Spice simulation of closed loop response of a linear regulator

you probably need to look to the op amp vendor data first and make sure that you have the op amp properly compensated per their recommendations. otherwise go to the same source to get a best guess at the phase-gain characteristics of the amp and plug that into your simulation.

: I'm spinning my own 5V linear voltage linear regulator. I'm using a NPN : pass transisitor (non-ldo configuration) and a opamp, and 2.5V reference.

: It is the textbook circuit with a voltage divider on the output (two 1K : resistors) with the center going to the in- on the opamp and the 2.5V : reference on in+ and of course the output drives the base of the NPN through : a current limiting resistor. essentialy an emitter follower.

: What I'd like to do is simulate the loop response because with no : compensation the output oscillates after a step load change. Also : oscillates with heavy load.

: How do I simulate the gain and phase in spice so I can check the phase : margin at various loads and compensate properly.

: thanks.

You want to simulate the loop gain, correct?

There are at least 2 different ways:

1. Do a google search of Middlebrook (or is it Middlebrock?) stability analysis.
2. Consult your SPiCE documentation about how to use AC resistors to break the loop at AC. Basically, you allow the circuit to bias at DC, but you break the loop and run it open-loop at AC.

Joe

If you are using LTSpice, there are 'loop gain' circuit elements that you can use for this. Check out the yahoo ltspice group.

Jim Thompson has a version on his website that works with pspice:

However, you can also probably just break the loop using an AC source at the V- input, because the impedance at the opamp is high enough.

Actually, though, folks were talking about this on another thread. A longish run from the smoothing caps, added to a very low impedance through the opamp, can cause the pass transistor to oscillate. Use a 100 ohm resistor between the output of the opamp and the NPN base, and make sure there is a 0.33uF cap between the collector lead and ground of the NPN transistor to shunt any oscillations to ground. I suspect this is the reason that 7805s and their brothers need decoupling caps if the leads from the power supply are too long; it is to counteract the inductance in the supply run.

--
Regards,
  Bob Monsen

The most beautiful thing we can experience is the mysterious. It is
the source of all true art and all science. He to whom this emotion is
a stranger, who can no longer pause to wonder and stand rapt in awe,
is as good as dead: his eyes are closed.
    Albert Einstein (1879 - 1955)

[snip]

Or use the LoopGain part on my website... my adaptation of Middlebrook's method to simulation.

...Jim Thompson

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