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24th June 2019, 18:59 #1
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Different connections to Common node in differential pair
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/tec...erentialpair/
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 (Commonmode 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

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24th June 2019, 19:30 #2
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Re: Different connections to Common node in differential pair
1. use to improve linearity. it is an easy way, low cost, and possible to implement degeneration without high common mode reduction if you use 2 tail current sources for Q1 and Q2, and the degeneration resistor is between them: https://www.researchgate.net/profile...ivesource.png
2. as you said to improve CMRR and keep consumption independent from Vcm
3. it is called quasidifferential pair, low supply voltage usually a reason why people implement it.
So use scenario 1 when commonmode voltage varies in any way (DC or AC), use 2 when Vcm and CMRR is not relevant, scenario 1 if Vcm is constant and VDD is very low."Try SCE to AUX." /John Aaron/
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24th June 2019, 19:38 #3
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24th June 2019, 20:48 #4
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Re: Different connections to Common node in differential pair
yes, scenario 3, sorry for the typo.
I think it is reasonable when VDD is about a Vgs+Vdsat=Vth+2×Vdsat. This is the minimum VDD for an active currentmirror loaded quasidifferential pair.
Vth depends on the process, Vdsat on the sizes too. Hard to say definite value without knowing the process."Try SCE to AUX." /John Aaron/

25th June 2019, 03:43 #5
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Re: Different connections to Common node in differential pair
The longtail pair is supposed to operate so that an increase in one transistor conducting results in the other transistor decreasing (and viceversa). Thus it rejects a commonmode signal, and reveals the difference between the input signals.
A current source is an easy method to create proper biasing for the transistors. Otherwise it would require DC voltage to be added to the input signals, and/or a negative supply rail. The current source can be replaced by a resistor but it requires careful adjustment of component values.
To check whether your longtail pair is working properly, connect input A to input B through a resistor. Monitor voltage across the resistor only.

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27th June 2019, 18:42 #6
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