why cant use miller compensation in single stage opamp

miller compensation

In Gray it is said single stage opamps,such as the telescopic cascode or folded cascode,have only one gain stage;therefore Miller compensation is not possible.
Why? Can I attach a miller cap between negative input and single ended output?

miller compensation in amplifiers

Because at high frequencies because of the forward transmission through the Cc(basically a zero @ high frequencies) there would be no gain left in a single stage.
However in a two stage, you would design it in such a way that stage1 still has gain.
Also in general, stability is not an issue in single stage opamp(consider a differential to single ended opamp).
It has two poles : One @ high impedance output node which is the dominant pole and another @ low impedance current mirror node which is the non dominant pole.So you would have a PM of about 90 degrees in single stage, so why bother to compensate it further and slow down the transient response?

op amp miller pole

leohart said:

In Gray it is said single stage opamps,such as the telescopic cascode or folded cascode,have only one gain stage;therefore Miller compensation is not possible.
Why? Can I attach a miller cap between negative input and single ended output?

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I think we use miller compensation for improve the stablity of the circuit. but the miller compensation will reduce the gain of amplifier, so if the single stage opams should be easy to be stable, so miller compensation is not useful here.

single stage op amp stability

right, there is such big stability issue in case of single stage opamp...

dominant pole compensation

I know there is no stability problem in single stage, and of course Gray knew...so why did he bother to say "cant use miller compensation in single stage opamp" isnt it weird?

ps:

A_U_J said:

Because at high frequencies because of the forward transmission through the Cc(basically a zero @ high frequencies) there would be no gain left in a single stage.
However in a two stage, you would design it in such a way that stage1 still has gain.
quote]
Isnt what does miller compensation do is just eliminate gain at high frequency to make sure gain<0dB before the phase reach 180degree?So what does your comment mean for gain at that time?

anyway thx for your comments, they are good,

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miller compensation in op-amp

i don't get what u mean.
adding miller capacitance is to do pole splitting between two nearly dominant pole.
so in ur description u r adding the miller between the input (ideally: no pole there) and the output (usually the high impedance node(dominant pole)) what will u gain from that (except maybe making the dominant pole less dominant and generating a new dominant pole !!)

what is miller compensation on amp

hi,A_U_J,can u explain it for me?

hi,safwatonline,do you mean in two stages, we do miller compensation,then we get a dominant pole in the input of the second stage.but in a single stage,the dominant pole is at the output which diffs from the two stages,so miller compensation cannot work properly.

miller compensation in op-amp.

Miller compensation is done is introduce a zero at the pole position and cancel the effect of the pole. but in a telescopic cascode , between which two nodes can you introduce a zero to cancel it out? introducing Cc between negative terminal and output seems a bad idea.

miller compensation,what is dominant pole?

leohart said:

hi,A_U_J,can u explain it for me?

hi,safwatonline,do you mean in two stages, we do miller compensation,then we get a dominant pole in the input of the second stage.but in a single stage,the dominant pole is at the output which diffs from the two stages,so miller compensation cannot work properly.

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Yep.

what is miller compensation

You can put the miller capacitor between input and output of a single stage amplifier. It's just not useful at all, since the dominant pole is usually at the output already, and there's no second pole being pushde higher in frequency.

Don't forget that the stage that drives your amplifier input has finite ouput impedance.

The miller capacitance that you will generate *at the input* will severely limit the BW of the system... The output pole was pushed a bit higher in frequency, while the input pole was brought much lower. Ideally the input pole was *not* the BW limit before the miller capacitor was inserted (otherwise your single-stage has too much BW w.r.t. the signal BW), so the poles are now closer together. There's also the transmission zero, but as long as the miller capacitor is much smaller thant the load capacitor it'll not make the bad result much worse.

op amp miller capacitor output impedance

Yes, in single stage amp, output node will be dominant pole. But miller effect will push output node pole to high frequency that will worse the stability.