The 741: Practical Considerations

Though in some applications the 741 is a good approximation to an ideal op-amp, there are some practical limitations to the device in exacting applications.

The input bias current is about 80 nA The input offset current is about 10 nA The input impedance is about 2 Megohms The common mode voltage should be within +/-12V for +/-15V supply The output impedance is about 75 ohms. The voltage gain rolls off 6dB per octave starting at 100kHz. There is a finite input offset which must be zeroed by a resistor between pins 1 and 5. The input offset is typically 2mV to <6mV. The slew rate is 0.5V/microsecond. There is some temperature dependence

Op-amp Input Current

One of the practical op-amp limitations is that the input current is not exactly zero as we assume in the current rule. There is a tiny input bias current for an op-amp which is about 80 nA for 741 type op-amps. For FET- input op-amps it may be a few picoamps. The superbeta Darlington LM11 may have an input current of 25 picoamps and the MOSFET ICH8500 is one of the very lowest at 0.01 picoamp. Judgements have to be made because those with the lowest input bias currents cannot operate at high speed. For high speed one may choose an op-amp with higher bias current at the cost of seeing some voltage drop across the resistors of the feedback network, bias network or source impedance. This may restrict you to smaller resistors and place practical limits on gain, or may produce some variations in output voltage.

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.

Op-amp Input Impedance

One of the practical op-amp limitations is that the input impedance finite, though very high compared to discrete transistor amplifiers. For the 741 the input resistance measured to one input with the other grounded is about 2 Megohms. For FET input devices it is typically 10^12 ohms. In practice this finite impedance is usually not as big a problem as the input bias currents since the use of negative feedback raises the effective input impedance even higher.