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help for simple amplifier

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yawijaya

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Hi guys, this circuit should work (i think) based on textbook. But it doesn't work in real life(no sine wave output across R5)...any clue here?Thanks!
 

Hi,
clear can it (so) never work:)
If you takes inputs 2/3 >> need pin 1 as output to use.
Assumed, that your IC is an LM324, than these exist only in DIP/SO14 package, is a 4x OpAmp...
Other IMPORTANT problem: Pin 11 is GND & pin4 is Vcc/+12V...
Possibly you have killed your IC if it is some 14 pin typ.
K.

Added after 3 minutes:

Otherwise is LM 325 an voltage regulator, in 14 pin Dip or in 10 pin TO can...
Because its very others, what is your IC type on the workbench pls?
 

Hi, i'm sorry but i don't get what you're trying to say about the

"clear can it (so) never work:)
If you takes inputs 2/3 >> need pin 1 as output to use."

and this is just the model i grabbed out of OrCAD, so this is only a picture, and it might be wrong. And i know 100% for sure that on the real board i wire everything correctly, so wiring is not an issue here.
 

Here is my analysis:

Vp: + input of opamp)
Vn: - input of opamp)
Vo: output

Vp=12*800/(800+800)=6V (DC) (voltage divider)
Vn=Vo*100/(100+200)=1/3*Vo

Vp=Vn (Opamp)
=> 1/3*Vo = 6V => Vo=18V which is not right because Vs=12V, it does not have enough power to put Vo up to 18V, therefore what u will see in oscilloscope will be straight line in 12Vdc.

I think you should try to change R3 and R4 to R3=5k, R4=1k. What I though is Vp=12*R4/(R3+R4)=12*1/6=2V
=>1/3*Vo=2V => Vo=6V which makes Vsin oscillate between 6Vdc.

That's my thought....
 

You can also put a largish (10uF or so) capacitor in series with R1 to get a gain of one at the DC bias point (6V) while keeping the gain of three at the signal frequency.

Note that you probably also want a larger capacitor than 1nF at the input. The capacitor together with the bias network forms a high pass filter with a corner frequency of about 400 kHz. This will reduce the magnitude of the 100 kHz sine wave to roughly 25% of its original value.

Regards,
Chris
 

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    yawijaya

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cdh7 said:
You can also put a largish (10uF or so) capacitor in series with R1 to get a gain of one at the DC bias point (6V) while keeping the gain of three at the signal frequency.
Click to expand...

Yes, that´s 100% correct. The normal procedure for single supply is to bias the pos. input with 0.5*Vsupply and to provide a gain for DC (only for DC because of the mentioned capacitor!) of unity.
 

Hi yawijaya,
I wished to tell only, that your pinout isnt real for LM324 &LM325 too!
Other question that LM325 not a OpAmp is, their is a switched pwr supply...
This was all.
K.

Added after 2 minutes:

Maybe your simulator dont accept a false pinout & makes a unrealistic/wrong answer over your circuit?
 

cdh7 said:
You can also put a largish (10uF or so) capacitor in series with R1 to get a gain of one at the DC bias point (6V) while keeping the gain of three at the signal frequency.
Click to expand...

Ha, thanks cdh7, it works. I don't know why i never thought about it before. And for karesz, yes i know that it is a wrong model, i just put it over there so i can draw the schematic, not for simulation
 

The LM324 is an old low power quad opamp so its frequency response is bad. At 100kHz its output into a high resistance is only 1V p-p max. Its gain at 100kHz is only 5.

You have pin 3 at +6V and the circuit has a DC gain of 4 so its output will try to be 24VDC which is impossible.

The opamp has a max output current of 10mA to 20mA so its max output voltage into 100 ohms is +1VDC to +2VDC.
 

Thanks for the reply, but i have found the problem on the circuit, pointed by cdh7 and LvM. And once again, the circuit i posted is NOT the circuit i'm using, i'm just putting it to show the configuration of the op amp i use. the op amp i use is opa2674. Thanks
 

The lousy old opamp in your schematic is not the one you are using. They are completely different.

Your problem was that the input was +6V and the DC gain was 4 so the output was saturated high. A capacitor fixed it so that it has a DC gain of 1 and an AC gain of 4.
 

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