# Different connections to Common node in differential pair

1. ## Different connections to Common node in 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

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2. ## 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...ive-source.png
2. as you said to improve CMRR and keep consumption independent from Vcm
3. it is called quasi-differential pair, low supply voltage usually a reason why people implement it.

So use scenario 1 when common-mode 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.

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3. ## Re: Different connections to Common node in differential pair

Originally Posted by frankrose
scenario 1 if Vcm is constant and VDD is very low.
Do you mean scenario 3? How much low, <1 V or < 0.8 V?

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4. ## 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 current-mirror loaded quasi-differential pair.
Vth depends on the process, Vdsat on the sizes too. Hard to say definite value without knowing the process.

5. ## Re: Different connections to Common node in differential pair

The long-tail pair is supposed to operate so that an increase in one transistor conducting results in the other transistor decreasing (and vice-versa). Thus it rejects a common-mode 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 long-tail pair is working properly, connect input A to input B through a resistor. Monitor voltage across the resistor only.

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6. ## Re: Different connections to Common node in differential pair

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.
We still add DC voltage for biasing even if the tail current source is used.

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