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Loop Gain of an Op-Amp

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hemant03

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Hi,
I have been trying to simulate a circuit in LT Spice to find out the open loop gain.
To my surprise, when I plot Vint, I get a negative value of the gain (in db). I am using a small signal ac source of 1V.
I am attaching the schematic herewith. Can anyone tell me why the gain is negative.
Is it a known issue with LT Spice?

Thanks,
- Hemant
 

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Use double supply - and everything will be OK.
For single supply you need special circuitry fot correct biasing.
 
Yes that worked but what special circuitry would be needed for biasing. Do you mean op-amp as an inverting/non inverting configuration at dc?

Thanks,
 

Yeah, try giving Vcc/2 at the Non-inverting terminal.

Also, try running the transient analysis with 20mV sine wave as signal input, of some frequency.

BTW, what does this circuit do?
 

ninju said:
Yeah, try giving Vcc/2 at the Non-inverting terminal.
Click to expand...

Don't do that.
Vcc/2 multiplied with the gain of 3 gives a quiescent dc output (bias point) of 1.5*Vcc (that means: supply rail).
However, with a capacitor from R1 to ground the dc gain is unity.
 

**broken link removed**

check out this link on single supply op-amp circuits.
 
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