Common Mode Voltage Limits

One of the practical op-amp limitations is that the inputs must stay within a certain voltage range (usually significanly less than the supply voltages) for proper operation. Op-amps are subject to drastic gain changes and bizarre behavior if these ranges are exceeded. For a 741 operating on +/-15 volts, the common mode input swing should be within +/-12 volts. Voltages over 15V may damage the op-amp, regardless of the supply voltage. Some op-amps are designed to allow common mode voltages down to the negative supply voltage (LM358, 3130/3140) or up to the positive supply (301/307 or 355-357).

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Op-amp Output Impedance

One of the practical op-amp limitations is that there is a finite output impedance. For the 741 it is about 75 ohms but can be as high as several thousand ohms for some low power op-amps. The effective output impedance is further lowered by the use of negative feedback, so the focus becomes not one of the number of ohms looking into the output, but what limitations are placed on the output current. Limiting the output current also limits the allowable output voltage swing: the lower the load resistance, the lower the allowable voltage amplitude. For a load >2K, the 741 can swing to within about 2 volts of the supply. This is about all that is permitted by common mode limits for a 15 volt supply so the output impedance is not a serious limitation. For lower load impedances, however, the safe output voltage swing will be progressively curtailed. Op-amps with MOS transistor outputs(CA3130 and CA3160) can swing all the way to positive and negative supply voltages.

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Op-amp Voltage Gain Limitations

One of the practical op-amp limitations is that the open loop gain which is so high at frequencies in the kHz range (100,000 to a million or more) drops to a gain of one at some high frequency (say 1 to 10 MHz). Many op-amps contain internal compensation to cause a 6dB per octave rolloff of gain (like a low-pass filter) at some chosen frequency to stabilize the unit against high frequency oscillations. For the 741 this rolloff starts at 100 kHz. From the beginning of the rolloff the RC type filter action leads to phase shift starting at 90 and increasing to 120 to 160 as the gain approaches one. If the phase shift reaches 180 the feedback becomes positive and the system can oscillate.

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Op-amp Input Offset Current

One of the practical op-amp limitations is that the input bias currents for the two inputs may be slightly different. Even though the inputs are designed to be symmetrical, slight differences which occur in the manufacturing process may give slightly different bias currents. This offset current is typically on the order of a tenth of the input bias current, with 10nA being a representative offset current for a 741.

Even with identical source impedances, this offset current will produce a slight voltage between the input terminals, contrary to the ideal voltage rule.

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