tow thomas biquad OTA-C topology

[jimito13]

Hi everyone,

I would like to ask if somebody could provide me a screenshot with the equivalent OTA-C topology for the Tow-Thomas biquad filter,corresponding to the attached picture.

Thanks in advance for any provided help.

 

[JoannesPaulus]

Here is one example...

 

[jimito13]

At first,JoannesPaulus thanks a lot for you reply.I have seen this example already via google search but i have a couple of questions :

First one :

g1-C1 is the first integrator that corresponds to opamp1,R1,C1 to my schematic that i uploaded above.Where is the resistance R3 to the equivalent circuit with the OTAs?

Second one :

What is the role of g3?Does it play the role of the inverting unity gain configuration of opamp3,R5,R6 (R5=R6) of my picture?


Third one :

Which is the role of the feedback connection from v02 to inverting input of the first OTA in the schematic you uploaded?And finally where is the R2 from my pic to the pic you uploaded?

Thanks in advance for any helpful answer.

 

[LvW]

Hi jimeece13,

may I try to answer your questions?

g1-C1 is the first integrator that corresponds to opamp1,R1,C1 to my schematic that i uploaded above.Where is the resistance R3 to the equivalent circuit with the OTAs?

No, g1-C1 mimics the second integrator. Both integrating stages are interchanged.
This is no problem for series connections.Therefore, no equivalent for R3.

What is the role of g3?Does it play the role of the inverting unity gain configuration of opamp3,R5,R6 (R5=R6) of my picture?

No, the inverter is not required since it is easy to implement a non-inv. integrator in OTA technique. This has been done with the second stage.
The role of g3 is to provide damping of the second integrator (role of R3 in the opamp realization). Such an OTA configuration always can be used to simulate a grounded resistor.

Which is the role of the feedback connection from v02 to inverting input of the first OTA in the schematic you uploaded?And finally where is the R2 from my pic to the pic you uploaded?

Such a feedback, of course, is always necessary for an active filter with pole Qs higher than 0.5. The same applies to the opamp configuration.
The resistor R2 was necessary for combining two voltages at the inverting opamp terminal. This is not the case in the OTA structure since the input is at the pos. terminal. More than that, watch the additional inputs which are available only in OTA technique.
Perhaps this helps.
Regards

 

[jimito13]

Thank you very much LvW,you were absolutely clear.I will simulate the equivalent circuit with cadence and i will come back if i have any more questions.

Added after 1 hours 5 minutes:

Well,i will show you a fully differential equivalent circuit,for the first TT structure i uploaded,that i designed by hand.LvW do you agree that i made the transformation correct or you find any mistakes??Thanks for your help in advance.

The pic needs a rotate that i didn't manage to do after scanning the original drawing...sorry for that...

 

[LvW]

Yes, I think the structure is OK. It should work for proper dimensioning.

 

[jimito13]

LvW what do you mean with the phrase "proper dimensiong"?

I implemented the OTA-C structure that i uploaded above,in cadence virtuoso and it seems to work.But,how can i adjust the passband frequency of my low-pass filter since R,C elements doesn't exist in the feedback path around the OTA.I have the same question for the gain,how could i adjust the gain if i need more than 0dB?

Thanks in advance for any helpful answer.

 

[LvW]

LvW what do you mean with the phrase "proper dimensiong"?

I implemented the OTA-C structure that i uploaded above,in cadence virtuoso and it seems to work.But,how can i adjust the passband frequency of my low-pass filter since R,C elements doesn't exist in the feedback path around the OTA.I have the same question for the gain,how could i adjust the gain if i need more than 0dB?

Thanks in advance for any helpful answer.

Of course, you need the transfer function of the circuit for "proper dimensioning" - that means selecting all parameters according to your requirements (pole frequency and pole Q). Since the OTA's take over the role of resistors (voltage-current conversion) you have to adjust the transconductance gm of the 3 OTA's.
As you know, also commercial OTA's have a separate pin for gm tuning.

 

[jimito13]

Ok LvW,i got it!I aslo found a helpful tutorial from google search that connects the gm,C1,C2 of each transconductor with the transfer function (ω0,Q,Gain) of the filter that is close to the the ideal prototype realization.Thanks for your help.

Regards

 

[jszair]

Ok LvW,i got it!I aslo found a helpful tutorial from google search that connects the gm,C1,C2 of each transconductor with the transfer function (ω0,Q,Gain) of the filter that is close to the the ideal prototype realization.Thanks for your help.

Regards

I'm also studying tow-thomas filter. Could you post the link to the tutorial you are referring to? Thanks!!

 

[jimito13]

Hello jszair,

I send you the tutorial i told you above.Hope it is helpful to your study.

Regards,
Jimito13