Question on analog opamp tail current

[jktheone]

Dear fellows,
I have one question just like that shown on the topic,when lots of papers introduce tail current of opamp, they often use a bias voltage and a MOS transistor to represent the tail current, I am wondering how the bias voltage comes out? Does it come from bandgap voltage, or from the one transfered by LDO and bandgap? Because bandgap can produce one voltage, can it be used in many applications in one single chip? That introduces me another question, if a current reference is designed, can its current be used in many circuit blocks?

 

[nitishn5]

The bias voltage is generated using a bias circuit.
Assume that a bandgap circuit generates a current(A voltage could easily be converted to a current as well).
The using a bias circuit, the bias voltages are generated.

The simplest could be just a current mirror. The current from a bandgap is given to a mos diode which generates the Vgs voltage. This is then given to the gate of the opamp tail current mos.

If you want to generate more voltages for example the NMOS current Source, PMOS current source, and also cascode devices as well, you would need more advanced bias circuits.

The following link gives a Bias circuit which generates 4 voltages by mirroring a constant current(assumed to be from a bandgap).
https://www.edaboard.com/threads/304811/#post1305925

A bandgap voltage could be used directly as a reference assuming that it can handle the loading. Otherwise it would need a buffer.
A current can easily be mirrored multiple times to be used in many circuit blocks.

Smaller chips might have only one bandgap circuits while bigger ones would have multiple ones for many reasons such as isolation.

 

[jktheone]

Thank you, it is very helpful, but I have a little question, if the bandgap needs an op amp to operate, so how the tail current in this op amp comes out? Because if we use this bandgap output voltage to generate a bias in this op amp, this will not work because the steady state can not be constructed and the bandgap output voltage can not be generated. Please help me to figure out this problem, thank you.

 

[yuvan]

There are many self biased bandgap voltage, which biases it's own tail current but will require very carefully designed start up circuitary. Or you can generate current from constant gm(PTAT) bias circuits which don't need any reference currents.

 

[jktheone]

Thank you on your explainations, I think I know how the circuit exact works, self-biased circuit can start the op amp, this is why many books omit this little trick. Thank you.