Generating the bias voltage of an op-amp

[jimmyshu]

Bandgap Reference

I agree with jnuhope.
Generally, opamp is biased by current not voltage, to get a constant Gm. So you need to design a current source independent with temperature and power supply.
What do you want to bias by voltage reference? The gate of current source MOS?

 

[pradeep.bhardwaj23]

Re: Bandgap Reference

please tell me also

 

[firry]

Bandgap Reference

Bias the opamp by current is better than bias it by voltage. when you layout, you should consider the parastic of the metal and the voltage drop on it.

 

[turtleden]

Re: Bandgap Reference

use the resistor network!

yes, it's the easiest way

 

[linkfox]

Re: Bandgap Reference

I agree with jnuhope.
Generally, opamp is biased by current not voltage, to get a constant Gm. So you need to design a current source independent with temperature and power supply.
What do you want to bias by voltage reference? The gate of current source MOS?

yes, one method is using beta-multiplier which has been mentioned earlier

 

[qqic]

Bandgap Reference

use an opamp and resistor to boost it

 

[ashish_chauhan]

Re: Bandgap Reference

If you are trying to bias opamp, you don't need bandgap. Just use current mirrors. Otherwise, how are you going to generate the bias for the opamp used in bandgap?

if biasing of bandgap opamp is considered ... you can use self biased opamp structure.

 

[seanyang]

Re: Bandgap Reference

All right, just using the architecture like LDO form.
But must sure no cuurent drive from out pin.
Or you will have stability issue.

 

[amarendrap14]

Re: Bandgap Reference

generally bandgap also provide current reference (which can be 5-8%) accurate..
so best way is to bias through current(biasing is simpler and good control over power dissipation).. . also accuracy requirements for current is generally less compared voltage..

LDO are used for high current capability design (power carrying)..better use opamp-R combination if you really need voltage biasing(expect some 1-2 %) degradation due to this..

 

[northeast1]

Re: Bandgap Reference

you can use 1.4v for reference to generator higher reference voltage.

 

[starrinesss]

Re: Bandgap Reference

yeah but when i fabricate the chip, the resistance will vary within 20%...a 4k resistor might only have 3.2k resistance. the result wont be very accurate...i need something that can generate a very accurate bias voltage...something that only have error within 0.1%...

It depends the match of 2 resistors, if there is a kind of res with less than 1% mismatch that have the same geometrical size, that will fit your design.
And you'd better use large W the res

 

[vbhupendra]

Re: Bandgap Reference

you can use techniques shown in allen holberg or razavi to get voltage ref higher than bg.

 

[joezl82]

Bandgap Reference

prefer use gand gap
ldo in not suit in this application

 

[mahgoub]

Re: Bandgap Reference

i dont understand if he is not going to use bandreference how is the voltages are referenced to it

 

[peni]

Re: Bandgap Reference

use Bandgap generate fixed voltage , than use LDO to generate higher voltage you want

 

[lijianheng]

Re: Bandgap Reference

Try this one

 

[mydreamhouse]

Re: Bandgap Reference

you can use LDO or some regulator

 

[engronger]

Re: Bandgap Reference

you amp need a so accurate bias?, usually, what a amp really need is a bias current and some times a reference input voltage, if you need a bias current , you can use a bandgap, and then a current mirror is used to give the bias.

if you need a reference input voltage, and it must be large than 1.2V, you can also use a bandgap, you can read more paper, there are some circuit can give you >1.2V & <1.2V . LDO is not a choice can be choosed. LDO is not used to give a bias ,it is used to give a power supply to core chip.

if you need a bias to some other gate, maybe a rough bias is ok. you can use MOS and resistor to create it .

So I am trying generate the bias voltage of an op-amp. So far the only circuit I know that can be used to generate the bias voltage is bandgap reference. But bandgap reference can only generate a voltage that is around 1.4V. I need a bias voltage that is higher than 1.4V...What circuit should I use?

 

[xin]

Re: Bandgap Reference

Normally current mirror is used to bias the opamp. The current in current mirror is often generated from bangbap with a feedback network.

 

[northeast1]

Re: Bandgap Reference

So I am trying generate the bias voltage of an op-amp. So far the only circuit I know that can be used to generate the bias voltage is bandgap reference. But bandgap reference can only generate a voltage that is around 1.4V. I need a bias voltage that is higher than 1.4V...What circuit should I use?

You and add a buffer follow the bandgap, and generator what reference voltage you want.