hello

I have a circuit
im not using the specified mcp6002(general op amp, single supply) or components, namely the diode or feedback resistors. I have an OA91 diode (germanium) 4760 in place of 5K and 678 in place of 648 resistors. the caps are polyester box type(I don't know too much about these) they are the correct value and non polarized so I thought they would be ok.
I put in the correct supply and a signal generator from arduino, 1.1VPP at 4kHz. I used my little oscilloscope to measure the sinewave going in and the flat dc coming out. I got the correct values going in and about .38mV coming out. I measured this on a cheapo multimeter and got the same output. the gain I got was 8 using, 1+ R2/R1 * Vi. I used Vi as 0.5Vp as it was rectified through the diode. though I didn't remove the diode voltage drop which would put it down to 0.2Vp. the output at pin 1 should be 2.4vP
I don't know if the caps are the problem, polyester supposedly not good at high frequency which is where I will be using them.
not sure what else I may be doing wrong. any help would be great

thanks
getting closer to Christmas, WOOOOOOOOOOOOO

simon

 

"Form follows function."

Since you changed the form, it stands to reason that it changed the function.

Do you want the function or not?

 

What opamp are you using? That is likely the most critical component in that circuit.
It must be a modern low-voltage CMOS rail-to-rail type.

What is the input referenced to? Ground? That means it swings 550mV above ground and 550mV below ground? Or is it riding on a DC level?

 

What opamp are you using? That is likely the most critical component in that circuit.
It must be a modern low-voltage CMOS rail-to-rail type.

What is the input referenced to? Ground? That means it swings 550mV above ground and 550mV below ground? Or is it riding on a DC level?

hello

the op amp is a lm324
the output should be a dc level I think 2.4 volts

 

hello

the op amp is a lm324
the output should be a dc level I think 2.4 volts

When powered from 5V, the LM324 cannot pull its output pin any higher than ~2.5V, so it will not work in this circuit. Try a LMV324, instead.

I asked about the INPUT level.

 

There was a reason for the choice of the op amp. The MCP6002 is a rail-to-rail single supply op amp that is perfect for this application. Use it.

Bob

 

When powered from 5V, the LM324 cannot pull its output pin any higher than ~2.5V, so it will not work in this circuit. Try a LMV324, instead.

I asked about the INPUT level.

hello MikeML

the input is 1.1vpp from the signal generator, this goes through the diode which I imagine has a diode drop of 0.3 volts and rectifies the voltage.
I did just order some of the mcp6002 from RS components they will arrive on Monday hopefully.

thanks
simon

 

It is not a great circuit. Do want the level of V(out) to change with different amplitude of the input signal?

 

While the op amp is an important feature, the feedback resistor (which you said you left out), is critical. Feedback is the only thing that prevents the op amp output from either pegging at low, high or (less likely in your case) oscillating wildly. Use the feedback resistor.

 

While the op amp is an important feature, the feedback resistor (which you said you left out), is critical. Feedback is the only thing that prevents the op amp output from either pegging at low, high or (less likely in your case) oscillating wildly. Use the feedback resistor.

His feedback resistor is 4760 in place of 5K.