help for simple amplifier

[yawijaya]

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!

 

[karesz]

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?

 

[yawijaya]

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.

 

[adinogcas]

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....

 

[cdh7]

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

 

[LvW]

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.

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.

 

[karesz]

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?

 

[yawijaya]

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.

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

 

[Audioguru]

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.

 

[yawijaya]

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

 

[Audioguru]

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.