what is the function of base current compensation in amplifiers?

[shaopianzi]

Dear all:

Could you tell me " What the function of base current compensation in amplifiers?
Such as the yellow part in the schematic.

Regards & Thanks!

 

[keith1200rs]

It reduces the input bias current. Input bias current of a bipolar amplifier can be a nuisance and source of offsets and errors where you have high resistances connected to the amplifier.

Keith

 

[shaopianzi]

It reduces the input bias current. Input bias current of a bipolar amplifier can be a nuisance and source of offsets and errors where you have high resistances connected to the amplifier.

Keith

Thanks for your detailed answer.
Because of the zero input current, the compensation circuit indeed can reduce the offsets of the reference voltage.
But if there are some concerns about the base current compensation function? such as offset?

 

[keith1200rs]

Base current compensation is usually not perfect. My designs have usually been totally bipolar so you still have a residual current (but at least 10 times better than with the compensation). It adds noise. It is affected by current mirror and/or transistor matching. With CMOS processes you don't see it very much now.

Keith

 

[shaopianzi]

Base current compensation is usually not perfect. My designs have usually been totally bipolar so you still have a residual current (but at least 10 times better than with the compensation). It adds noise. It is affected by current mirror and/or transistor matching. With CMOS processes you don't see it very much now.

Keith

Sorry, "you stlill have a residual current( but at least 10 times better than with compensation) ", what this means?

 

[keith1200rs]

The base current, even after compensation will not be zero. If the base current was 50nA without the compensation, with the compensation it will not be zero - it might be 5nA or 1nA or maybe less - it depends on how good the compensation circuit is. It also depends on if you look at "typical" values or "worst case" i.e. with transistor matching effects. As an example, an all bipolar design I did many years ago had a bipolar input stage with 2.5nA of base bias current and 50pA after compensation for "typical" values. The worst case was 5nA without the compensation and around 150pA with compensation. So, it is better than a 10x reduction in base current but the circuit isn't perfect, particularly when you take into account component tolerances.

Keith

 

[shaopianzi]

Yes, in all cases the current is 30x reduction with compensation in the circuit i attached before.
But the compensation becomes worse in high temperature(higher than 110'C), because the Vgs of Pmos is reduced with temperature increasing, so the bipolar which helps copying the current works in saturation region in high temperature. But the base voltage cannot be pulled low very much, because the input pair have to work in active region and needing more margion, too.
Anyhow, there is a tolerance.

Thank you very much.

 

[shaopianzi]

There is a capacitor in the base current compensation circuit.
What the function of this capacitor is used for?
If it is used for stability, i found the PZ simulation with the capacitor is worse than without the capacitor???
If it is uesd to kill the gain, what is this function realized???

Thanks!!