The TLC083 with a 1-V step response has an 80% overshoot with a natural frequency of 3.5 MHz when
configured as a unity gain buffer and with a 10-kW feedback resistor. By adding a 10-pF capacitor in parallel with
the feedback resistor, the overshoot is reduced to 40% and eliminates the natural frequency, resulting in a much
faster settling time (see Figure 49).
The TLC08x is a family of high-speed low-noise CMOS input operational amplifiers that has an input
capacitance on the order of 20 pF. Any resistor used in the feedback path adds a pole in the transfer function
equivalent to the input capacitance multiplied by the combination of source resistance and feedback resistance.
For example, a gain of –10, a source resistance of 1 kW, and a feedback resistance of 10 kW add an additional
pole at approximately 8 MHz. This is more apparent with CMOS amplifiers than bipolar amplifiers due to their
greater input capacitance.
This is of little consequence on slower CMOS amplifiers, as this pole normally occurs at frequencies above their
unity-gain bandwidth. However, the TLC08x with its 10-MHz bandwidth means that this pole normally occurs at
frequencies where there is on the order of 5-dB gain left and the phase shift adds considerably.
The effect of this pole is the strongest with large feedback resistances at small closed loop gains. As the
feedback resistance is increased, the gain peaking increases at a lower frequency and the 180° phase shift
crossover point also moves down in frequency, decreasing the phase margin.
For the TLC08x, the maximum feedback resistor recommended is 5 kW; larger resistances can be used but a
c apacitor in parallel with the feedback resistor is recommended to counter the effects of the input capacitance pole.
1. “Any resistor used in the feedback path adds a pole in the transfer function
equivalent to the input capacitance multiplied by the combination of source resistance and feedback resistance.” 你好，反馈回路上的电阻，不是增加一个零点吗？怎么说增加一个极点呢？求指导，谢谢！
2. “the overshoot is reduced to 40% and eliminates the natural frequency” 你这里的“natural frequency” 是啥意思呢？？？谢谢！
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