why use current source for a load ???

[balavinayagam]

Hi all. i have one basic question. Why do we prefer to use a constant current source for an active load in most of the circuits ?



Thanks in advance

 

[keith1200rs]

I am not sure what circumstances you have in mind, but a lot of circuits have a voltage output so if you want an active load you cannot use a voltage load - the current would but infinite - only limited by spurious resistances. With a current load the power into the load will vary linearly with the output voltage. Another possible (passive) load would be a resistor, but then output power would vary with the square of output voltage. However, a resistive load (often with a parallel capacitor) is often used as well.

Keith

 

[FvM]

Do we use active loads for most circuits? Depends on, in analog IC designs we probably do, in discrete circuit design we rather don't.

Do we mostly use current sources as active loads? Which other kinds of active loads do you have in mind?

Apart of other motivations, in analog IC design a transistor is cheaper (in terms of area usage) than a resistor, so active loads are actually everywhere.

 

[balavinayagam]

thanks for the reply. but i have another question
under what circumstances what loads are preferable?

i think there are many active loads like saturated nmos or depletion type mos or pmos grounded like in pseudo nmos ? so in what condition do we prefer what?

thanks

 

[keith1200rs]

In IC design current mirrors are often used as loads because it effectively gives a high resistance load which gives higher gain. It also keeps the size low.

Keith.