Open loop gain and phase margin analysis !

[biolycans]

Hi !!!!

I would like to know how can I measure the open loop gain of an operational amplifier CMOS.

REgards,

Joak

 

[Audioguru]

Why measure it? Simply look at the spec's and graphs on its datasheet.

 

[biolycans]

Why measure it? Simply look at the spec's and graphs on its datasheet.

Hi,

I designed an opamp CMOS two stages. I am doing the characterization of the opamp.

I have an example to measure the open loop gain but there is something I am not understanding. First I found the offset of the opamp then I use an AC 1 stimulus. What I am not understanding is the AC 1, because if I use a signal of 1 V amplitude in the AC sweep (open loops system), then the opamp should be saturated.

 

[FvM]

So you are actually asking about simulated "measurements".

There are two points to consider:
- you need to understand the nature of small signal AC analysis. Voltage levels are meaningless in small signal, they aren't but scaling factors. Selecting a source voltage of 1 V has the advantage that output voltages can be directly read as gain numbers.
- to get right loop gain, the amplifier must be correctly biased. A smart way is to do it in a feedback setup that achieves right DC operation point but doesn't affect the loop gain.

 

[biolycans]

So you are actually asking about simulated "measurements".

There are two points to consider:
- you need to understand the nature of small signal AC analysis. Voltage levels are meaningless in small signal, they aren't but scaling factors. Selecting a source voltage of 1 V has the advantage that output voltages can be directly read as gain numbers.
- to get right loop gain, the amplifier must be correctly biased. A smart way is to do it in a feedback setup that achieves right DC operation point but doesn't affect the loop gain.

Hi, thank you for your response. I know that in the small signal analysis I should work with small signals.

I understand that there are two ways to achieve the right DC operation point. One is for example I connect the inverting input to ground and the not inverting input to a voltage source, then I do a dc sweep to find the OFFset that produce a zero voltage in the output.

The other way is what you told me, a feedback setup, using a huge resistor in the feedback and a huge capacitor to ground.

Related to the 1V source, if I don´t have a feedback, If I put a 1V AC signal with the offset I measured I am in the right operating point but I don´t understand why I have to use a 1 V signal to measure the open loop gain instead of a small signal (because I am in an open loop configuration).

I would be very grateful if you can help me.

Regards,

JC

 

[FvM]

I know that in the small signal analysis I should work with small signals.

"Should work" misses the point . In small signal analysis, any signal is treated as if it was small signal, because a pure linear circuit model is used for the calculation.

This is however a two step action:
- First determine a DC operation point using a non-linear model
- Secondly use a linear circuit with gains, node impedances etc. determined in the first step

In so far AC source levels used in small signal analysis are arbitrary. You may use µV, V or kV as you please. Using V simplifies the gain calculation, that's all.

But if you miss the correct operation point, all results are wrong.

You can use an open loop circuit if you don't mind to adjust the bias point manually, e.g. by a DC input source. But you should verify that the amplifier output voltage is in the expected range by a .DC analysis.