Open loop gain and phase plotting in a closed loop system

Hi everyone,

I am checking the stability of a closed loop system. At same time I am debugging it - for that I am using AC analysis. In this system I have a differential pair and I would like to measure its open loop frequency response to analyse the pole position and gain.

One of the inputs in connected to a reference voltage (where I am putting the AC source) and the other is a feedback point, coming from a resistor string.

Does anyone knows how to measure the open loop gain of the differential pair while operating on the closed loop system (this is just to maintain the same bias point and loadings).

Regards.

In order to see the open-loop gain you always have to break the loop somehow.

Is the feedback just to set the DC point for the loop?

In quite a few simulators you have access to special switches that can set the DC operating point (established through the feedback). In spectre: analogLib/spt-switches.

Another approach could be to use a laaarge inductor that, during DC, operates as a short, and then during AC (for reasonable frequencies) behaves as an open-circuit.

By letting the DC point be set by the loop and then breaking it during AC you are mostly pretty close to the expected performance. You might miss some effects due to parasitics, etc., that disappear in the loop while breaking it.

I've generally used the approach of putting the device in
closed loop (A=100, maybe) with resistor feedback, and
use a vcvs across the inputs to get true real time input
difference. db20(mag(VOUT)/mag(VID)) and
phase(VOUT)-phase(VIN)+180 (or minus, whatever makes
the Bode plot look sane at low frequency).

If you are seeing true input difference then I don't think
it matters whether the "big picture" is open loop or closed
(provided that input null and output at appropriate common
mode position, are had).

A 1H inductor is a good way to "soft break" the loop.
Presuming there's nothing interesting going on below 10Hz
or so.

CAMALEAO said:

Hi everyone,
I am checking the stability of a closed loop system. At same time I am debugging it - for that I am using AC analysis. In this system I have a differential pair and I would like to measure its open loop frequency response to analyse the pole position and gain.
One of the inputs in connected to a reference voltage (where I am putting the AC source) and the other is a feedback point, coming from a resistor string.
Does anyone knows how to measure the open loop gain of the differential pair while operating on the closed loop system (this is just to maintain the same bias point and loadings).
Regards.

Click to expand...

It would be much easier to give you a substantial answer if you could provide us with a circuit diagram.
Furthermore, please note that for a stability analyis you need the gain of the complete open loop (callred "loop gain") . The gain of the amplifier without feedback (open-loop gain) is not sufficient!

Hi, LvW et all,

Thank you for your inputs and participation. I am trying to learn how to draw block diagrams structures to represent the actual schematic. Sorry for not making myself clear. Basically, my understanding is that you can represent a circuit using the normal schematic approach, small-signal approach and sort of block diagram representing the several gm or amplifier stages.

What's your opinion about this? For example, the block diagram representation is the same as the small-signal blocks? Can we say that? Are they equivalent? How?

So basically what I want to understand is the following:

For example, if you look at t his paper: Paper 1 you will see in the first figure a block diagram representing a circuit. This block diagram is very useful, I think, to derive some equations if you are trying to understand the circuit but I don't understand how the author has put together this.

1. He draws a block diagram without showing any circuitry.
2. Then he presents 2 circuits but he doesn't say anything if it related to the block diagram constructed in figure 1.
3. Another thing that is confusing is the fact that each block has got a sign inside. Why?
4. Are they even related?

Another paper as for another example is this: Paper 2

In this one the author starts with a block diagram, with the respective symbols inside the block. Then he draws the small signal circuit and the schematic using transistors. But the block diagram doesn't picture the capacitors, only 1. Why?

Another good example of a paper using block digrams is this one: Paper 3

In here the author uses only diagram blocks. Why doesn't he apply that with a circuit to demonstrate which block represents which part of the circuit? What does that mean? That we can use whatever circuit we want as long as we follow the signs? But in my case I don't understand the signs.

Finally found a presentation where the author has got the block diagram side-by-side with the circuit. But the problem with the signs follows! Don't understand why do we have those signs: Presentation 1

This one is a good example. Here the author uses the small signal to derive the equations but the other author from paper 3 uses only block diagram! How come?

Hope this was clear. Thank you very much in advance.

I like to mention that none of your linked papers contain something we call "block diagram" - all are circuit diagrams.

In general: In circuit diagrams, we have parts represented by symbols (note that intergated circuits like opamps etc. are considered as a part with a corresponding symbol). These drawings allow application of all the Kirchoff laws (KVL, KCL).
This does NOT apply to a block diagram. All variables are considered as signals only. For example: A current will NOT be split into three currents when we have a common node. More than that, variables in a block diagram must not be necessarily voltages or currents. Other physical quantities are allowed like pressure, temperature, ....
Such a block diagram visualizes the various mathematical relations between physical quantities in a complex system.