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Design >> Analog Design >> OTA error amplifier in LDO
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Message started by tenso on Mar 13^th, 2016, 6:47pm

Title: OTA error amplifier in LDO
Post by tenso on Mar 13^th, 2016, 6:47pm

I had some questions in LDO design.

1) Why is it advantageous to use OTAs when designing the error amplifier in LDOs?
I know that we utilize OTA when we are driving small capacitive loads and voltage amplifiers when driving high impedance loads. Since the output of the EA is connected to the gate of the pass transistor device, is an OTA a better option?

2) Is the pass device of LDO always operated in the linear region? I read in a Masters thesis that it is sometimes operated in the saturation region. Is this right?

Title: Re: OTA error amplifier in LDO
Post by raja.cedt on Mar 14^th, 2016, 4:25am

1. OTA/OPAMP is always depends on what kind of you have been driving. In case of LDO no matter what kind of block you design it works as VCVS, because of high impedance. You deliberately want to add some o/p stage to make to VCCS when you are driving heavy current loads.

2. O/p device, generally will be saturation contrary to what you wrote, this give's good psrr and load regulation. of course when you wanted to build a very low dropout, where you can't afford 100mV-150mV due to heavy current and supply limitation you will have to live with triode device. But as far I know no would want to deliberately keep it in triode.

Best regards,
Raj.

Title: Re: OTA error amplifier in LDO
Post by tenso on Mar 14^th, 2016, 10:19am

raja.cedt wrote on Mar 14^th, 2016, 4:25am:

Thanks for the reply. I think I should have modified my first question, is it advantageous to use OTAs instead of OP-AMPS for Error amplifiers ? I have seen papers where they use OTAs as the error amplifier (Millikan, for example).

It is interesting you say that the preferred mode of operation is saturation and not linear/triode for the pass device. I thought that the pass device was used as a resistor in a voltage divider setup and so it used in the triode mode.

Could you clarify your second statement? If I want low dropout, then I use it in triode mode?

Title: Re: OTA error amplifier in LDO
Post by raja.cedt on Mar 15^th, 2016, 2:32am

Please go through basic analogue course, that answers both of your Questions. basically opamp and OTA are same, from the architecture point of view.

Title: Re: OTA error amplifier in LDO
Post by raja.cedt on Mar 15^th, 2016, 2:34am

Please go through basic analogue course, that answers both of your Questions. basically opamp and OTA are same, from the architecture point of view. Take a simple folded cascade opamp, which is a pure OTA because it's huge o/p impedance.

In general don't think about OTA & OPAMP story, just do an opamp for LDO it works.

Best regards,
Raj.

Title: Re: OTA error amplifier in LDO
Post by buddypoor on Mar 15^th, 2016, 7:04am

raja.cedt wrote on Mar 15^th, 2016, 2:32am:

Please go through basic analogue course, that answers both of your Questions. basically opamp and OTA are same, from the architecture point of view.

I rather think, OTAs are much simpler in architecture and, thus, better suited for integration purposes.

Title: Re: OTA error amplifier in LDO
Post by tenso on Mar 15^th, 2016, 8:33am

raja.cedt wrote on Mar 15^th, 2016, 2:34am:

Please go through basic analogue course, that answers both of your Questions. basically opamp and OTA are same, from the architecture point of view. Take a simple folded cascade opamp, which is a pure OTA because it's huge o/p impedance.
In general don't think about OTA & OPAMP story, just do an opamp for LDO it works.

Best regards,
Raj.

Thanks for the feedback. Reading up more I see that the only difference is that an op-amp is an ota with a buffer stage added ( so that the output resistance is much smaller than the load resistance). You get a voltage output and use it drive large capacitive and resistive loads (opamp)

I guess the reason I got confused is that a lot of courses and books refer to the architecture with just differential stage and the second gate gain stage as op-amp and some refer to the same as OTA.