Hello,
I have an AC signal source that varies in frequency - 5hz - 200hz. The minimum voltage measures at around 75 millivolts. I need to amplify this to drive a digital transmitter which has a minimum input voltage of 2.5v.
signal quality is not a big concern, as I am converting it to square wave before input to the transmitter.
Searching the web, I have found a couple of circuits that claim to amplify weak signals - the latest is a detector/amplifier intended for a crystal radio. It seemed that this should do the trick, but I can't get it (or the others) to work.

Anyway, I would appreciate your input on how to approach this problem. It seems like it should be relatively simple but, alas, I'm baffled.

THX!

- k -

 

Just to be sure, is your signal bipolar or unipolar?

If bipolar, you can very easily use pretty much any opamp to increase the signal. Search non-inverting amplifier and see how to set the resistor values. A gain of ~33 at a maximum frequency is only 33*200= 6600Hz gain-bandwidth product. Try to find a precision opamp if you can with low input noise and low voltage offset, and at least 100KHz g-bw product.

Steve

 

Have a look at the attached circuit; it's a variable gain inverting AC amplifier. It will not amplify DC signals.

I used an LF353 because they're so widely available. A better choice would be Linear Technology's LT1007.
You could also use a TL082 from Radio Shack. I wouldn't use a 741 or 324 because they're ancient technology.

C1 and R1 establish a high-pass filter; signals below about 3.3Hz will be eliminated. Don't change these. If C1 is removed from the circuit, it won't work.

R2 establishes the gain for the circuit. The output can be adjusted for a gain between 1:1 and 100:1.

R3 and R4 sets the reference voltage for the non-inverting input to 1/2 of the supply voltage.

C2 isolates the circuts' DC voltage levels from the load. It must be used, and be at least as large as C1.

 

thanks to both! I will implement your suggestions and repost my results.

I appreciate your time to answer!

- k -

 

I have ordered some LF353 OPAmps and expect arrival tomorrow. In the mean time, I built the circuit with an LF357 I had in my box of stuff. it appears to work, but the output is a scraggly square wave. the square part is actually a good thing, since that's what the transmitter needs for input. is the distorted waveform a result of the substitute OPAmp? If not, how do I 'square-up' the square wave?

 

The "scragglyness" of the output square wave is most likely due to the caps you used.

Electrolytic caps can have quite a bit of resistance and inductance along with the capacitance. This causes attenuation of higher frequencies, which is causing the "scragglies". Poly caps would likely give you a much cleaner waveform.

As an alternative, you could try using several smaller caps (tantalum, ceramic) in parallel - just as long as they all add up to 47uF.

 

thanks for the clarification! I will try your suggestions this evening.

 

The LF357 is de-compensated so its minimum allowed gain is 5.
It is very high speed and might oscillate at a high frequency on a breadboard or if it does not have a 0.1uF ceramic disc supply bypass capacitor with short leads very close to the IC.