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FAQ-OPAM-0005
- Q:
- The chart of "Voltage gain vs. phase frequency characteristics" shows all 40dB gain. Doesn't it also explain voltage gain Av figures?
- A:
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The characteristic example of op. amp voltage gain/phase vs. frequency is generally of non-inverting amplifier circuits that are set at 40dB closed loop gain. Only high frequencies reflect open loop characteristics. That is why the low frequency area of gain frequency shows 40dB without variation, as seen in Fig5_1.This is because this is the simplest way to evaluate. Characteristics of a high frequency area match characteristics of an open loop, which enables you to assess the movement of the feedback loop.
Also, when you need the characteristic chart of open loop in a low frequency area, you can estimate it using Fig5_1. -
Gain frequency characteristics:
Make an A-B line at gain = Av, and extend that line with -20dB/dec slope (B-C). The curved line A-B-C shows an estimate value of open loop gain frequency characteristics. (Frequency-axis is logarithm and gain-axis is dB value. This should be a straight line). -
Phase frequency characteristics:
Make a segment D-E with phase 0°, and a segment G-F with phase 90°. If the frequency of point-B is set as f1, and point-E (f3 = f1/10) and point-F (f2 = 10 x f1) are set, then the curved line D-E-F-G shows an estimate value of open loop phase frequency characteristics. (Frequency-axis is logarithm, phase-axis a straight line). 
Fig5_1．Approximation process of low frequency characteristics of op. amps
