# Electronic – Op amp circuit stability in linear regulator circuit

bode plotlinear-regulatorltspiceoperational-amplifierstability

I am trying to design a stable linear regulator using MOSFET and op amp. Schematics is shown below:

I am using LTSpice for simulations. This circuit is mostly for educational purpose: I want to understand how to use bode plots to design a stable op amp circuit with feedback. I have studied a bunch of theory on this topic but I don't have any practice regarding op amp circuits.

So first of all I broke the feedback loop, grounded inverting input of the opamp, ran ac analysis measuring node "out". I put DC offset of the V5 to 0V and results are shown below:

!!See fig 1!! (sorry I had to put figures in one image because my reputation is lower than 10 and I can't attach more than 2 images)

Gain is not going higher than 0dB.

Afterwards, I have used V4 for injecting AC into the feedback loop (DC value = 0, I have restored the feedback loop). Plot of V(out)/V(vn) is show below:

!!See fig 2!!

Phase margin might be not that good but anyways…

Also I have done a plot of V5 with AC 1 and without using V4 in the loop:

!!See fig 3!!

Here I actually see the peak near 200 kHz but… Transient analysis is showing the following:

!!See fig 4!!

A bit closer…

!!See fig 5!!

So the circuit is oscillating and frequency is 1.8 MHz. I don't understand why. Obviously I am laking some knowledge concerning AC analysis… I understand that I should just increase capacitance C1 to 40uF and it will solve the problem but I want to know why I should do that (i.e. how can I see it on the bode plot). The only plot which showed some problems was the transient plot to step response.

Figures:

UPDATED:

The reason I am trying to use an op-amp in this circuit is that I already have a following linear regulator in my device and I want to upgrade it:

It works fine but sometimes it is not charging the capacitor C1 fast enough so I thought about using op-amp to make the response faster. And the most important characteristic here is to make the C1 charge from 8V to 10V as fast as possible.

R1 is 2 km long cable.