For example, if we are using the ADA4004-1 op amp in all these circuits, the open loop gain is 400000 as shown in the spec sheet (Table 2): ADA4004 Datasheet

The open-loop breakpoint is at 30 Hz (GBWP=12 MHz, also in Table 2) so the plot should be constant at 112 and then fall off at -20dB/decade after 30Hz.

However, how do you plot open loop gain for an inverting amp, difference amp, and low pass filter? Shouldn't it just be the same plot for all of them?

  • \$\begingroup\$ With negative feedback the gain is now reduced to become the impedance ratio \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 22 '18 at 17:10
  • \$\begingroup\$ How do you plot it like on graph paper or do you mean something else like how does the math work. If so, please be clear. \$\endgroup\$ – Andy aka Apr 22 '18 at 17:22
  • \$\begingroup\$ Ok, so I am also asked to plot the closed loop gain, and I think my confusion comes from me not totally understanding the difference between the two. Is the closed loop response the same as the frequency response? And if I were to plot both the open and closed loop gain for the circuits stated above, would the open loop gain be the same plot for all of them and the closed loop gain be the frequency response? \$\endgroup\$ – Kevin J Apr 22 '18 at 17:29
  • \$\begingroup\$ yes plotting on paper \$\endgroup\$ – Kevin J Apr 22 '18 at 17:30

Kevin - the term "frequency response" tells you how the gain and the phase depends on the frequency of the incoming signal. However, now we have to discriminate between two alternatives: "loop gain" (gain properties of the open loop) or "closed-loop gain". Of, course in both cases the results of all three circuit alternatives (as mentioned by you) will be different because the feedback path is not the same.

In case of an inverter, the feedback path is a simple voltage divider (two resistors) and in case of a lowpass it is a frequency-dependent circuitry (at least one capacitor).

Remark: It is important to be familiar with the terminology:

  • open-loop gain: Gain of the active device WITHOUT any feedback;

  • loop gain: Gain of the whole open loop including active device and feedback circuitry;

  • closed-loop gan: Gain of the complete circuit with feedback applied.

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