Help with the BiCMOS bias

[ddrr]

bicmos ptat current source

Hi All,

Does the bias below still need a start-up circuit??

thanks a lot!

 

[yxo]

bias without start up

Are you sure it is works?

 

[ddrr]

Are you sure it is works?

Yes, I think.

In the expected condition, it generates a PTAT currrent.

 

[erikl]

Does the bias below still need a start-up circuit??

No, I don't think so. There are 2 negative feedback loops to provide a stable operating condition. The operating current of the left branch, however, is rather undefined.

 

[saro_k_82]

There is one positive feedback loop as well. Still it would work without startup but the major problem is it's supply sensitivity. With small changes in the supply the current will vary a lot., I don't think this circuit can be used even with an LDO. On the other hand one can get desired output current by placing this circuit in a loop and controlling the supply voltage

 

[sutapanaki]

I have seen this circuit being used to establish a PTAT bias current in the right branch of the circuit. The interesting thing is that this current does not depend on the current in the left branch. Usually, and I mean in what I've seen, the left branch just carries a small start-up current. So, if this is what you're asking, yes, the left branch carries the start-up current for the circuit.

 

[rfsystem]

Is that a PTAT?

There is 3 diode path from VDD to VSS. It will govern the currents.

 

[wpchan05]

It there is PTAT current, how can you mirror it out?

 

[sutapanaki]

It is actually very easy to prove it is a PTAT current source, takes just couple of lines of algebra (neglecting the base currents). See the attached file.
The circuit as originally posted is drawn in a bit of a strange way. The drains of the NMOS devices go directly to supply. I assume those NMOS transistors are there to increase the output resistance, but then it makes no sense not to use their drains. Anyway, if you connect a diode connected PMOS transistor in the right branch of the circuit, you can mirror the PMOS current. Also you can have a scaled combination of a resistor and an npn bjt and connect the base of that bjt to the base of the bottom bjt in the right branch of the circuit and mirror this way.

 

[yxo]

true. But nmos should be replaced by Pmos, and diode connected transistor have to be on the right side.