Question on a buffer circuit

[afujian]

Hi all, I got a buffer circuit recently,this buffer is for the reference voltage in a DAC with a class AB output stage,but now I don't know the function of the diode connected transistors marked with red circle,so could you please give me a hint?

 

[erikl]

Bias voltage generation. 2*V_th shift.

 

[afujian]

hi erikl,sorry for the late reply.
If a 2*Vth shift is generated,there should be some current flow through the diode-connected transistors.However ,for example,the branch1 and branch2 are totally matched,so the current of the two branches should be matched too,hence,no current should be flow through the transistors in the circle 1,and no shift should be generated,right?
I updated the schematc with some new marks as below.


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[erikl]

If a 2*Vth shift is generated,there should be some current flow through the diode-connected transistors.

Right!

... the branch1 and branch2 are totally matched,so the current of the two branches should be matched too,hence,no current should be flow through the transistors in the circle 1,and no shift should be generated,right?

What makes you think so? There's a voltage difference between node A and VDD, which makes a current flow through the transistors in circle 1, its value depending on their W/L sizes. This current flow generates a voltage drop of 2*Vth and so arranges for a node voltage of VDD-2*Vth at node A. Try and analyze this by simulation!

 

[D.A.(Tony)Stewart]

When gate and source are shunted, it is a depletion mode current source.
When two gates are running in tandem, they perform a current mirror operation.
Together they become a level shifter.

 

[afujian]

erikl and SunnySkyguy,thanks for your answer,I got it.