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Design >> Analog Design >> Bipolar input switched capacitor opamp
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Message started by thechopper on Oct 14^th, 2008, 6:58pm

Title: Bipolar input switched capacitor opamp
Post by thechopper on Oct 14^th, 2008, 6:58pm

Hi all,

I was just wondering what the drawbacks are in using a bipolar differential pair for a fully differential opamp that is going to be used in a switched cap application (BICMOS process).

I know is not the typical approach but the reason I'm think of using bipolar rather than MOS input is to minimize offset and 1/f noise contribution from this opamp to the overall system performance Other approaches for minimizin this contritbution like chopping or auto-zeroing are not an option since I do not have room for any post-filtering.

As a first drawback I just thought of the droop the base current unbalance might generate on the capacitors assuming the amplifier will have some small input referred offset.

Any ideas or comments are welcome.

Thanks in advance
Tosei

Title: Re: Bipolar input switched capacitor opamp
Post by Tlaloc on Oct 14^th, 2008, 9:16pm

The base current requirement certainly would be a major drawback. I guess, though, that a small bias current and large capacitances would mitigate the problem.

Another problem is capacitor mismatch. If your capacitors mismatch, then the common mode voltage times the mismatch parameter shows up as a differential signal. This may be a small effect, but it may or may not be negligible.

Are gain-and-offset correction techniques not good enough using MOSFETS? Bipolar inputs may see an order of magnitude improvement in offset, but these other two effects seem like they would worsen the overall picture. Also, a small collector current, i.e. a small base current, would see larger input referred noise.

Adam

Title: Re: Bipolar input switched capacitor opamp
Post by thechopper on Oct 15^th, 2008, 5:57am

Hi Adam,

Thanks for your comments. You brought a good point on the capacitor mismatch, which certainly will have some impact apart from the base currents as I pointed out.
The idea is to use small capacitor values with a stray insensitive switch arrangement for the caps, so that is something that will end up hurting me.

Any of the dynamic offset cancellation techniques would be very effective in removing the offset components from the mos input pair. However, I would have to kill somehow the high frequency noise or ripple - if I use chopping - or deal with an excessive base band noise if I use auto-zero (due to noise folding).
That is why I considered the option of going with bipolar input which, at a first degree would not need any technique for reducing the offset or noise components (compared to the MOS input).

I agree at the end of the day the other effects might worsen the overall picture as you suggested.

Thanks
Tosei

Title: Re: Bipolar input switched capacitor opamp
Post by Tlaloc on Oct 15^th, 2008, 3:11pm

One other thing that I thought of is the matching of your input pair. Certainly the Vbe will match very well for the bipolars, but what about the beta? I can't think of any data that I've seen on beta matching. Of course with mismatched beta, the droop in your input voltage would have a differential component and not just a common mode shift.

Quote:

or deal with an excessive base band noise if I use auto-zero (due to noise folding)

This is something that has been on my mind recently. Certainly, using auto-zero when compared to a continuous time amp will see a large noise effect due to folding, but since you are using a switched cap circuit, isn't the noise already going to be folded even if you don't use auto-zero?

Title: Re: Bipolar input switched capacitor opamp
Post by thechopper on Oct 16^th, 2008, 5:22am

Adam,

thanks again for your feedback. Matching of beta and VBE in bipolars is driven essentially by the same parameter: base charge --> A mismatch on the betas will create (for perfectly equal collector currents) a mismatch on VBE. Also, a difference on the saturation current density will create a VBE mismatch. Such saturation current mismatch will generate a mismatch on the collector currents and ultimately on the base currents --> similar to a beta mismatch.

That was my first thought about using bipolars for the SC amp --> having beta mismatch will generate a different droop on both input branches that will hurt me (even assuming ideal matching for the caps).

As for the noise fold-backing, I forgot to mention something important: the incoming signal going into the SC amp is already filtered (AAF) such that the SC amp does not fold back any noise component coming with the input signal. Therefore, not auto-zeroing the amplifier will essentially not generate any extra baseband noise, while auto-zeroing it will fold back all the noise components generated inside the amplifier.

Thanks
Tosei

Title: Re: Bipolar input switched capacitor opamp
Post by solidstate on Oct 19^th, 2008, 6:37am

There was a paper at last year's ISSCC that might be useful to you:

A Chopper-Stabilized Lateral-BJT-Input Interface in 0.6Â¿m CMOS for Capacitive Accelerometers
Dongning Zhao; Zaman, M.F.; Ayazi, F.

It's available on IEEEXplore.

Title: Re: Bipolar input switched capacitor opamp
Post by thechopper on Oct 20^th, 2008, 7:20am

Hi Solidstate,

Many thanks for the info.

Regards
Tosei

Title: Re: Bipolar input switched capacitor opamp
Post by raja.cedt on Oct 22^nd, 2008, 11:26pm

hi i would like to add one more point that when there are BJT, there nothing like floating node?then analysis becomes very difficult.Moreover bjt will off when there is no current through it even though there is enough base drive.where as mosfet will turn on if u gave enough vgs,hence during one phase every switch wil be in off condition.Where as in bjt case whichever cap charge first that switch will turn off,so that orher nodes swing may impact this voltage level hence SNR becomes low.correct me if am wrong
Thank you

Title: Re: Bipolar input switched capacitor opamp
Post by thechopper on Oct 23^rd, 2008, 6:27pm

Hi raja.cedt

I think you are referring to using BJT as switched in the SC amp.
Instead I was referring to using BJT in the input pair of the amplifier. Certainly the switches are much easire to realise with MOS transistors although bipolar switches have been used in the past. However I was not asking here about using BJT switches as I explained above.

Thanks anyway
Tosei

Title: Re: Bipolar input switched capacitor opamp
Post by raja.cedt on Oct 23^rd, 2008, 10:14pm

hi,
i miss understood the question .based on u r answer i got one question.
is it possible to put 1st stage with BJT and 2nd stage in MOS tech,is this BICMOS tech ?

Title: Re: Bipolar input switched capacitor opamp
Post by thechopper on Oct 24^th, 2008, 11:49am

Hi raja.cedt,

Yes, this is a BICMOS process and certainly I'm thinking of using a MOS active load for the first stage and 2nd stage also in order to minimize noise and offset contributions from the internal opamp stages.

Regards
Tosei