Inverting/Noninverting Amp - please help

[awpitt13]

I've been assigned a project that requires me to build a circuit that can recognize high frequency sound waves (16KHz - 20KHz). The setup is a electret mic to an amp to a high pass filter to my microcontroller (that determines the frequency of the wave). The input to the microcontroller is 0 - 3.3V.

My problem is getting the amp to work correctly. Because I only need to recognize a frequency of the sound wave I can use either an inverting or non inverting amplifier. Because the circuit will be battery powered i am using a single supply for my op amp (LM741CN) and not worry about the bottom part of the wave below 0V because my microcontroller can only accept 0-3.3V anyway.

I've been testing my amp with little success. To simulate input I am using a function generator. To simulate a battery I have a power supply set at 9V and plug the (-) terminal to common ground and the Vcc to the (+) terminal.

Here is my pinout for my chip (LM741CN):
33kohm resistor from pin 2 to 6 (inverting input to output)
Pin 3 (non inverting input) is grounded
Pin 7 (V+ Pin) to +9v of the power supply
Pin 4 (V- Pin) to ground (Negative terminal on the supply)
The two offset null pins I don't touch
Waveform from function generator through a 2.2kohm resistor to pin 2 (The inverting input pin)

So my gain should be 33/2.2, but instead of doing this the output from pin 6 gives me a voltage just below 9V. I've replaced the op amp several times with new ones (of the same type) but get the same results.

Ive noticed that when i slowly increase the power supply to the chip from zero to 9v the output from pin 6 goes from the original waveform signal (not even inverted) to being clipped from the bottom until it reaches the peak of the original waveform when the supply voltage is equal to the Vpp of the waveform. Past that voltage the waveform disappears completely and the voltage rises as a constant value that is about equal to the supply voltage.

These amps seem like such simple circuits, I have no idea what is wrong and I'm worried I've wasted so much time on some simple oversight like how I am handling the power supply.

I will be happy to provide pictures of my circuit on my breadboard or clarify anything.

 

[Grossel]

The problems you have (but cannot see) is this:

1: The 741 opamp cannot provide output voltage that goes from -V to +V. I don't remember how much voltage gap is, I guess as in your case, the opamp can provide between 2.5 to 6.5 volt output. That factor alone may lead the circuit to not function as intended.

2: The slew rate of the 741 is pretty poor, and that will effecly limitate gain for high frequenzy.

Conclusin is: I think you need another opamp circuit. If datasheet says they have rail to rail output, it means they can provide an output voltage at whole range from -V to +V.
You also can try to change the voltage supply to +12V and -3V (or more negative).

[edit]
I've to admit I think there is no such thing as rail to rail output. Only input may have the properties of being rail to rail input.

Can you upload the schematic?

 

[awpitt13]

I did some research and there are some chips that are close to rail to rail output.

TLV2772
Digi-Key
Precision Amplifier - High Output Current - TLV2772 - TI.com

DS4812
**broken link removed**

Would one these work? I don't know enough about slew rate to know what would be high enough for a 20kHz waveform. Otherwise I'll go ahead and do a dual supply like you suggested.

Here's the circuit schematic
http://img.photobucket.com/albums/v520/awpitt13/schematic.jpg

Thank you for your time Grossel

 

[crutschow]

The maximum slew rate of a sine-wave is 2π x f x Vp where Vp is the peak value of the sine-wave. Thus for a 20KHz, 3.3Vp sine-wave the slew rate is 414,690 V/s or 0.4V/µs. Any op amp with a slew rate greater than that should work for you.

 

[audioguru]

Your opamp has its DC inputs at ground. But its datasheet shows (its common-mode input voltage range) that the inputs must be at least 3V above its negative supply pin (ground in your circuit) and 3V less than its positive supply pin. The inputs do nothing in your circuit.

1) Your generator is probably grounding the input to the 2.2k resistor. Use an input coupling capacitor.
2) Use a voltage divider to bias the (+) input at half the supply voltage.

You don't need a rail-to-rail opamp. Simply use an opamp that works as high as 20kHz. The lousy old 741 opamp works up to only 11kHz.
A TL071 opamp is inexpensive, works to 100kHz, has low noise and low distortion.

 

[awpitt13]

Thank you all for your input, it seems there is much I still need to learn about op amps, but that goes for electronics in general.

I will try the dual supply method and/or a different op amp that is more suited for my circuit then decide which I will implement.