Purpose of 4 transistors in Driven Right Leg circuit

[anshulvats]

Referencing existing thread:

https://www.edaboard.com/threads/197407/

Here is a picture of a part of a circuit, which contains a right leg driver.

My understanding of DRL is a circuit which send a signal to a human body, therefore this human body would have the same 'common voltage' as the circuit, which helps to increase CMRR.

However, I would like to know how the below DRL works, especially at the point A, B and C.

Also I have seen quite a few of DRL circuits, and they all do some sort of filtering (Low pass filter), I would like to know what value of cut off freq is aiming actually.

what is the function of those 4 transistors in that circuit??

 

[schmitt trigger]

The bottom pair is a mirror image of the top pair. Thus there are two identical, but independent circuits, one for each channel.

The circuit essentially is a clamp for positive and negative voltages. When the R211, R212, base node gets higher than about 0.6v, the transistor collectors will clamp the signal.