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Rail to rail input folded operational amplifier

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Junus2012

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Dear friends,

in the circuit shown below, the output cascode stage of the folded cascode amplifier has two miror, one up and one down, and there is no current source, how the current is then being defined to 2 Ib? we usually have in the output cascode one mirror and the other side is current source

Thank you

foolded.PNG
 

Junus2012 said:
... how the current is then being defined to 2 Ib?
Click to expand...

By W/L ratio (=1) between (P3,P4):(P1,P2) and (N3,N4):(N1,N2) resp. Don't forget: V2 & V3 must be supplied externally. Also, the common drain voltage of P5,N5 (≈VDD/2) supplies all the gates of N3,N4 & P3,P4, hence this design isn't suited for high VDD voltages, cf. Fig. 3 of the attached document.
 

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erikl said:
By W/L ratio (=1) between (P3,P4):(P1,P2) and (N3,N4):(N1,N2) resp. Don't forget: V2 & V3 must be supplied externally. Also, the common drain voltage of P5,N5 (≈VDD/2) supplies all the gates of N3,N4 & P3,P4, hence this design isn't suited for high VDD voltages, cf. Fig. 3 of the attached document.
Click to expand...

Dear Erikl,

Thank you for your reply

Do you practically prefer to deal with such circuit ? or to use the traditional folded one

my second question, does it worth to have a constant gm amplifier? because it makes the design more complicated and consumes more power, or you think it is not that mandatory
 

Junus2012 said:
Do you practically prefer to deal with such circuit ? or to use the traditional folded one
Click to expand...

Depends. I used it for supply voltages of 1.0±0.2V and it worked well, as the Vth voltages of the process suited fine. With higher VDD this simple (but admittedly very elegant, because of its perfect symmetry) topology probably isn't well suited, because the circuit unnecessarily will end up with too much current consumption. Actually you should have a process with Vthp+Vthn ≈ VDD .

Junus2012 said:
does it worth to have a constant gm amplifier? because it makes the design more complicated and consumes more power, or you think it is not that mandatory
Click to expand...

Again depends. If you are bound to get rather flat open loop gain over your ICMR, you need constant gm architecture. If the application allows for scaling down the gain considerably by neg. feedback (closed loop gain « open loop gain), you can achieve the necessary gain-over-ICMR flatness by this feedback. In this case you don't need constant gm topology.
 
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