2

enter image description here

About the circuit above, I can't understand from the row "VA=2Vos".

If that's because most of the voltage between 1K ohm resistor and capacitor is on 1K ohm then I can't understand why VA should be double of Vos.

Please help me understand this circuit.

devnull
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Greenbanana
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  • Va is a low resistance node and Vin- is assumed to draw no current. The negative feedback matches Vin- to Vin+(=Vos) to have null error and thus Va must be double Vin+ – Tony Stewart EE75 Apr 25 '21 at 11:52

1 Answers1

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Assuming ideal non-saturated operation, at DC:

enter image description here

The two currents are the same (current to/from V- pin is zero), so is the voltage across both resistors. Since the ideal model is assumed, \$V_+ = V_- = V_{OS}\$, so:

$$\frac{V_A}{2}=V_{OS}$$

The current indicated will be 2000 times smaller than the current across the 1 kOhm resistor. So, considering only this larger one, for simplicity, the voltage across the third resistor would be 1000 times larger than \$V_{A}\$, so, also disregarding \$V_A\$ (when compared with \$V_O\$):

$$V_O \approx 2000 V_{OS}$$

enter image description here

That's only the DC analysis and considering the input resistance of the op. amp. >> than 1 MOhm.

devnull
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