What's the use of MOSFET M8 and M9 in the figure?

[yaobo]

whats the purpose of m8 and m9

The circuit diagram shown in figure1 is an N-type V-I conventer,which does not require balanced inputs, I'm just not sure about the use of MOSFET M8 and M9,
level shifting or sth else?
Thanks a lot

 

[happy2005]

M6-M5, M6-M7, M12-M11, M13-M14 are current mirrors. M8 is a current reference for M12 mosfet. In the same way, M9 is a current reference for M13. All these transistors are on the same chip, so (W/L) ratios can be controlled very precisely in the chip. I think M8 and M9 supply a dc current reference for M12-M11 and M13-M14 current mirrors respectively.

 

[kwkam]

I think M8 is current mirror of M5. It like to control the current matching between M11 and M12.

 

[wwfieee]

both m8 and m9 are active loads for the current mirror below them. think of them as resistors providing reference current to be mirrored.

 

[sutapanaki]

Guys,

This circuit is loaded with current sources M5 and M7. In this case you have to care about the voltage at the drains of M1 and M4. If for example, the currents sourced by M5 and sunk by M1 don't match, the voltage can go too high or too low and put the tramsistors out of operating region. M8, M11 and M12 form a feedback (similar to a common mode feedback in fully differential amplifiers) to control that voltage. You don't need such a control for the current outputs since usually they are loaded with small impedance levels.

 

[kokabanga]

that is just to provide balanced layout that is all!

 

[kwkam]

Thanks, sutapanaki.

I strong agree with your idea. But I think the M8, M11 & M12 will form a loop to create oscillation.

 

[sutapanaki]

Thanks, sutapanaki.

I strong agree with your idea. But I think the M8, M11 & M12 will form a loop to create oscillation.

No, they won't. It is a negative feedback. Of course, every negative feedback loop can oscillate if not properly compensated. But otherwise, it should work in a stable way - just maintain 2Vgs from gnd to the drain of M1 and M4.