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jimmyshu
Joined: 08 May 2007
Posts: 15
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 26 Dec 2007 17:42 explain current sources |
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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!
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naalald
Joined: 02 Dec 2006
Posts: 300
Helped: 23
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26 Dec 2007 18:54 Re: Who can help me explain what's the current source for? |
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Hi,
I think you're right. The current mirror limits the inverter peak current.
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milkdragon
Joined: 03 May 2007
Posts: 64
Helped: 6
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26 Dec 2007 19:55 Re: Who can help me explain what's the current source for? |
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| 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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Ghitani
Joined: 03 Jul 2007
Posts: 32
Helped: 2
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26 Dec 2007 22:53 Re: Who can help me explain what's the current source for? |
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| milkdragon wrote: |
| 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 |
No, I think the reverse is true: This circuit will limit the current in expense of
circuit speed
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firry
Joined: 03 Dec 2005
Posts: 24
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27 Dec 2007 2:43 Who can help me explain what's the current source for? |
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| 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.
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mycelldevice
Joined: 16 Oct 2007
Posts: 6
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27 Dec 2007 3:02 Re: Who can help me explain what's the current source for? |
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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
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jeffsky520
Joined: 11 Oct 2006
Posts: 71
Helped: 5
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27 Dec 2007 4:22 Who can help me explain what's the current source for? |
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| Tiny current limits the inverter speed, so i think it is designed aiming for limiting peak current to achieve low power .
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jimmyshu
Joined: 08 May 2007
Posts: 15
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27 Dec 2007 4:35 Who can help me explain what's the current source for? |
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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!
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27 Dec 2007 4:35 Ads |
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safwatonline
Joined: 19 Nov 2005
Posts: 1418
Helped: 172
Location: EGYPT
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27 Dec 2007 5:15 Who can help me explain what's the current source for? |
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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
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