741 Op-amp Characteristics

The 741: Practical Considerations

741：实际考虑

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.

尽管在某些应用中，741运放可以作为理想运放的近似，但在对精度要求较高的应用中，该器件仍存在一些实际限制。

The input bias current is about 80 nA
输入偏置电流约为80 nA
The input offset current is about 10 nA
输入偏移电流约为10 nA
The input impedance is about 2 Megohms
输入阻抗约为2兆欧
The common mode voltage should be within +/-12V for +/-15V supply
共模电压应在±12V范围内，适用于±15V供电。
The output impedance is about 75 ohms.
输出阻抗约为75欧姆。
The voltage gain rolls off 6dB per octave starting at 100kHz.
电压增益从100kHz开始每 octave 下降6dB。
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.
pin 1 和 pin 5 之间必须接入一个电阻以消除有限的输入偏移，该偏移通常为 2mV 到 <6mV。
The slew rate is 0.5V/microsecond.
slew rate 是 0.5V/微秒。
There is some temperature dependence
存在一些温度依赖性

Op-amp Input Current

运算放大器输入电流

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.

运算放大器的一个实际限制是，输入电流并不完全为零，正如我们在电流规则中所假设的那样。运算放大器中存在一个微小的输入偏置电流，对于741型运算放大器来说，大约是80纳安。对于场效应管输入的运算放大器，这个值可能低至几皮安。超级β达林顿LM11的输入偏置电流可能为25皮安，而MOSFET ICH8500则是极低的，只有0.01皮安。必须做出判断，因为那些输入偏置电流最小的运算放大器无法在高速下工作。为了高速，可能需要选择一个输入偏置电流较高的运算放大器，但这样会付出一些代价，即在反馈网络、偏置网络或源阻抗上会有一些电压降。这可能会限制你使用更小的电阻，从而对增益施加实际限制，或者可能导致输出电压出现一些变化。

Op-amp Input Offset Current

运算放大器输入偏置电流

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.

运算放大器的一个实际限制是两个输入端的输入偏置电流可能略有不同。尽管输入端设计为对称，但制造过程中可能产生的微小差异可能导致偏置电流略有不同。这种偏移电流通常约为输入偏置电流的十分之一，10nA是741运算放大器的代表性偏移电流。

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

即使输入端阻抗相同，这个偏移电流仍会在输入端子之间产生微小的电压，这与理想电压规则相违背。

Op-amp Input Impedance

运算放大器输入阻抗

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.

运算放大器的一个实际限制是输入阻抗有限，尽管比离散晶体管放大器高得多。对于741运算放大器，当一个输入端接测、另一个接地时，输入电阻约为2兆欧。对于场效应管输入器件，其通常为10^12欧。在实际应用中，这个有限的阻抗通常不像输入偏置电流那样成为大问题，因为负反馈会将有效输入阻抗提高得更高。