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93 Threads found on edaboard.com: Stage Cmfb
I am designing a 2 stage fully differential op-amps shown in the schematic: 118256 The resulting Bode Plots are shown here (magnitude and phase of vo_2_r - vo_2_l): 118257 My question is about the stability analysis, what should i do regarding the cmfb circuit, i heard about the iprobe com
I am building a 2 stage op-amp similar to that in the thread referenced. Mu question is about the cmfb, the solution in that thread is implemented using IDEAL voltage controlled voltage source, now if i want to realize it on transistor level, what should i do? or should i implement anothe
The common mode gain of an amp stage across frequencies depends on the ugb of the cmfb loop of that stage. In two stage amps, the differential loop forms a positive feed back for common mode signals. Hence it is essential to ensure that the common mode gain of your open loop amp is less than one across frequencies, which (...)
Hi, what are poles and zeroes in a compensation network like Type-3 compensation for 2 stage cmfb amp. how we can boost phase margin without affecting Bandwidth. by using miller compensation cap, BW decreases. thanks Safiya
I have designed 5MHz bw 2stage cmfb diff amp. but i could not adjust the Phase Margin using R,C values for the compensation, in my case when i achive PM = 60 , bw reduces to 800kHz. is there any method to compensate this amp of BW 5M or 3MHz with enough PM. Regards, Safiya
Hi guys, I want to design a full-differential two-stage OPA. The first stage is telescope cascode structure and the second on is usual common source structure. I want to use switched-capacitor cmfb to make the output common-mode voltage stable. I am wondering now that how many SC-cmfbs I should add in the OPA. One (...)
This is a simple two stage OTA ... with fully differential output at first stage and single ended at the second stage. Gain will be approx gm^2 (ro || ro)^2 .. assuming the all the ro and gms are same. The MOS arm connected to VB2 is used for cmfb (Common Mode Feedback) as the first stage is fully (...)
I think the circuit already has a cmfb around the first stage as mentioned in the text. For the 2nd and 3rd stages they don't need cmfb because they are self-biased and not supplied by current sources from both top and bottom. This is assuming good matching of the current mirrors between the 2nd and 3rd stages.
Fully differential opAmps always need a cmfb. Here, the cmfb stage is already integrated: M41 - M46 . The common mode feedback works from M46 into M36 , whereas M43 to M34 as well as M44 to M33 provide positive 1/N feedback to the output stage.
Hello everyone, I'd'like to design a switched capacitor amplifier 94797, But I don't know how does this amplifer work. Here is my own point of view: in the S1 phase, the circuit in the sampling phase, VCM and LM49 supply a cmfb; in the S2 phase , the circuit is amplified stage. However when I use the spectre simulation
You might be able to get a common mode of Vdd/2 by appropriate sizing and also using cmfb. You cannot fix the common mode without using cmfb otherwise it would vary with corners. Another option is to have a second stage. It would be easy to use cmfb to get Vdd/2.
I am designing a 2 stage Miller compensated OTA. I have problem in fixing the output common mode voltage. There was no problem when i designed a differential output OTA, as a cmfb loop ensured proper output DC voltage. But when i tried to design a single ended OTA, i dont get the desired common mode voltage. The first stage common mode (...)
Hi, I was wondering how the polarity of your cmfb works. I am assuming you are just sensing with resistors. If you sense the output of the second stage and feed that back to the first stage then it would cause positive feedback. Consider the output CM voltage from the second stage lowers a bit. This means your feedback (...)
I am designing a 2 stage Class AB op-amp , with cmfb network, The OP-Amp will be used as an integrator (with Capacitive -ve feedback). But i have a question regarding the stability , GBW and PM calculations: I am not sure whether i need to check the stability and PM condition, 1) Using phase and gain of the differential output . With NO
but the first stage was not fixed ......why does this happen By nature of a two-stage amplifier. It's neither feasible nor useful to bias the first stage output to Vdd/2. The bias point of the first stage is commanded by Vth of the second stage transistors.
Hello, I'm drawing a 2 stage fully differential miller OTA with 2 cmfb networks. These cmfb networks are ideal and contain a vcvs with a gain "Acm". However, when I put 2 instances on my schematic and use the ADE (Analog Design Environment) to retrieve the variables I only get 1 variable Acm for the 2 blocks. I wish to control them (...)
Hi,everyone I've got some questions about cmfb stability in two-stage op amplifier. 1) Figure 1 is a schematic of two-stage opa with continuous cmfb circuit. I insert a device of iprobe (in analogLib library) to simulate the loop gain and phase of cmfb circuit. Figure 2 shows a voltage buffer (...)
Where did you get this strange amplifier topology? For reasonable gain and common mode range, I would expect p channel transistors in place of M1 and M2 and the output stage driven from the top (M10/M12) rather than from the bottom (M5/M6).
Hi, this topic was discussed before, it did show 5 threads when i typed the tilte. here is one i almost used the same cmfb scheme as the last post for the class AB output stage, that is tuning the PMOS gate by adjusting the current into the output nodes. check the picture below 81313
I think this is a good question. C2 directly loads the output stage all the time, while C1+C2 loads at half clock cycle. for faster settling time to arrive ideal Vcm, C1 has to be larger than C2, right? did k.martin tell the reason why c2 should be approx. 10 times of C1?
hi, but i think for the class AB output stage this kind of cmfb is not good, it has the linearity problem at large signal swings, I prefer to use resistive one for continous time output stage cmfb. can you tell me which paper is that talks about SC-cmfb for class AB output stage? i think (...)
while dseigning,an op-amp,how to decide the cmfb voltage that is to be fed to the first stage of the op-amp? The cmfb control voltage should be able to keep the output voltage of the op-amp within an acceptable output range, i.e. more or less in the center of the available output voltage range.
Surely they are related. Please consider what happens if the input stage is driven by a high common mode impedance. Also remember that MOSFETs have finite input impedance at higher frequencies. How about sketching an equivalent circuit of the common mode amplifier and feedback path?
I was wondering why analog design books dont present common mode feedback circuits for single ended two-stages op-amp. Can someone propose a cmfb circuit for design of single ended differential CMOS op-amp? Thanks.
OK, so you want balanced output like this? I didn't think that would be useful for a sample and hold circuit. Anyway, I think both circuits can have the same speed. Implementation of cmfb will be different though, so a lot depends on the first stage. ---------- Post added at 08:08 ---------- Previou
what do you mean by second stage in ur pic? Folded cascode is single stage amplifier, though with a rather comlicated load... as for ur pic, ur input is too small: only 20uV peak to peak. The output then is acceptable since even a 60db gain would not be able to rise output voltages to opamp saturation. One thing u must improve upon is the jitter
thanx I am not use cmfb simply Folded Cascode Input and Common Source Output stage with 4 pf load capacitor my gain is 73 db. Now what is the error ? cmfb is require ? now I want to design 4.3MHz LPF using these opamp but it is not working can you solve these problem.
yeah I tried it and I got gain as expected Oh good! It's your lucky day then ;p If you look at your output stage, have you ever wondered who controls your output current? Basically, both the PMOS and NMOS current mirrors are trying to force a current at the output. Device mismatch, short channel, systematic offs
Dear All, I am designing a push-pull regulator for digital circuits. The schematic is as attached. It consists of pre-amplifier followed by comparators (push and pull side) and finally switched source-follower output stage (push and pull side). I have verified that all the feedback loop (pre-amp for cmfb and replica bias loop for push and pull
Hi all I have problem design 2 stage fully diff opamp with sc cmfb. I am using 0.18um from tsmc process. When I run w/o sc cmfb the gain about 65db, phase margin 64 degree and GBW~540Mhz. If sc cmfb is connected the gain a little bit reduce to ~60db but outputs from negative and positive have diff result for GBW and phase (...)
HI I have designed a capacitive feedback OTA ( OTA is folded cascode single stage OTA with ideal cmfb). When I simulate the loop gain of OTA (using iprobe and stb analysis in cadence) I get a 'High Pass' response at low frequencies. I am not sure why this is happening. Please help to solve this issue. Thanks
can anyone help out with the design equations for the below 2 stag
Hi, Please can any body help me in designing common mode feedback circuit for two stage amplifier.
i have designed a two stage fully differential opamp with hybrid ahuja compensation (that is, two compensation capacitor used in the two cascode nodes of each branch), the first stage is folded cascode, the second is common source, and the cmfb is continuous common mode feedback which have two input pairs, one pair connected to vcm, the (...)
i design a 2 stage fully differential opamp. the cmfb of the first stage is continuous-time cmfb and the second stage is the SC cmfb. the problem is:i dont know how to simulate its SR,settling time,ac etc. thanks for you help. Added after 3 minutes: or you can give me
Hi,everyone I've got some questions about cmfb stability in two-stage op amplifier. 1) Figure 1 is a schematic of two-stage opa with continuous cmfb circuit. I insert a device of iprobe (in analogLib library) to simulate the loop gain and phase of cmfb circuit. Figure 2 shows a voltage buffer (...)
I designed a fully-differential two-stage OTA. It is a telescopic pmos input one(cascoded) with a nmos common source amplifier. The cmfb feedback to the bottom nmos transistor of the first stage. I have done the biasing and cmfb circuit, I compensated the circuit with lead compensation. The openloop small signal gain is (...)
hello, I am designing a two stage operational amplifier.I always see the cmfb to control the out of the first differential input stage. How about the comon source stage? Why not use the cmfb to control the output of the common source stage? Thanks!
hi, i'm confused by a problem like this: i design a fully 2-stage opamp with continue cmfb. and the ac analysis show dc gain is 90db ,also have go0d phase response. but after post-simulation ,the gain is only 50db! and To make things worse, the phase have Obvious Curved-up before the gain to 0db ! . Just for clarification:
wangh,I met the same situation with you. You have to match the current of the cmfb stage and the feedback opamp stage.
I think if you are using the two stage amplification in the cmfb amplifier, the same problem of two stage Opamp stability problems are appearing in your circuit. May be your test bench could be missing some capacitances required. Connecting the two differential outputs with two identical capacitors to the cmfb node may solve (...)
Dear All I have a standard two stage standard OTA, first stage is folded cascode, output of first stage is given to gate of NMOS transistor with PMOS current source on top. I am using Switched cmfb to adjust the voltage at the gate of first stage PMOS current sources. I would like to know what are (...)
For a folded cascoded two stage opamp, Q1: Typically, we just add one cmfb from 2nd stage output to 1st stage current source. Why don't we add 1 cmfb for each stage? Is there any drawback? Q2: If we just need one cmfb, why do we sense the 2nd stage (...)
I am going to design a cmfb in two stage folded cascode differential opamp. according to the , somebody said "we need to have two cmfb circuit in two stage." is it correct that two cmfb circuit is needed in two stage folded cascode differential opamp?[/
Hi friends, I am starting to design a folded-cascode fully differential amplifier OTA with cmfb. I am confused and don't know how to start because of the large number of mosfets. I know that the ideal flow is to start with hand analysis then move to the simulation stage, but I have two problems with hand-analysis, first is the
i have designed a folded cascode fully differential opamp used for pipelined adc(1.5 bit per stage),but there are some problems: when i test it in open loop with ideal cmfb(vcvs source),the GBW is 103M,dc gain is 80 db,-3db bandwidth is about 10khz;however,when i test it in closed loop with sc-cmfb,there are some strange problems: first,my (...)
For two stage fully differential op amp, both first and second stages are fully differential, do both stages need their individual common mode feedback circuit? or just the second stage need common mode feedback circuit. Thanks!
u have to use cmfb ckt for 2 stage fully diff op amp.
You can design a voltage either from follower or Vt for reference input of cmfb's in every stage.
Hello, i am going to design a three or four stage single-end OpAmp. Input Vdc=1V,Output Vdc=1V. Someone can tell me, if a cmfb is needed for each stage or only a cmfb from Voutput to first stage? Thanks a lot!