A question on Class AB opamp

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yschuang

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**broken link removed**Hello, all

I have a question on the size of some transistos and marked them in red.
The question is why they are not equal and why they are sized like that way(50/2, 24/2).
What I thought is try to set sources of M20 and M28 equal but that is not true.
Thanks a lot.
 

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I reviewed the circuit, Both M28 and M20 should be different size, Because Vgs of M28 is lower than M20's, so The size of M28 is larger than M20's for equal current.
 

pol_sky said:
I reviewed the circuit, Both M28 and M20 should be different size, Because Vgs of M28 is lower than M20's, so The size of M28 is larger than M20's for equal current.
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Vgs of M28 and M20 will be the same if the currents through M28 and M20 and size of M28 and M20 are the same.
Currents through M28 and M20 are the same. The size of M28 is larger than that of M20. The Vgs of M28 is definitely smaller than that M20.
I = 0.5uCox(W/L)Vgst2
Anyway, thanks for your post.

 

Vg to VDD voltage is equal both M28 and M20, But Drain of M13 to VDD voltage is larger than drain of M14 voltage, so Vgs28>Vgs20, in order to make the current both M28 and M20 be same, so size of M28 is larger than M20'.
 

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