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Design >> Analog Design >> How to calculate bias current
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Message started by icekalt on Jun 15^th, 2009, 5:45am

Title: How to calculate bias current
Post by icekalt on Jun 15^th, 2009, 5:45am

Can someone explain@help to determine the value of Ibias? You can find the circuit in the attachment. VDD is 3.3V and the MOS technology used is 0.35µm.

thx in advanced

Title: Re: How to calculate bias current
Post by raja.cedt on Jun 15^th, 2009, 6:03am

hi,
thats mainly in your hand only, i guess this is a first stage of an comparator, so based on the input resolution you might have some gain and you know how much time in your hand for first stage so based on that how much gm you need and for that gm you can find how much current in the input differential pair and double of that current is what you want.Listen this is the not only the way...
Thanks,
rajasekhar.

Title: Re: How to calculate bias current
Post by pancho_hideboo on Jun 15^th, 2009, 6:12am

icekalt wrote on Jun 15^th, 2009, 5:45am:

Can someone explain@help to determine the value of Ibias?

As draft design, you can determine Ibias if you determine required intrinsic gain and required slew rate and load capacitance.

Title: Re: How to calculate bias current
Post by raja.cedt on Jun 15^th, 2009, 6:31am

hi,
i forgot to ask ,how did you come up with diff amp w/l ratio,don't you consider current in that case?

Thanks,
Rajasekhar.

Title: Re: How to calculate bias current
Post by icekalt on Jun 15^th, 2009, 6:49am

raja.cedt wrote on Jun 15^th, 2009, 6:31am:

hi,
i forgot to ask ,how did you come up with diff amp w/l ratio,don't you consider current in that case?

Thanks,
Rajasekhar.

i just randomly selected it..:)

Title: Re: How to calculate bias current
Post by icekalt on Jun 15^th, 2009, 6:56am

pancho_hideboo wrote on Jun 15^th, 2009, 6:12am:

icekalt wrote on Jun 15^th, 2009, 5:45am:

Can someone explain@help to determine the value of Ibias?

As draft design, you can determine Ibias if you determine required intrinsic gain and required slew rate and load capacitance.

hello pancho_hideboo,

are you referring the load capacitance to the diode connected loads(two PMOS)?

Title: Re: How to calculate bias current
Post by pancho_hideboo on Jun 15^th, 2009, 7:09am

icekalt wrote on Jun 15^th, 2009, 6:56am:

are you referring the load capacitance to the diode connected loads(two PMOS)?

Right although they are not diode connected loads.

http://www.zen118213.zen.co.uk/RFIC_Circuits_Files/MOS_OP-Amplifier_Intro.pdf
http://www.zen118213.zen.co.uk/RFIC_Circuits_Files/MOS_OP-Amplifier_Example.pdf

Title: Re: How to calculate bias current
Post by uu@uk on Jun 15^th, 2009, 8:30am

Hi,

What is the amplifier for? If it is driving capacitors and you need high swing at the output. The slew rate may also give you some idea about the bias current.

Thanks

uu@uk

Title: Re: How to calculate bias current
Post by thechopper on Jun 15^th, 2009, 1:20pm

Hi,

On top of what was already suggested, also input noise and offset levels might be controlled by means of Ibias.

Regards
Tosei

Title: Re: How to calculate bias current
Post by icekalt on Jun 16^th, 2009, 5:34am

uu@uk wrote on Jun 15^th, 2009, 8:30am:

Hi,

What is the amplifier for? If it is driving capacitors and you need high swing at the output. The slew rate may also give you some idea about the bias current.

Thanks

uu@uk

thx uu@uuk. The amplifier is for the comparator. It has to amplify the difference of two signals.

Title: Re: How to calculate bias current
Post by icekalt on Jun 16^th, 2009, 5:37am

What is the best analysis to simulate that circuit? transient or ac or dc sweep??

Title: Re: How to calculate bias current
Post by icekalt on Jun 16^th, 2009, 5:44am

pancho_hideboo wrote on Jun 15^th, 2009, 7:09am:

icekalt wrote on Jun 15^th, 2009, 6:56am:

are you referring the load capacitance to the diode connected loads(two PMOS)?

halo pancho_hideboo,

I just have read through the documents. Do you have example for differential amplifier used in comparator? the one that you give me is used in op-amp..thx

Title: Re: How to calculate bias current
Post by uu@uk on Jun 16^th, 2009, 10:14am

icekalt wrote on Jun 16^th, 2009, 5:37am:

What is the best analysis to simulate that circuit? transient or ac or dc sweep??

If your amplifier is for a comparator, I think you need both transient and DC analysis. transient could tell you the rise/fall edge rate. DC analysis can check the FETS are all working in the right region. good luck.

Title: Re: How to calculate bias current
Post by icekalt on Jun 17^th, 2009, 7:15am

uu@uk wrote on Jun 16^th, 2009, 10:14am:

icekalt wrote on Jun 16^th, 2009, 5:37am:

What is the best analysis to simulate that circuit? transient or ac or dc sweep??

Thx uu@uk. Can you explain me more about rise/fall edge? How to know if our MOS transistors working in the right region based on simulation? thx