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a question about an op amp design

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airboss

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Hi,

I'm a beginner to analog design. I'm working on a project with a spec as follows:
*****
supply 1.8 v
input range p-p 0.9+-250mv pp500mv
output range p-p diff 1v
unity gain bandwidth 1G
phase margin >60
power consumption<20mw
load 1.5pico
*****
My first question is, here, input differential voltage is p-p 500mv and output range is 1v. Does that mean, the Av = 2?

2nd, I want to use telescopic cascode. Do I need to worry about the input signal range? In Razavi's example, he doesn't take the input range into account. But why?

3rd, here I have to get GBW = 1G. How can I start to meet this requirement?


Thanks for any advice in advance.
 

I think that the closed loop gain would be equal 2. This doesn't mean that Opamp's gain is 2.

Also, a 1G Gain Bandwidth product is weigh too high. Most probably it will be impossible ( but can't be 100% sure of that ).

I think for the given speed and input range, the telescopic is not a very good candidate ( very limited i/p range ). Perhaps you may try a current mirror Opamp ( but compensation won't be easy ).
 

    A

    airboss

    Points: 2
    Helpful Answer Positive Rating
use folded cascode opamp with gain boosting amplifier (which is also a folded cascode opamp). see baker li for more details.
 

Hi,

would it be easire to use 2 stages?

or can anybody share how we can determine the number of stages and the topology we can use?



thank you.
 

I don't think that use 2 stage is easier

If you design your ampli with a good method and if you respect all the constraints, you will have a good amplifier!

I think too that the telescopic is not a very good idea!
 

airboss said:
Hi,

would it be easire to use 2 stages?

or can anybody share how we can determine the number of stages and the topology we can use?



thank you.
Click to expand...
Two stage opamp can't meet your speed requirement.What technology is your choice,0.18um or 0.13um?
 

airboss said:
Hi,

would it be easire to use 2 stages?

or can anybody share how we can determine the number of stages and the topology we can use?

thank you.
Click to expand...

I think it's diifcult to get 1GHz UGB with reasonable gain using two stage. Best option is folded cascode with gain boosting. Here you need gain boosting because as you pump-in more current to get the desired UGB, your gain reduces.
 

    A

    airboss

    Points: 2
    Helpful Answer Positive Rating
you can read up on the high speed low offset op amp
 

First, output voltage swing is twice of the input voltage doesn't means that the Av is 2. you can't get the Av,especially the dc gain from this spec.

Second, because the supply voltage is 1.8v and the output range is 1v, especially the input range is 0.5v, telescopic architecture maybe can't accomadate these specs. Folded-cascode is a good choince.

Last, the ugb is 1G,so gain-boosting technique is needed.
airboss said:
Hi,

I'm a beginner to analog design. I'm working on a project with a spec as follows:
*****
supply 1.8 v
input range p-p 0.9+-250mv pp500mv
output range p-p diff 1v
unity gain bandwidth 1G
phase margin >60
power consumption<20mw
load 1.5pico
*****
My first question is, here, input differential voltage is p-p 500mv and output range is 1v. Does that mean, the Av = 2?

2nd, I want to use telescopic cascode. Do I need to worry about the input signal range? In Razavi's example, he doesn't take the input range into account. But why?

3rd, here I have to get GBW = 1G. How can I start to meet this requirement?


Thanks for any advice in advance.
Click to expand...
 

    A

    airboss

    Points: 2
    Helpful Answer Positive Rating
Hi,

Now I get an extra condition. The DC gain is 65 dB at least. Is it good to keep working on folded cascode + gain boosting or 2 stage is fine now?



thank you.
 

Hi,

as I said in my earlier post it's better to work on folded cascode. There, by reasonable gain I meant gain>60dB.

Regards
 

in general, 2-stage has superior frequency response, while the fold has better common input range.
merely for 65db, either can achieve readily in case of present processes
yet if you require a 1g unit-gain bandwidth with that "high" dc gain, as posted above, gain-boost is use to attain bandwidth goal.

airboss said:
Hi,

Now I get an extra condition. The DC gain is 65 dB at least. Is it good to keep working on folded cascode + gain boosting or 2 stage is fine now?



thank you.
Click to expand...
 

    A

    airboss

    Points: 2
    Helpful Answer Positive Rating
youyang said:
in general, 2-stage has superior frequency response, while the fold has better common input range.
merely for 65db, either can achieve readily in case of present processes
yet if you require a 1g unit-gain bandwidth with that "high" dc gain, as posted above, gain-boost is use to attain bandwidth goal.
Click to expand...

Hi youyang,

do you mean, use gain-boost in 2-stage opamp???
 

hi ipsc,
i just make a comparasion of the 2 circuit in terms of their advantages.

to your design, the small power supply and input common range makes 2-stage a not good choice, that is the spec on input common range should be the firstly-considered factor in choosing your circuit struct.
from this point, you can choose folded, or rail-to-rail
then the high unit gain bandwidth, which is approximately the product of dc gain and -3db bandwidth,
can be attained by achieving a high dc gain.
from this point, you can choose gain-boost and other techniques.




ipsc said:
youyang said:
in general, 2-stage has superior frequency response, while the fold has better common input range.
merely for 65db, either can achieve readily in case of present processes
yet if you require a 1g unit-gain bandwidth with that "high" dc gain, as posted above, gain-boost is use to attain bandwidth goal.
Click to expand...

Hi youyang,

do you mean, use gain-boost in 2-stage opamp???
Click to expand...
 

    A

    airboss

    Points: 2
    Helpful Answer Positive Rating
Hi,

I guess this question sounds a little stupid, but....
I've heard rail-to-rail a few times but I don't really know what it is. Can you give me some examples?


thanks
 

airboss said:
Hi,

I guess this question sounds a little stupid, but....
I've heard rail-to-rail a few times but I don't really know what it is. Can you give me some examples?
thanks
Click to expand...

rail-to-rail: VDD-to-VSS.
Input rail to rail: Common mode range VDD-to-VSS.
Similarly ouput rail-to-rail: Ouput swing VDD-to-VSS.

input rail-to-rail folded cascode is a special case of folded cascode where the input diff-pair/stage comprises both NMOS pair and PMOS pair and has input common mode range VDD-to-VSS.

Hope this helps.
 

You can find good examples in Huijsing book, which is avialable in download section.
 

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