Why is there S2 in this SC amplifier from Razavi's book?

[nxing]

Hello everybody,

I am studying Razavi's book about the Swithced-Capacitor circuit, which is showed below. During the discussion of timing issue, it said that if we turn off switch S2 slightly before S1, the charge injection from S2 will only introduce offset. I am wondering why it's necessary we have S2 here? Is it for the bias purpose only? How about we only use S1 and S3?

Thanks for help!

 

[flatulent]

Re: SC Amplifier

You need a path for the current going into the capacitor. If it tries to cram into the amplifier input (switch s2 being missing) the output will rail.

 

[winsonpku]

Re: SC Amplifier

i think s2 is used to bias the negitve input of the amp.if not s2,the negative input no dc voltage.and it can also short the negtive input with the output,which can reduce the effect of parasitic capacitor on these two ports.then the output will accurately holds the input sampling voltage.
if no s2,the negative input must be baised by another switch.

Hello everybody,

I am studying Razavi's book about the Swithced-Capacitor circuit, which is showed below. During the discussion of timing issue, it said that if we turn off switch S2 slightly before S1, the charge injection from S2 will only introduce offset. I am wondering why it's necessary we have S2 here? Is it for the bias purpose only? How about we only use S1 and S3?

Thanks for help!

 

[yibinhsieh]

Re: SC Amplifier

In your schematics, the S2 has two function. One is for offset cancellation and second is for reference level of your input sampled signal. The S2 is not necessary. But you shoud have a reference volatge for the capacitor when you sample the input signal.

Yibin.