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gate capacitance value of a CMOS transistor ?

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Octago

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cgd cap transistor

Now I want to study if my circuit is able to drive the input of a differential amplifier

So I am wondering normally how much is approximately the gate capacitance of a CMOS transistor?
 

gate capacitance 0.18um cmos

Depending on the operation mode the capacitance seen from the gate depends on the gate-source voltage of a mosfet. For an NMOS or PMOS device it is W*L*Cox where Cox depends on the technology you use. If your load device operates in saturation region its Vgs is above threshold unless it operates in subthreshold. Therefore W*L*Cox is probably your expected capacitance.

You can google "moscap" or "mos capacitance" for a short tutorial and checking capacitance versus Vgs curve for nmos and pmos will be useful.

Regards
smoked
 

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    Octago

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gate capacitance values

the capacitance depend on the size of the transistor and yhr working region. for differential amplifier,it mostly worked in saturation region,the capacitance is Cgs+A*Cgd,the Cgs is larger one,A*Cgd is the miller effect cap.Cgs include the 2/3WLCox and Cov.
 

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    Octago

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input capacitance cmos

freelancer said:
the capacitance depend on the size of the transistor and yhr working region. for differential amplifier,it mostly worked in saturation region,the capacitance is Cgs+A*Cgd,the Cgs is larger one,A*Cgd is the miller effect cap.Cgs include the 2/3WLCox and Cov.
Click to expand...

HI
as you say,if an amp have a 1000 dc gain,then it's input capacitor would be
Cgs+1000*Cgd, right? and the second term will be very larger than first term.
am i right ?
THX.
 

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    Octago

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transistor cmos capacitance

spring1860 said:
HI
as you say,if an amp have a 1000 dc gain,then it's input capacitor would be
Cgs+1000*Cgd, right? and the second term will be very larger than first term.
am i right ?
THX.
Click to expand...

Not really... That A specifies the gain of input pair alone! So you might have a dc gain of 1000.. But it might be due to a cascode or 2 stage or wat ever topology u select!

But question is about input capacitance! For that you must consider the Cgs and Cgd.. Since Cgd is affected by miller effect, you must consider the gain of the input pair also! I mean gm1 * R1, where R1 specifies the resistance of the input pair! Its better to keep the gain of input pair less to avoid larger parasitic cap at the input! Say if the gain is some 15 or 20, you have Cgs + 20 * Cgd!

Hope this helps!
 

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    O

    Octago

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cmos input capacitance

Octago said:
Now I want to study if my circuit is able to drive the input of a differential amplifier

So I am wondering normally how much is approximately the gate capacitance of a CMOS transistor?
Click to expand...

why not drive the differential amplifier to detect the loss.
 

    O

    Octago

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dineshbabumm said:
Not really... That A specifies the gain of input pair alone! So you might have a dc gain of 1000.. But it might be due to a cascode or 2 stage or wat ever topology u select!

But question is about input capacitance! For that you must consider the Cgs and Cgd.. Since Cgd is affected by miller effect, you must consider the gain of the input pair also! I mean gm1 * R1, where R1 specifies the resistance of the input pair! Its better to keep the gain of input pair less to avoid larger parasitic cap at the input! Say if the gain is some 15 or 20, you have Cgs + 20 * Cgd!

Hope this helps!
Click to expand...

yes, i know your mean, thank you very.
 

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    Octago

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Best thing will be to add the initial stages of differential ammplifier as load and run the sims.
 

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    Octago

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IN a typical UMC 0.18um process.for a NMOS transistor w=1um,l=0.18um
cdtot 1.3468f
cgtot 1.6677f
cstot 2.5686f
cbtot 2.5073f
cgs 1.1647f
cgd 364.8981a
PMOS transistor w=1um,l=0.18um
cdtot 1.3827f
cgtot 1.6899f
cstot 2.7874f
cbtot 2.7184f
cgs 1.2433f
cgd 354.6213a
 

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