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Design >> Analog Design >> Telescopic ota and its CMFB amplifier
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Message started by cetinkayahkn on Jan 28^th, 2017, 9:52am

Title: Telescopic ota and its CMFB amplifier
Post by cetinkayahkn on Jan 28^th, 2017, 9:52am

Dear All,

I would like to ask you a question about a
telescopic ota and its cmfb circuit as depicted
in the figure, attached.

In this configuration, I used it in a unity gain
capacitive feedback. In typical condition, cmfb could
set the output to the desired vocm value.
When I applied an offset voltage to one of the
input transistors, one output of the amplifier jumps
down, and other jumps up. This means that DC
stabilization is deteriorated.

Why CMFB can not set the output in an offset condition?
How could I correct it? Is it normal, or something is wrong
in the circuit?

I would like to wait for your sincere helps

Regards

Title: Re: Telescopic ota and its CMFB amplifier
Post by ULPAnalog on Jan 28^th, 2017, 11:05am

Is it an AC coupled amplifier? (Did you mean Cin = Cf to achieve unity gain?) If so, how is the input common mode set? Do you have mechanism to get the DC negative feedback (through pseudo resistors or something) to fix the input common mode?

Offset that you applied, is a differential signal so there is nothing cmfb can do.

Perhaps it would help if you can post the feedback configuration.

Thanks and regards

Title: Re: Telescopic ota and its CMFB amplifier
Post by cetinkayahkn on Jan 28^th, 2017, 11:57am

Dear ULPAnalog,

Firstly, I thank you for your questions.

I posted a more detailed schematic.
The amplifier is in a unity gain capacitive feedback.

I put a DC voltage source as depicted in the
attached file. Actually, I thought that Vicm could be set
by CMFB by mirroring the current on two diodes which
was already in a negative feedback but it did not solve the offset problem.

I simply model 2 mV offset, occurring in a mismatch condition.

However, in the offset condition, CMFB does not work, one output
slips down, and the other slips up.

How could I set vicm in a way that could set output correctly even
in a offset condition.

I thank you very much for your helps.

Regards

Title: Re: Telescopic ota and its CMFB amplifier
Post by ULPAnalog on Jan 28^th, 2017, 12:20pm

Ok. I believe your input to the opamp is floating at DC and hence the common mode is not being set correctly. Did you check the DC node voltage at the inputs of opamp (without the offset). They should read 0V.

You can modify the circuit as attached and make the resistance large say 100G. Check the input common mode in that case again (of course if the gate leakage is large, then you will have a trouble, you might then want to reduce R and see). Apply offset and check the behavior.

Bottom line is that the input common mode of the opamp needs to be set correctly.

Title: Re: Telescopic ota and its CMFB amplifier
Post by cetinkayahkn on Jan 28^th, 2017, 3:12pm

Dear ULPAnalog,

Thank you for your directive answer. It helps a lot.

The circuit is bias well now. I use resistors as you depict
them in your figure. Those resistors biased the gate of the input
transistor as vocm.

Now, I would like to eliminate the tail current as depicted
in the figure in this message. However, it is hard to guarantee
the current flowing on each branch, and biasing with resistors
in the same topology does not help. Since the gates of the input
transistors are not set to vocm.

Dear ULPAnalog, I hope you would like to enlighten how
to bias a tailless pseudo differential amplifier :)

Thank you in advance

Regards

Title: Re: Telescopic ota and its CMFB amplifier
Post by ULPAnalog on Jan 29^th, 2017, 10:22am

You could redesign the input pair such that the nominal current it carries when the input common mode at DC is equal to output common mode as the resistive feedback forces that.

Alternatively, you can use a configuration in which the input common mode can be set independent of the output common mode,say by adding resistors at the input of the opamp and tying them to say Vi,cm. One such configuration is shown in the paper "0.5-V analog circuit techniques and their application in OTA and filter design" by S. Chaterjee et al. Although it is for DC coupled resistive feedback OTA, you can extend it to your case with modifications.

Title: Re: Telescopic ota and its CMFB amplifier
Post by designerH on Mar 6^th, 2017, 11:09am

Dear AnalogULP,

I thank you for your informative explanations. The paper
was quite helpful to understand how to set the bias of
the amplifier correctly.

What is the difference between CT-CMFB and SC-CMFB?
How are the Vocm and Vicm of an amplifier set by SC-CMFB,
correctly, in process variation including mismatches. Since, in any
process variation, CT-CMFB sets the bias point in
skew.

An MDAC of a pipeline ADC can be given as an example
for that.

I would like to thank you in advance for your reply.

Sincerely