Why is there a 2mA tail current in this structure?

[overmars]

Look at this structure, if M1 and M2 are equal , the current flow through M1 and M2 should be the same. So why there is still a 2mA tail current? I think it shall be 0.
Please help.Thank you.

 

[abel51]

Re: a simple question

I think M1 is reverse biased since the base voltage polarity is 0. But M2 is forward biased. Therefore only M2 is conducting. And the current through M2 is 2mA wich should also be equal to Iin.

 

[flatulent]

Re: a simple question

Something is missing from the circuit such as parasitic diodes. The circuit above the 2 mA current source is floating without any current paths to ground. The 2 mA current source will have a infinite voltage across it in its attempt to move charge from the floating transistors to ground.

 

[iggyboy]

Re: a simple question

There is a 2mA tail current because of a 2mA current souce in the tail. No matter what is done above.

 

[jayc]

Re: a simple question

This is a classic negative resistance generator. There is actually nothing missing or incorrect about the diagram, but some interpretation is involved.

The 2mA tail source is only for DC biasing, and you will need it if you want to calculate the value of the result in the end. There is other DC biasing circuitry not shown because it does not effect the operation of the circuit. The current source labeled "i_in" is your input (small signal) test current source and "v_in" is the corresponding voltage to be measured across the source. If you work through some KCL and try to calculate R_in = v_in/i_in, you will find the result to be -(gm1||gm2). In the case that
M1 and M2 are the same size, each will have a DC current of 1mA flowing through and
the small signal resistance seen at the cross-coupled drains will be R_in = -gm/2.

hint: draw the small signal model