circuitking
Full Member level 5
Hi I have read few posts about this but could really get deep insights. Below are my questions
image taken from https://www.allaboutcircuits.com/technical-articles/the-basic-mosfet-differential-pair/
1. Why should we degenrative resistor (Say Rdegen) in differential pair at all? - I know it provides negative feedback and stability can be improved. Is it used here also for the same purpose ?
2. Why is the current source is used at Vp?- I found out that if Rdegen is used, when Vcm (Common-mode voltage) changes then voltage across Rdegen changes and Current in Q1 and Q2 changes. So to prevent this changes and make the bias conditions stable even when Vcm changes, Rdegen is replaced with current source. And also use of current source decreases Acm(common mode gain) and inturn increases CMRR.
3. I have seen people connecting the common node (Vp) to ground directly that means no tail current or degenerative resistor
So when to choose scenario 1,2 and 3 and what changes we see in the circuit in these cases. Thanks
image taken from https://www.allaboutcircuits.com/technical-articles/the-basic-mosfet-differential-pair/
1. Why should we degenrative resistor (Say Rdegen) in differential pair at all? - I know it provides negative feedback and stability can be improved. Is it used here also for the same purpose ?
2. Why is the current source is used at Vp?- I found out that if Rdegen is used, when Vcm (Common-mode voltage) changes then voltage across Rdegen changes and Current in Q1 and Q2 changes. So to prevent this changes and make the bias conditions stable even when Vcm changes, Rdegen is replaced with current source. And also use of current source decreases Acm(common mode gain) and inturn increases CMRR.
3. I have seen people connecting the common node (Vp) to ground directly that means no tail current or degenerative resistor
So when to choose scenario 1,2 and 3 and what changes we see in the circuit in these cases. Thanks