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very common traditional OTA issue

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Junus2012

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Hello

for this common traditional OTA, there is something confusing, some of the references considering the GBW as gm1/2pi.cl like Prof.Jakob ( i attached this from his book) others are multiplying it with K.

The other thing still some of the Editors like Prof Holberg are calling it two stages OTA while I see as well in other many references it is one stage OTA.



So what you see??
Welcome to discussion
 

well if you read the page you attached, he did say assume K=1.

i think of this as two stage, with second stage being a current gain stage. you can think of first stage generating a ss current gm*vin and you use current mirror to multiply it by K to the output. so the second stage current gain is K.

if you follow his steps with K=/=1 you can write vout= K*gm*vin *1/(jw*CL) and solve the same equations and you should see K in your final answer
 
Thank you circuitrookie,

I also read that but i am inquiring why he didnt put it in the general equation , any way lets tke the factor of K as you also said.

Moving to the second part, if you say that the mirror will be considered as a gain stage then why we didnt consider it for the folded, the folded still to be said that is single stage amplifier and no doubt about that. isnt it????
 

senan said:
Thank you circuitrookie,

I also read that but i am inquiring why he didnt put it in the general equation , any way lets tke the factor of K as you also said.

Moving to the second part, if you say that the mirror will be considered as a gain stage then why we didnt consider it for the folded, the folded still to be said that is single stage amplifier and no doubt about that. isnt it????
Click to expand...

folded cascode only has one gain stage. the input pair simply inject current into the main branch which gets multiplied by the output impedance to get the output voltage. i don't think the technicality of one or two stage matters as long as you understand how they work.
 

Yes I know how it work but the problem I am writing a technical data report about this circuit and I must be accurate with my say, if I tell it is one stage and it is two stage then I will lose some points,
the thing that make me more believe that this circuit is one stage is that "self compensated circuit", if it is two stage then it need a compensation capacitor which this doesnt need

circuitrookie said:
folded cascode only has one gain stage. the input pair simply inject current into the main branch which gets multiplied by the output impedance to get the output voltage. i don't think the technicality of one or two stage matters as long as you understand how they work.
Click to expand...
 

senan said:
Yes I know how it work but the problem I am writing a technical data report about this circuit and I must be accurate with my say, if I tell it is one stage and it is two stage then I will lose some points,
the thing that make me more believe that this circuit is one stage is that "self compensated circuit", if it is two stage then it need a compensation capacitor which this doesnt need
Click to expand...

why does two stage always need a compensation capacitor? if the two poles are far away apart, you dont need compensation. for your circuit, the interstage node has impedance of 1/gm which will push the pole quite far away, leaving your output pole as dominant pole
 
Thnax circitrookie

your answer helped me , any way Jakob book consider the circuit as single stagee and this is why got confused
 

senan said:
Thnax circitrookie

your answer helped me , any way Jakob book consider the circuit as single stagee and this is why got confused
Click to expand...

sorry i don't know the technicalities behind considering it a single or second stage. Razai's book p309 consider this as two stage though, i would trust him and maybe put a reference to it at your paper then no one can deduct point from you.

edit. my apologies the topology is not exactly then same on p309, but on p 339 he has the same topology and he used terms like "first stage"
 

Yes that is real two stage as you have later corrected , in usual two stages op-amp the conversion to single ended is done at the first stage, with Razai he did the conversion in the second stage

circuitrookie said:
sorry i don't know the technicalities behind considering it a single or second stage. Razai's book p309 consider this as two stage though, i would trust him and maybe put a reference to it at your paper then no one can deduct point from you.

edit. my apologies the topology is not exactly then same on p309, but on p 339 he has the same topology and he used terms like "first stage"
Click to expand...
 

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