But I'm not asking why resistor is used instead of current source. I'm asking how does that resistor keep the current roughly/almost constant? How does it act as if it is a current source?

It does NOT keep the current constant, so I used the phrase 'dynamic range' which goes with current regulation. This is ok for RF designs, but hurts low cost audio designs.

Oh so its purpose is not to keep the current constant at all? I really thought it was. But what is its real purpose? I thought the idea of diff pair is to decrease Ic1 when Ic2 increases or the other way around. But that can only be done by a current source. Could you explain how Re makes things work here? Thanks

I edited my answer to clarify that issue. Constant current sources and sinks give you wide dynamic range and operating voltage.

No forget about the current source version; Im wondering how does the pair with resistor work.

Re is your current sink-you must have one, but it is not constant current.

Hmm I still dont understand how this works. I thought the whole idea is to make Ic1+Ic2 = constant

isnt that?

Q1 and Q2 will not have as much voltage range with just a fixed resistor.

How does Re makes make Ic1+Ic2 = constant?

This is much harder to underatnd than the current source version

They are independent of the Re value and if it is constant or not.

But do you agree Ic1+Ic2 = constant?

If you assume that the be current of Q1 and Q2 is the same.

If tehre is diff voltage applied to the inputs Ic1 will increase and Ic2 decrease

How come currents of Q1 nad Q2 can be equal now?

Making sense?

Nayway I have to go sleep I dont hink I will get it soon. I gues I have to read about it from a text

I assume that is why he says it limits the dynamic range. You can't apply too much of a differential voltage because it will cause the current to change

It is really difficut for me to underatnd hoe it works

They are equal unless you do something to imbalance them, like the base-emitter voltage

the resistor is a cheap, very approximate current source

Like a balance beam with equal weights on both sides.

immbis how does that resistor act as a current source?

It is actually a current sink.

But not regulated

sparrky says it is not a current source since it doesn keep the Ic1+Ic2 constant

Thanks huys not possible to underatnd for me by chat I gues I have to dig it somewhere else Thanks I have to go sleep it is very late here

That seems correct. It's only an approximate current source. I am no expert on differential pairs.

Good night!

I would assume that the current is almost constant because the voltage is almost constant because you design the circuit that way

If Re is only a resistor it is subject to voltage changes.

This is ok for RF circuits.

But not precision op-amps

It is getting late