# Current mirror biasing circuit

I have just seen the comparator as figure 1 below and I am wondering why don't we use a simplified version as figure 2?

Hope anyone could make it clear. Thanks.

Figure 1:

Figure 2: simplified version

• You are in principle right that you could use the 2nd circuit also but there's something you overlooked ! Look carefully at the geometries of the two PMOSFETs you eliminated. What does this PMOS mirror do with the current Ibias ? So circuit 2 is NOT identical unless you make the value of Ibias.... Another reason for using circuit 1 is that you only have this Ibias available from the negative side so you're forced to use a PMOS mirror. – Bimpelrekkie Oct 10 '16 at 7:29
• Yes, actually I ignored the current value or size of MOS, just consider topology. With negative side, do you mean that Ibias is as a current sink? – anhnha Oct 10 '16 at 8:16
• I also think that normally Ibias is implemented by a resistor. – anhnha Oct 10 '16 at 8:27
• With negative side, do you mean that Ibias is as a current sink? Yes I mean that the current is delivered from a NMOS mirror for example. I also think that normally Ibias is implemented by a resistor If you can live with up to +/- 20 % variation in bias current over process variations (I'm not even mentioning temperature dependency and supply voltage dependency) then yes, you could use a resistor. But often that bias current will be derived in a different way. Using a resistor is the easiest but usually also gives the least constant Ibias. – Bimpelrekkie Oct 10 '16 at 11:33
• How can you implement that Ibias so it doesn't depend on power supply, temperature and process? You could write a thick book about that ! There is no source which depends on nothing so a compromise has to be made. Usually some form of bandgap circuit is used to create a temperature compensated voltage, then by combining different types of resistors (with positive and negative temperature dependencies) you could make a reasonable stable bias current. But sometimes, you don't want that, sometimes you need a current which increases over temperature: PTAT ! – Bimpelrekkie Oct 10 '16 at 12:38

Basic function of current mirror here is to "copy" the Ibias current to main circuit via dividing by 25 (due to transistor sizes 600 vs. 24).

In order to maintain stability of circuit, we generally use transistors in saturation. Very small current may cause transistor not to saturate.

We can use transistors to obtain Ibias instead of resistors. If you maintain saturation of transistor (see diode connected), you can get stable and constant current.