Opamp current biasing to PTAT

• Mar 6, 2006

• #1

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aryajur

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Opamp Biasing current

Why would we want to have the tail current to bias an opamp to be PTAT?? The gain of a diffamp is gm ro , and
gm = Id/(Vgs-Vth) (Id is the tail current)
Vth has a negative temp coefficient so for gm it makes sense to have Id as PTAT to prevent gm from falling. But ro = 2/λId, that would decrease when we increase Id. Is there ome consideration for the temperature coefficient for λ if there is any?

 

• Mar 6, 2006

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sunking

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Opamp Biasing current

which is more important? gm or ro?
of course gm.

 

• Mar 6, 2006

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aryajur

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Re: Opamp Biasing current

Why is gm more important?

 

• Mar 6, 2006

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Syukri

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Re: Opamp Biasing current

gm is the input gain....

we wanted all the input to be asrobd by the op-amp to have high current amplifying

 

• Mar 6, 2006

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sixth

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Re: Opamp Biasing current

aryajur said:

Why is gm more important?

Click to expand...

Becuase the GBW is determined by Gm/C. For an opamp, the constant gbw is more important than the constant gain.

 

• Mar 6, 2006

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rfsystem

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Re: Opamp Biasing current

If the MOS diff pair is saturation region

gm(difpair)=Ibias/(4*VDSAT)

but if the pair is in substhreshold region

gm(difpair)=Ibias/(4*m*VT)

where m is the subthreshold slope. So if subthreshold region is used the PTAT is a perfect compensation. For the saturation region I think there are modified current bias cells. These are regulating a current trough a MOS matched to the diffpair operating in linear mode such the voltage drop VDS is equal to a fraction of a bandgap voltage.

 

• Mar 6, 2006

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vbhupendra

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Re: Opamp Biasing current

we use tail current source to minimize the input common mode variation.

 

• Mar 6, 2006

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opamp741

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Re: Opamp Biasing current

If u are concern about ur UGB then using PTAT is a good idea.
at Slow corner (125 degree) u will have a higher Bias current.
which will cause u a better gm and hence UGB.
while is Fast or typical corner u can meet ur speed spec with
lower current that a PTAT can provide u.

 

• Mar 13, 2006

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jinxingsun

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Opamp Biasing current

gm also determines the noise!

 

• Mar 14, 2006

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Re: Opamp Biasing current

jinxingsun said:

gm also determines the noise!

Click to expand...

actually gm/Id determines the noise

 

• Mar 14, 2006

• #11

opamp741

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Re: Opamp Biasing current

qslazio said:

jinxingsun said:

gm also determines the noise!

Click to expand...

actually gm/Id determines the noise

Click to expand...

i am unable to understand ur statement. can u elaborate further.

 

• Mar 23, 2006

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laglead

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Re: Opamp Biasing current

For an opam input stage, its current gain (gm) does matter in opamp compesation as well as bandwidth. So, its we should make its gm relatively stable with temperature variation.

 

• Mar 26, 2006

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montage3000

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Opamp Biasing current

to make gm constant

Added after 2 minutes:

make gm does not change with temperature

 

• Mar 27, 2006

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jordan76

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Re: Opamp Biasing current

rfsystem said:

......These are regulating a current trough a MOS matched to the diffpair operating in linear mode such the voltage drop VDS is equal to a fraction of a bandgap voltage.

Click to expand...

Hi rfsystem,

Could you please elaborate more about the above? Thanks!

regards,
jordan76

 

• Mar 29, 2006

• #15

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elbadry

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Re: Opamp Biasing current

A PTAT current would make gm constant FOR A BIPOLAR DIFF. PAIR ONLY

This is because for a bipolar device: gm = Ic/VT ( VT: thermal voltage and is directly proportional to absolute temperature)

Hence if I is proportional to absolute temperature, the term Ic/VT would "ideally" be constant with temperature...

 

• Apr 1, 2006

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jeff_zx

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Re: Opamp Biasing current

elbadry said:

A PTAT current would make gm constant FOR A BIPOLAR DIFF. PAIR ONLY

This is because for a bipolar device: gm = Ic/VT ( VT: thermal voltage and is directly proportional to absolute temperature)

Hence if I is proportional to absolute temperature, the term Ic/VT would "ideally" be constant with temperature...

Click to expand...

but the gain gm×ro is still not constant for bjt?!