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simulating opamp virtual ground

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yefj

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Hello, I have build the opamp as shown bellow, the right input is 180 phase to the other AC input.
negative feedback is supposed to do virtual ground,which means that if i connect the output to the inverting input as shown bellow i get totay different DC values on both inputs of the opamp.
Where did i go wrong?
Thanks.
1624216501890.png


1624217638971.png
 

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It is not opamp, nor pseudo differential Amplifier. Both branches are completely independent, so there is no reason to have gate voltage of one mosfet following gate voltage of other one.
 

Yes i will add PMOS load connected by gates together.
Thanks.
--- Updated ---

UPDATE:
I have updated the schemtics as shown bellow,but stiil the DC input voltages differ.
Where did i go wrong inplementing virtual ground?
Thanks.

1624219648992.png
 
Last edited:

A differential detector commonly is a long-tail pair. The tail can be a resistor, or transistor, or current source. Attach it to a negative supply voltage, so that your input signals can be zero volts (ground) and still cause the amplifier to operate. Then your output voltage is referenced to 0V ground.

Adjust operation so that the left half of the detector influences the right half (and vice versa). When current increases in the left half, it causes current decrease in the right half. Etc.
--- Updated ---

Basic example of differential detector. 2 NPN, with tail resistor.
long-tail pair 2 NPN (tail resistor) 5V supply sine-n-sawtooth create SPWM.png
 
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To make a virtual ground, the amplifier must have high differential gain and sufficient common mode rejection. Your simple amplifier has neither.
 

As noted, you can't have a differential amp if the transistor sources are connected to ground.
The sources need to be connected to a common, high impedance current source.
 

You connected two common source amplifiers in parallel.
introduce a current tail providing both branches to create differentially
 

Basic virtual ground

1624316428571.png


Above we have a Aol of 100K, a feedback of 10%, so overall G = 10. But notice the Vir Gnd V,
its 99 uV. In other words at the diff input the two inputs are essentiually same V due to the
feedback.

Now lets reduce the Aol (Slider Gain) to 100 -

1624316655613.png


Here we see large error in Vout, and we are losing the Vir Gnd, its jumped to 90 mV, almost
100X what it was when we had large Aol.

The importance of Vir Gound, one example, is the classic summer -

1624316930705.png


Basically when Virtual Ground is solid then the current thru each input is unaffected by
the other input currents, eg I = V1/R = V2/R = V3/R in this case.

So lots of Slider G (forward path gain) here makes the simple summer work well. Or higher
Feedback Factor (which applies more error correction when closed loop G
is lowered).


Regards, Dana.
 

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