741 Op-amp Characteristics

Common Mode Voltage Limits

共模电压限制

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).

运算放大器的一个实际限制是，输入电压必须保持在一定的电压范围内（通常显著低于电源电压）以确保正常工作。如果这些范围被超越，运算放大器会受到剧烈的增益变化和异常行为的影响。对于使用±15V供电的741运算放大器，共模输入摆幅应保持在±12V以内。超过15V的电压可能会损坏运算放大器，无论电源电压如何。一些运算放大器设计允许共模电压降至负电源电压（LM358，3130/3140）或升至正电源（301/307或355-357）。

Op-amp Output Impedance

运算放大器输出阻抗

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.

运算放大器的一个实际限制是存在有限的输出阻抗。对于741来说，约为75欧姆，但对于某些低功耗运算放大器来说，可以高达几千欧姆。负反馈的使用进一步降低了有效的输出阻抗，因此关注点不再是对输出端看进去的欧姆数，而是输出电流的限制。限制输出电流也限制了允许的输出电压摆幅：负载电阻越低，允许的电压幅度越低。对于负载大于2K的情况，741可以摆动到电源电压约2伏以内。这大约是15伏电源下共模限制所允许的全部范围，因此输出阻抗并不是一个严重的问题。然而，对于较低的负载阻抗，安全的输出电压摆幅将逐步受到限制。具有MOS晶体管输出的运算放大器（CA3130和CA3160）可以摆动到正负电源电压。

Op-amp Voltage Gain Limitations

运放电压增益限制

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.

运算放大器的一个实际限制是，其在kHz范围内的开环增益虽然很高（100,000到一百万或更高），但在某个高频（例如1到10 MHz）时会下降到增益为一。许多运算放大器内部包含补偿电路，以在某个选定频率处导致增益每 octave 下降6dB（类似于低通滤波器），以稳定单元免受高频振荡的影响。对于741运算放大器，这种滚降从100 kHz开始。从滚降开始时，RC型滤波器作用导致相位偏移从90°开始，并随着增益接近一而增加到120°到160°。如果相位偏移达到180°，反馈将变为正反馈，系统可能会振荡。

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。