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High gain opamp with telescopic first stage and a common-gate second stage

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stanlangley

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Hi guys,

Have you ever seen a high gain opamp with telescopic first stage followed by a common-gate second stage? How does this architecture work? Any reference? Thanks.

Stan
 

high gain opamp

maybe what you said is just a folded-cascoded circuit...
 

high gain differential pair

I have more or less seen it yes. It is a folded-cascode with an extra cascode in the input stage. I can't say I have a reference.

Each cascode gives a paracitic pole, so you should be carefull with those. And the input range is limited like a telescopic amp.

But the gain is high, the bandwidth is ok, and if you can connect the gates of the first cascode (in the input) in a way that the input transistors are biased at a constant Vds. Which leads to a more constant gm, which is the reason why I played with it.
 

op amp high gain

I agree with "drDOC"'s opinion, I only have some more comments:
1. In general, cascoding input differential pairs is always to enhance PSRR performance, instead of open loop gain;
2. The bulk of every transistor in cascoded input differential pairs should be connected to its source, to prevent decoupling of Gnd/Vdd's noise.
3. If the tail current is set to be small, to make sure the input differential pairs working in sub-threshold region, to enlarge gain-bandwith, slew rate will be an important parameter. It will not only be reduced greatly, but also be asymmetric.

I've got such reference but it's too large to transfer.
 

why cant op amp have very high gain

I kind of did the something similar when i connected a Telescopic output to a second stage buffer to get swing but it is very tricky to get the Phase and Gain Margins and you will have a lot of poles in ur circuit....

if u get any reference let mw know as I had hooked up the circuit as i liked and it worked :D
 

high gain op. amp.

Hi, Windgoal

Can you explain more details about your first point and third point?
As I think, the cascode can boost output resistance, so do the open loop gain.
In your third point, you mean that it should be care of not driving the input pairs into sub-thresold? As you said, to enlarge bandwidth, the SR will be pay much attention, then it will result in large current. Is it right?

windgoal said:
I agree with "drDOC"'s opinion, I only have some more comments:
1. In general, cascoding input differential pairs is always to enhance PSRR performance, instead of open loop gain;
2. The bulk of every transistor in cascoded input differential pairs should be connected to its source, to prevent decoupling of Gnd/Vdd's noise.
3. If the tail current is set to be small, to make sure the input differential pairs working in sub-threshold region, to enlarge gain-bandwith, slew rate will be an important parameter. It will not only be reduced greatly, but also be asymmetric.

I've got such reference but it's too large to transfer.
Click to expand...
 

asymmetric opamp

1. In general, cascoding input differential pairs is always to enhance PSRR performance, instead of open loop gain;
Click to expand...

Can you explain more details about your first point
Click to expand...

I think I can answer that one. You cascode your input transistors to keep their Vds constant. So if VDD changes, the Vds stays constant, which helps PSRR a lot it also leads to a more constant gm.

3. If the tail current is set to be small, to make sure the input differential pairs working in sub-threshold region, to enlarge gain-bandwith, slew rate will be an important parameter. It will not only be reduced greatly, but also be asymmetric.
Click to expand...

As for the third point. Just simulate your SR and its asymmetric behavior and see if you are happy with it. I do not think it works differently from the ordinary folded cascode.

Anyway optimizing these kinds of circuits always leads to tradeoffs. What a nice job we have.
 

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