Problem with getting gain in a simple inverting op-amp

[manofwax]

Dear All,
I was trying to build a very simple inverting op amp. Input level is 0.04V and i want the output level to be 10V, and it's operating at 40kHz. But for some reason, I couldn't get any gain, but instead, it's attentuating. Please Help. Thanks.

 

[Kral]

Re: Op Amp

manofwax,
You are probably using an Op-Amp that has a low gain bandwidth product. You are trying to get a closed loop gain of 250 @ 40KHz. This means that the open loop gain of your op amp must be significantly higher than 250 @ 40KHz.
Regards,
Kral

 

[manofwax]

Re: Op Amp

Kral,
I tried to get a gain of 2, by changing the resistor values. Rf = 2K, R1 = 1K. But somehow it's still attentuating.
Regards,

 

[Kral]

Re: Op Amp

manofwax,
What Op-Amp are you using?
Regards,
Kral

 

[Gorgon]

Re: Op Amp

What is the supply voltage(s) for this construction. If it's a single supply, you'll need rail to rail input at least.

TOK

 

[amriths04]

Re: Op Amp

try operating at reduced frequencies. if the gain improves, then you need to use another higher speed opamp than the one you are currently using. or just look into the data sheets of your opamp and look for the gain-bandwidth product and check out whether it is below 40khz.

if G-BW is above 40khz, check whether your +Vcc is atleast >10V.

regards,

 

[zhangseong]

Re: Op Amp

if u are using pspice, uA741 opamp, here is a reference for you

 

[manofwax]

Re: Op Amp

Thanks a lot. I will try again tomorrow.
I'm using LM324, what rail to rail?
The way I set it up is:


[Signal Gen]-->[inverting op amp]-->[Oscilloscope]

Signal Gen: 33kHz
inverting op amp: Rf = 2K, R1 = 1K

I just want to see how the gain varies when I change the value of Rf. But somehow, this formula is not working for me, Vout = Rf/R1* Vin.

 

[nguyennam]

Re: Op Amp

For the frequency, it doesn't matter which Opamp you choose since most general purpose opamps have the unity gain bandwith of 1MHz the lowest.

The resistors are calculated as usually.

If it is 10VAC RMS output needed, it means 14,14V peak, so you need the opamp with split power supply of at least +/-15V, or LM741 can be used in the worst case. For the latter case, you should use the power supply of +/-16V or higher with other high voltage opamp, or use LM741 in its +/-12V power supply to drive the transistor output stage in +/- 16V upwards power supply.

nguyennam

 

[JoseLeonardo]

Op Amp

you can read Design with OP Amp and Analogic Integrated Circuit, by Franco.