[SOLVED] Can I remove tail current of telescopic ota to get larger swing?

[fuxinmingming]

Hi guys,

I designed a telescopic differential ota, its swing is Vdd-5Vdsat, however, it is too small.
Can i remove the tail current and connect the differential pair just to the ground to get larger swing? In that case, the swing becomes Vdd-4Vdsat. What is the defect if I remove the tail current ?

 

[JoannesPaulus]

If you remove the tail current, you do not have a differential amplifier anymore. If you need a large output swing you should add an output stage or look into a different architecture (for example, folded cascode).

 

[fuxinmingming]

Hi Joan,

Since the input signal is differential, the output(output left and output right) also is differential even if I remove the current source.

Since power is another concern, I can't use two stage or folded cascode.

 

[JoannesPaulus]

Since the input signal is differential, the output(output left and output right) also is differential even if I remove the current source.

In principle, with all ideal components, that is correct but I am afraid it does not work that way. A circuit as you describe it is called "pseudo-differential": it looks differential but it really isn't.

 

[fuxinmingming]

Hi Joan,

Can you explain me what's the pseudo-differential in detail ? Why "it looks differential but it really isn't" ?

Regards

Ming

 

[checkmate]

With no tail current, the biasing current depends on your input voltages.
Not only do you run the risk of having a large biasing current (if you have large inputs), your small signal parameters are also fully dependent on your input, giving you no linearity and CMRR to talk about.