Which type of OTA should I use for designing a bandgap buffer?

[tyanata]

I have to design buffer for bandgap voltage=1.2V at 0.35LV technology.
The requrements are low noise, gain>65dB, PM>70deg, GBW>6MHz, SR>10V/uS, VDDmin=2.4V.

Can any one give me advice wich type OTA is most approprite for this goal,
Now I try to use folded casscode OTA as a first stage, after that invertr with Miller capacitor, and after that the source follwer. Is it the better case?

 

[swicap]

Re: Bandgap_buffer

I have to design buffer for bandgap voltage=1.2V at 0.35LV technology.
The requrements are low noise, gain>65dB, PM>70deg, GBW>6MHz, SR>10V/uS, VDDmin=2.4V.

Can any one give me advice wich type OTA is most approprite for this goal,
Now I try to use folded casscode OTA as a first stage, after that invertr with Miller capacitor, and after that the source follwer. Is it the better case?

bandgap buffer amplifiers are often loaded with large capcitors, so 70deg Pm is
difficult for two stage amplifier( if you set the pole of 1st stage as dominant pole).
folded cascode amplifier is more noisy than telescopic amplifier.

 

[jordan76]

Re: Bandgap_buffer

Hi

One thing to remember: in your application the gain is fixed(around 2), so this alleviates the stability problem somewhat.

As swicap pointed, load capacitances are often very large, so it is better to set the output pole as the dominant pole.

Therefore, besides two stage miller compensated OTA, another possible option is: put a low gain,wide band buffer amplifier before telescopic op-amp.

hope it helps

regards,
jordan76

 

[tyanata]

One of the problems

The telescopic stage is unapproprate because the circuit has to can wark at VDD=2.4V and on the output of the source follower we have about 1.2 by default + Vth worst case = about 0.8V + Vds sat about 0.1V , hence the input of the source follower could be at 2.1Vworst case.

 

[swicap]

Re: One of the problems

The telescopic stage is unapproprate because the circuit has to can wark at VDD=2.4V and on the output of the source follower we have about 1.2 by default + Vth worst case = about 0.8V + Vds sat about 0.1V , hence the input of the source follower could be at 2.1Vworst case.

since ur gain requirement is low, you needn't use telescopic amplifier.
A simple amplifier as first stage is ok, which also improves frequency response.

 

[ccw27]

Re: Bandgap_buffer

Agree, a simple 5 MOS diffamp buffer will do the job

 

[tyanata]

Decision

Finally I used telescopic OTA, after that PMOS source follower to preserve the OTA from the output follower, and after that the output NMOS follower. It became quite good.