Who can help me explain what's the current source for?

explain current sources

The circuit is shown in Fig 1. as you can see, this is a inverter, but I don't know why M3 is used in here. Obviously, M3 is a current source. Is it for biasing the inverter?
The circuit is a basic unit of a chip. In that chip a lot of digital circuits, such as AND, OR, D-Latch and so on, is similar to this inverter biased by current sources. By the way, the chip is extreme low-power, the total supply current is only 4uA including analog parts and digital parts. So I guess maybe it is for limiting the peak current. As I know, when vin is in switch-point, pmos and nmos are all on to produce a large current. So in this circuit the maxmum current is limited by source current, and we can design a very small biasing current source such as 0.1uA.
This is my guess, and I don't know whether it is right.
Welcome to criticize!
Thank you so much!

Hi,
I think you're right. The current mirror limits the inverter peak current.

I am not sure if it is correct. The added current source is to increase the Gm of the circuit, thus, speed and swing. This is good especially for high frequency circuit

milkdragon said:

I am not sure if it is correct. The added current source is to increase the Gm of the circuit, thus, speed and swing. This is good especially for high frequency circuit

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No, I think the reverse is true: This circuit will limit the current in expense of
circuit speed

I think the current mirror limits the inverter peak current. If VOUT is connected to a capacitor, the discharge time of that load capacitor will be long.

Hi all:
I think:
1) limit peak transient current, improve latch-up
2) make Vin's threshold voltage
3) will give more dc current to the IC
:|

Tiny current limits the inverter speed, so i think it is designed aiming for limiting peak current to achieve low power .

Thank you everyone!
I also think it is to limit peak current. In fact, this chip has no requirement in speed, and it needs delay intendedly. But power is important for it. So I think it is for geting low-power by sacrificing speed.
I also need other advice!

this is called current starved inverter and it is used to specify the maximum current drawn by the inverter, but it needs another current source above the PMOS for operation when charging