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There is No enough current running in my amplifier simulation

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sys_eng

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amp_simul.png
M19 gate is at 3.5V and the transistor is on. Why is it only drawing 6.6pA?
Also it doesn't add up. M19 drain current is not equal drain current of M15.
 

Because Vds voltage is almost zero..

But the only thing you can control is Vgs. not Vds.

- - - Updated - - -

Because Vds voltage is almost zero..

But the only thing you can control is Vgs. not Vds.

The gate voltage of M19 is 3.5V so it should conduct significant current. right?
 

Your Mosfets are drawn with 4 pins instead of the normal 3 pins. Then we cannot see which is drain or source.
Your supply voltage is not shown. If it is low then the P-channel Mosfets are turned off and they are the loads for the N-channel mosfets so of course the current is very low.
 

Your Mosfets are drawn with 4 pins instead of the normal 3 pins. Then we cannot see which is drain or source.
Your supply voltage is not shown. If it is low then the P-channel Mosfets are turned off and they are the loads for the N-channel mosfets so of course the current is very low.

how did gate of M14, M15 becomes 3.294V??

Both are PMOS and therefore got turned off therefore the circuit is not working.
 

well, after some tweaking, getting rid of the diode connected pmos and connected to a ground then everything working now. Weird.

It's drawing 52.6uA. It's working now.
amp_simul3.png
 

The original circuit is "working" as well. The question is what you want to achieve.
 

The original circuit is "working" as well. The question is what you want to achieve.

The first one is running at pA whereas the second one is running at uA. (right hand side)
I don't think pA is correctly working.
 

Believe me, tweaking your circuit like this is not the best practice.
Your first circuit works as well. It's much better to have a current mirror as a load in your differential stage.
If you want to get more help from us, you should give further information such as: VDD, threshold voltages and aspect ratio of mosfets (at least).
 

Look, in your first circuit you are tying the gate of M18 to VDD.
In principle, this is not the correct way of biasing because you want M18 to behave as a current source and create a virtual ground for balancing the current at each branch.
Basically, what you are doing is dropping the drain terminal of M18 to ground, so there is no Vds, then it operates in the linear region and not in saturation and the rest of your circuit is not operating as a consequence of this.
What you can do is to put an ideal current source, set the proper value (you need hand calculations for that) and mirror this current to M18.
This is the correct way of biasing since you ensure that your differential pair is drawing the current set by your current source and so you don't have floating nodes.
 

Believe me, tweaking your circuit like this is not the best practice.
Your first circuit works as well. It's much better to have a current mirror as a load in your differential stage.
If you want to get more help from us, you should give further information such as: VDD, threshold voltages and aspect ratio of mosfets (at least).

Look, in your first circuit you are tying the gate of M18 to VDD.
In principle, this is not the correct way of biasing because you want M18 to behave as a current source and create a virtual ground for balancing the current at each branch.
Basically, what you are doing is dropping the drain terminal of M18 to ground, so there is no Vds, then it operates in the linear region and not in saturation and the rest of your circuit is not operating as a consequence of this.
What you can do is to put an ideal current source, set the proper value (you need hand calculations for that) and mirror this current to M18.
This is the correct way of biasing since you ensure that your differential pair is drawing the current set by your current source and so you don't have floating nodes.
The main issue is M14 drain and gate stuck at 3.294V therefore M14, M15 current load off. No current flowing.

If I could solve when M16 gate at 0 V, M14 drain and gate would not be at 3.294V and be near 0 V.
 

The main issue is M14 drain and gate stuck at 3.294V therefore M14, M15 current load off. No current flowing.

If I could solve when M16 gate at 0 V, M14 drain and gate would not be at 3.294V and be near 0 V.

This is not your main issue.. The point is that you are not doing this in the right way. Just check whatever book on Analog IC design and you will see what I mean on my previous post. Bias your circuit properly and it should work. Good luck.
 

The main issue is that you are using a linear amplifier circuit far beyond it's linear voltage range. It's not clear in the slightest what you want to achieve. The circuit might be intended as switched current source, but of course, it can only source current while charging the load capacitor.

Useful design steps:
Clarify the intended circuit function.
Select transistor size and bias voltages respectively.
 

A very efficient technique to design analog ICs is the gm/ID methodology.
In this case, you need to calculate what is the tail current that you need given the required gm at the differential pair input you want.
Once you have calculated this, you can put an ideal current source for first order analysis and mirror the to the tail of the differential pair.
 

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