Dear Hagar,

The stability of any feedback network depends on the amplifier gain. It is so that the open loop gain increases by increasing the gain of the linear amplifier.

Since the gain of the amplifier depends on the DC operating condition then one has to bias the amplifier at the a suitable operating point.

I propose that you simulate the amplifier to get its frequency response

and also simulate the feedback network to determine its transfer characteristics.

In this way you can design you system with the required stability.

Best wishes

Thank you Dr. Zekry, but also I have a question could you please check the attached 2 graphs the first one shows that the loop gain is below 0 dB does that mean that the system is stable, and the 2nd graph shows that the loop gain is above 0 dB so does that mean that the system is unstable? Abdelhalim abdelnaby Zekry

Dear Hagar,

All you need is to apply the Barkhausen criterion” for sustained oscillations.

It is so that the negative feedback amplifier will be stable so long mag. AB is smaller than one at -180 degrees phase shift and it may oscillate if mag. AB is greater than one at -180 degrees phase shift.

Accordingly, so the amplifier at the top figure is stable. While it will be unstable in the bottom figure. The probable point of oscillations is slightly to the left of M2.

You can test the stability by an input impulse, if the amplifier is stable the impulse will decay and it will grow if the amplifier is unstable.

This means that you let the amplifier function as an oscillator.

Yes what do you said is okay.

But I see in the top figure, the amplifier shows zero response at specific frequencies.

Best wishes

Hi Hagar,

Yes it does. The operating point does affact the stability analysis of the circuit you’re doing. The STB analysis (also ac analysis) is calculated based on the operating point. The DC roots of your circuit will determine some small-signal related parameter such such as rout, gm, parasitic capacitance etc. That why during our design phase the circuit must be considered in terms of DC operating range.

As shown in the captured graph above, I believe that you’re designing in Candence. And the result shows that there is no phase margin in your design (0dB Gain should have phase less than 180 degree. Rule of thumb is >45 and

thank you so much Dr. Zekry, and Van, but what should I do, If changing a parameter affects the DC operating point, so is there any automated thing to recompute the dc operating point by sweeping the parameter in cadence @@

Abdelhalim abdelnaby Zekry Van Ha Nguyen

Hagar,

You can search in the manual of the cadence. What the parameters that can be scanned.

My comment is that what is the specifications of your amplifier? what amplifier do you want to achieve. You must not rely totally on the tool and leave the manual analysis and intuition. I see that you have an operational amplifier. the operational amplifier voltage gain is more or less independent from the load at the output of the amplifier. So all you need is to apply a moderate load on the output stage.

The operating point of the differential amplifiers depends on the operating power supply voltage. The power supply voltage is set by the technology generation of the circuits.

I think you must search for the highest gain curve. This curve will be determining for the stability.

Best wishes

Dear Dr. Zekry, this is the circuit that I aim to study metastability in it attached here as well as I changed the values of the R1 resistor sweep its value and the dc operating is very sensitive due to the high gain, so I am not able to adjust the dc operating point as you see in the previous comment sometimes the gain is below 0db and some times with above 0db and this is affected by dc operating point.

Hagar,

Nice, the things became more clear!

Please what is the function that must be basically performed by this circuit?

Best wishes