[Moved]: Folded Cascode with pmos input pair plus wide swing cascode current mirror

[AMSA84]

Hi guys, I want to design a folded cascode with a total current consumption <20uA.

THe design I made I am struggling to get 60º of PM. In this situation should I use a capacitor to improve the PM, at the output?

 

[AMSA84]

Re: Folded Cascode with pmos input pair plus wide swing cascode current mirror

I forgot to say another thing.

In one design that I have made, I got the desired specification: <20uA, PM > 60º, UGB ~ 10MHz, Gain > 60dB. (here the UGB is irrelevant). However there is something that concerns me. Is the shape of the phase response.

Please, see the attached:





To be honest, I would like to increase the to 70dB. Well, but right now this doesn't matter. The problem here is the shape of the phase plot that when compared to the phase response with a capacitor load (in this case I am using a transistor to simulate the next stage) in which the response is very good looking. That's why I was asking if I can use a capacitor in the output to compensate that response.

Please, feel free do comment the design and if in your opinio is OK. I never designed a folded cascode before.

BTW, I decided to go for a folded cascode, before you ask, because I need good PSRR because this will be used in a bandgap and my vdd and gnd will have some noise.

Regards.

 

[Dominik Przyborowski]

Re: Folded Cascode with pmos input pair plus wide swing cascode current mirror

First question from me. What is the reason to make a current mirror on NFETs not on PFETs like in typical FC opamp? In fact You decreasing your gain with actual connection.

This phase "bump" is typical for cascode loads. Check this paper for more info.

 

[AMSA84]

Re: Folded Cascode with pmos input pair plus wide swing cascode current mirror

Thanks for the reply dominik.

Please, can you go deeper on the "reason to make a current (...)"? What you mean?

Can you identify the NMOS current mirro that you are referring to? Perhaps are those identified as NMO23 25 34 35? Or NM 18 and 19?

- - - Updated - - -

Thanks for the paper, by the way!

 

[AMSA84]

Re: Folded Cascode with pmos input pair plus wide swing cascode current mirror

Forgot something. From ehat I understood from your questionar you mean that I cannot use a NMOS currwnt mirror for a PMOS input MOST or independently of whatever the input transistorsxare the current mirror should be allways PMOS?

In page 803 of bakers (3rd ed) you have a NMOS input with a PMOS current mirror and I ask why then, because from what I know it is used a NMOS current mirror in NMOS input transistor.

 

[Dev_RM]

Re: Folded Cascode with pmos input pair plus wide swing cascode current mirror

From your schematic , it seams that your bottom current current sources NM34,35 are not biased properly.
their vds is 78mV , is it worst corner or Typical ?

 

[AMSA84]

Re: Folded Cascode with pmos input pair plus wide swing cascode current mirror

Hi Dev, it's typical. However this is not yet the final circuit because I am evaluating/researching the other existing topology and trying to figure it out, if it is best to go with this or with the typical folded cascode that dominik has mentioned.

One thing that I am trying to figure out is what are the differences between this topology and this one:



By chance you know what are the differences?

Using this last circuit, the biasing voltages are expected to be around what? 0.6V for the bottom transistor and the other two around 1.2V?

Rergads

 

[KingOfHearts]

Re: Folded Cascode with pmos input pair plus wide swing cascode current mirror

What are the purposes of the inductors and the current source? You need some decoupling capacitors if the inductors are used to model the bondwires. 13uH is a very large inductance on-chip.

 

[Dev_RM]

Re: Folded Cascode with pmos input pair plus wide swing cascode current mirror

KingOfHearts,

That is not the inductance value, it is the dc current (from dc operating point).

 

[KingOfHearts]

Re: Folded Cascode with pmos input pair plus wide swing cascode current mirror

Thanks for corrections. My main concern is the purposes of adding the inductors and the current source.

 

[AMSA84]

Re: Folded Cascode with pmos input pair plus wide swing cascode current mirror

The inductors are only to simulate the bonding wires, though knowing that I should add the pad capacitance. But for now I will let it be as it is.

Regarding the current source, are you talking about the current at left of the folded cascode? I will change that for the current reference that I have designed. When I designed this folded cascode, I didn't had the current source reference.

Anyway, the main question here was to know if the phase response shape is suppose to be like that, because this is the first time that I am designing a folded cascode an the circuits that I have seen around IEEE and so they presented a high output capacitance (that is >1pF) and with that it is comprehensive that the output shape of the phase response is clean and looks nice. Because my output capacitance is small, they load capacitance will not be sufficient to compensate for the PM so I increased the cascode transistors sizes in order to compensate the folded cascode. From what dominik said, it is a normal shape.

Regarding the last issue, comparison with the typical tolopogy I have found the answer in Sansens book.

Regards.

 

[rokhsare_229]

Re: Folded Cascode with pmos input pair plus wide swing cascode current mirror

hi friends
what is difference between folded cascode amplifier ( where it has input NMOS) with cascode.
i know that folded cascode has lower supply voltage. What are difference in terms of noise and power consumption? Would i say the voltage is lower, so is less power ?

 

[Dominik Przyborowski]

Re: Folded Cascode with pmos input pair plus wide swing cascode current mirror

If You asking about differences between folded cascode and telescopic cascode, the answer is obvious.
For given current consumption and supply voltage in folded cascode, less current flowing through input transistors than in telescopic case, so transconductance of input fets is smaller and noises are higher (also contribution from load current source is higher in FC).

 

[rokhsare_229]

Re: Folded Cascode with pmos input pair plus wide swing cascode current mirror

Thank you so much
then, For given current consumption and supply voltage in folded cascode, we want to say the FC amp has lower gain, lower power and higher noise. Do I understand correctly?
(The difference between thees two amplifiers in below, in terms of gain and noise figure and power consumption Was desired.)

 

[Dominik Przyborowski]

Re: Folded Cascode with pmos input pair plus wide swing cascode current mirror

For given current consumption power is the same in both cases. Also gain is equal in both if transistors operating points are the same. In folded cascode less current flowing through input transistors what results in lower transconductance causing lower bandwidth and higher noise.