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Constant Gm bias circuit?

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katrin

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constant gm bias

I want to design a simple constant Gm bias circuit in order to provide a voltage supply to the main circuit
but how can I start to design such a circuit?

and what's the main advantage of this constant Gm bias circuit? Is it not variant to the process and temperature variations?
 

constant gm biasing

You can search constant-Gm in google, probably there will be a lot pop-up. Or just check any ananlog IC design book.
 

constant gm

yeah constant Gm circuit , mean that the Gm of the transistor will be constant against temperature and process variation but this circuit need external resistor so the Gm will depend on this resistor it shoud be 1% tolerance


check razavi's book about this circuit

khouly
 
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    katrin

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    zirimong

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constant gm bias circuit

So what's the main advantage of such Gm constant circuit?
Is is more robust to the process and temperature variations compared with the simple resistor and MOSFET biasing
 

constant gm current source

Depend on the application.

I could see tow application where constant gm is important

1. OpAmp
2. gm-C Filter

The second use reference matching architectures so individual constant gm is less important than C/gm time constant matching to a clock source.

For OpAmp constant gm reduces the spread of the frequency compensation. Typical a folded cascode represent a critical secondary pole. That is

Cmos/Gmos of the folded cascode device

If the compensation is also defined by

Cmos/Gmos of the input stage and the compenastion cap you get a robust design
 

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    katrin

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constant gm biasing circuit

khouly said:
yeah constant Gm circuit , mean that the Gm of the transistor will be constant against temperature and process variation but this circuit need external resistor so the Gm will depend on this resistor it shoud be 1% tolerance


check razavi's book about this circuit

khouly
Click to expand...

is it really process insensitive? i really need to know about this. can you provide me the source and where you quote this. thank you.
 

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    katrin

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gm constant biasing

The idea of constant gm bias circuit is to generate bias current(Iref) which is the function of a resistor and inversely proportional to µCox(W/L); then if you pass this current into a transistor whose gm equals sqrt(2*µCox(W/L)*Iref). You can see that the only variable left is the resistor( plus some constant factor).

You can refer to razavi's book for this.
 

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    katrin

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constant gm circuit

Always use off-chip resistor, if you wanna use the constant-gm bias in your design.

--Ruri
 

gm circuit

check out in ken martin text book u will get the circuit.
 

gm of a circuit

i think gm will not be constant against temperature and process variation
 

constant-gm

Sorry !! I am a fresh man.

Constant Gm , means which device's gm ???

gm = (ΔIds)/(ΔVgs).

Am I right ??

thank you:cry:
 

gm constant

Constant gm circuit is generally realized with PTAT current sources to compansate approx. -2mV/°C drop in Vbe that results lowering current.
 

constant gm bias layout

katrin said:
I want to design a simple constant Gm bias circuit in order to provide a voltage supply to the main circuit
but how can I start to design such a circuit?

and what's the main advantage of this constant Gm bias circuit? Is it not variant to the process and temperature variations?
Click to expand...

constant-gm is PTAT current, based on a constant R.

So, if the R is change with temperature, the GM will not constant.

Added after 1 minutes:

katrin said:
I want to design a simple constant Gm bias circuit in order to provide a voltage supply to the main circuit
but how can I start to design such a circuit?

and what's the main advantage of this constant Gm bias circuit? Is it not variant to the process and temperature variations?
Click to expand...

besides, what do you mean to provide a voltage supply to the main circuit.

Its a contant gm current source.
 

Constant gm bias circuits are generally supply independent circuits.Circuit consists resister R (which basically defines the module current) which is process and temp dependent. Due to that module current is supply independent but not process and temperature. You can get the further detail in 11th Chapter of Razavi.

This circuit is used in most of the designs (opamps,current mirrors,comparators etc) for biasing.

-Mani
 

gm is determined by geometric ratio only. It is independent of power supply voltage, process parameter, and temperature. This is only true to the first order of approximation, since the body effect is neglected. In addition all other transconductance are also stabilized since all transistor currents are derived from the same biasing network.
 

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