dB dB of gain is defined to be: dB = 20  Log(f)... than the original, the circuit might oscillate • If bread-boarding a new circuit, estimate the parameter values you need for the circuit (slew rate, input offset voltage, etc) and compare them to the op-amp s data sheet . CHAPTER 10 Op-Amp Limitations OBJECTIVES Describe and Analyze: • Input bias currents • Input offset current • Input offset voltage • Frequency response • Slew rate limitations. differential pair at the front end of an op-amp. JFET op-amps can have more input offset than bipolar op-amps. Input offset voltage for bipolar op-amps can be in the range of milliVolts. “slew-rate” of an op-amp is the maximum rate of change of VOUT: SR = VOUT / t • To keep bias currents low, the internal currents in an op-amp are limited. Also inside the op-amp is a a few