Hi all,

I need some help on the stability analysis of my cmfb circuit.
I used a simple resistive divider to sense the output common voltage, and compared it with the threshold voltage of a inverter (standard cell). Also, in order to do the 'stb' analysis, I put an 'iprobe' between the output of the cmfb circuit and the gate of tail current sink.
results shows that the gain crossover point is at around 600MHz and phase crossover point at around 200MHz. according to Razavi's analog book, the loop is unstable.

now, question no. 1: Is it necessarily true if the input signal I'm working with is around 100MHz, which means at that frequency the phase shift is less than 180 degrees?

then I did a transient analysis on this circuit. With small signal input (<50mV amplitude), the result shows the output is ok, no oscillation occurs. But with large signal input (>100mV), oscillation occurs.

question no.2: why is this happening? and if this is because the loop is unstable, is there anything I can do to compensate it?

question no.3: what's the feedback factor, β, in this circuit? According to Razavi' s book again, "the β is less than or equal to unity and does NOT depend on the frequency". But it seems to me that, when the sensed output cm voltage is fedback to the main amplifier, it goes through another amplifier, which is dependent on the frequency.

I have gone through several analog books, but none of them seems to talk about the stability analysis of the cmfb circuit. so if anyone has a idea on this topic, please help me out. thanks in advance.

Mike

Hi,
    To Compensate: If you don't need very large "Gain cross-over" frequency for the CMFB loop, you may reduce the input stage Gm of the CMFB loop [The input stage Gm for CMFB loop is formed by the tail current source of Main-Amplifier]. This can be done by reducing the aspect ratio of the tail transistor which is controlled by the CMFB return voltage. Proportionately increase the W/L of the    tail current portion which is controlled by the Bias Voltage.

Hope this would help.
Packiaraj.V.

Hi,
 The method proposed by avlsi seems the best method.


The method I use is the same. I use inductor and capacitor to test stability.

one end of inductor connected to the CMFB amplifier input and the other end to the forward amplifier resistor divider mid point.

Connect a capacitor to the inductor node connected to the CMFB amplifier input. Connect an AC source to the other end of the capacitor.

Measure for PM and GM at the inductor node connected to the mid point of resistor divider network.

Cheers,
SBR

The above said method is good, when u work with HSPICE. I use HSPICE for my work.

Hi SATurn,
Thanks fro the reply. My design should be low power. I'm burning 30 uA as Opamp's nominal current (tail current source of diff pair) however each branch of cmfb in above picture draws only 8 uA. If I force TN8 for 30uA  it will be beyond my current budget. So to have robust and stable one what kind of modification can I do on cmfb without increasing current consumption significantly?

thanks

using inductor and capacitor method doesn't break the DC operation point, but may ignore the loading (cap) effect. mock cap loading should inserted at least.
I think iprobe is more accurate.

avlsi wrote on Mar 26^th, 2007, 4:04am:

Hi, Ken,

I agree with you, but not 100%.
I wonder how does iprobe work? middle brook? I have used it for a while and pretty handy.

But, in certain cases, i.e, trans-impedance amplifer (opamp with a resistor feedback to inverting input), or standard interverting amplifers, by nature, they are shunt shunt feedback, meaning sense voltage, feedback current. Using iprobe ignores the nature. Its result is close to inserting a test ac voltage source and measuring voltage feedback. Bottem line is iprobe gives different Loop gain than using two port analysis, which is also introdued in your book.

Thanks,
Robert


Ken Kundert wrote on May 25^th, 2007, 3:09pm: