whats wrong with this current source

[deepa]

thisHTML:

Normal link:
https://obrazki.elektroda.pl/31_1165758194.GIF



transistor is in saturation ,what prevents it being used as a current source

 

[diemilio]

If you're using an enhacement channel P MOSFET you would need a value of Vgs smaller than Vth (which in this case would be a negative value) to turn it on. In your circuit Vgs=0, which means your MOS is in the cutoff region.

For example, if Vth=-2 V and Vdd=5 V, you would need Vg to be: Vg=-2V+5V=3V or LOWER!!! to always get a Vgs smaller than -2V. In your case you have a Vg=5V which doesn't work for you.

There are several things you can do to make your current source work:

- Ground Vg to get a current value of: ID=kn*(-Vdd+Vth)^2 (where Vth is negative)
- Change your enhacement MOS for a channel P JFET or a depletion mode MOS to get values of ID=Idss (where Idss is kn*Vth^2)

hope this helps,

diemilio

 

[deepa]

i am sorry to have drawn the circuit wrongly,,it is an NMos only,,
n yet this doesn seem to be used as a current source,,what is wrong with this

 

[raduga_in]

Firstly we need to understand that the NMOS/PMOS will
act as a current source when it's gate-to-source voltage must be constant; provided that we ignore the secondary
effects like finite ro and body effect. So, in this circuit
you can easily see that that cannot be the case your
gate voltage is fixed and source voltage can vary. Therefore it cannot act as a current source.

 

[deepa]

wont an NMOS or PMOS act as a current source when it is n saturation,
if Vgd<Vth ,we can say that it is in saturation so isnt the above transistor in saturation,hence a current source

 

[Guest]

the device you have defined, really is a diode conncted MOS transistor, since transistor is always in active region, noting I-V equation for a specified VGS=VDS=Device Vout, device has a different current value;
Since a current souce should have a constant current, Iout, for different Vout this device is not worth to use as a CS;
BTW, as you know, a CS must have large output resistance but this device for large amount of current output resistance is approximately 1/gm.
If answer is not very bad, it's not bad to help!

Regards,
SAZ

 

[venkateshr]

generally current sources can be classified into two things. current source and current sink.
former is sourcing current to bottom circuitry while the latter sinks some definite amnt of current. pmos /pnp is the candidate for source and nmos/npn is the candidate for latter as current value depends on Vgs(mos) or Vbe(bjt) which shd be constant.

in ur case Vs can vary according to current and due to othr circuits so this is not a current source but a simple diode connected load with 1/Gm as resistance.

there is no need to do any other thing than simple analysis . if u want to have const. current there shd be constant Vgs. if u can ensure this in some ckt then that is a current source.

 

[diemilio]

If you're using your current source on a DC circuit, and you can guarantee that the MOS is on sat region everything would be OK. On the other hand, if this current source is beeing used on an opamp, or any other circuits with an applied AC signal it would be very probable that your current varies as your input changes. Remember that ID deppends on Vgs, and since Vsource si exposed to AC variations your current would not be a constant value.

You can solve this problem by using a P channel mos that works on sat region.

diemilio.

 

[nguyennam]

i am sorry to have drawn the circuit wrongly,,it is an NMos only,,
n yet this doesn seem to be used as a current source,,what is wrong with this

I only know that JFET can be used as current source.

However, its ability is very limited. So you had better to use 2 BJT or 1 BJT + 1 diode to make a current source, you can figure it out in any manner as you want.

nguyennam

 

[pixel]

This is not current source. It has low output resistance.