Need a easy to build signal amplifier

This is an ordinary job for opamps. These are universal signal amplifiers that have two inputs (labeled + and -) and an output. The opamp has a very high differential gain (it amplifies the difference voltage between the two inputs) while using almost no current through these inputs. When the + input is more positive than the - input, the output heads positive.

You program an opamp for what ever gain you want with a few resistors. It can produce a positive or negative output for a positive input (called noninverting or positive gain versus inverting or negative gain). The input voltages and output voltage are constrained to stay within the supply voltages.

For a basic tutorial on these cheap and useful components and lots of circuit ideas, see:

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John Popelish

This site is cool! I really enjoy it. Thanks

Allen

try this single op amp circuit

a

millivolt

volts

of

Although the OP doesn't specify his needs very clearly, its clear that he is working with audio. Are most run of the mill opamps fast enough to handle high audio without distortion?

Thanks, Bob Monsen

What is the signal source?

The mic input is already made for very weak signals, below and above

1mV, so maybe you do not need any amplifier, just use the volume control on the mic channel in the computer.

If you really need an amplifier you should know that amplification is not the main problem with such weak signals, an op-amp can deliver a lot of amplification, keeping unwanted noise out is the big problem.

You should look for microphone pre-amplifiers for good low noise designs.

You need a low noise op-amp, like the 5534/5532, and you need to be very careful with how you arrange the input cable, shielding of the circuit, etc..

For such weak signals you need to shield the signal source, the cable, the pre-amp, and the cable into the mic input. Coaxial cable and metal boxes for the circuits, battery driven or use the voltage output available in the mic jack. There are 3 contacts on a mic connection on a sound card, ground, input, and 5-9Volt output which is used to drive an electret mic.

That voltage output can you use to drive your pre-amp, if you do not need so much current. Otherwise use a battery, and shield the battery too.

If you want to use the sound card as an oscilloscope there are problems with using the mic input, by the way. It often has an automatic gain control and that will make the gain adjust automatically, which means you cannot rely on the values you get.

If you want to use the sound card as an oscilloscope you should use the line input instead, and then you really need a pre-amp for such weak signals.

Your best option to begin with is to use the mic input without any pre-amp, and see what results you get. Check the soundcard docs to see if you can disable the automatic gain control. If not you cannot trust the values you get on the strength of the signal, but maybe it is not so important.

If you need to measure the signal strength, and the automatic gain control cannot be disabled on the mic input you need to build a sensitive mic pre-amp and use the line input instead.

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Roger J.

The LM324 quad and the LM358 dual op amps are cheaper, more compact, just as crummy and work as single supply op amps. Using a 741 is pure lazyness.

The guys who design for the military had an excuse - it takes a long time to qualify an integrated circuit for high reliability military applications, and their choice of op amps used to be limited.

Fortune 500 equipment represents a different problem, not unlike high end audio, where the customer wants something idiosyncratic to indicate their unique capacity to throw money down the drain, and that market is too small to pay for good design - whence loads of 741's.

------ Bill Sloman, Nijmegen

" Hello, i need a easy to build signal amplifier (i'm a begginer), not a Complex one, my problem is that the input signal is below a millivolt and i need to raise it to at least a copple of volts (tenths of volts will be perfect) anyway the output signal is going to the mic jack of my soundcard (because i intend to use it like an osciloscope)."

To which Kevin McMurtrie indirectly replied"

" I'd avoid the 741 op-amp. It's an ancient op-amp with quirks that can be frustrating unless you know how its internals work. Radio Shack sells a dual JFET input op-amp that's good enough for most uses. Some fancy op-amps are specially made for audio use (low distortion open loop gain)."

"Op-amps are generic term for a generic device that has a differential input of nearly infinite gain. The circuits using them rarely depend on a specific type. Use any basic circuit and any good op-amp."

Kevin -

I'd be interested in what quirks in particular you experienced with the

741. While it's true that the 741 has been around for awhile (I think it was a follow up of the 709), it is still made because it is such a good, reliable general purpose device. While I wouldn't use it for precision instrumentation, it is admirably suited for the purpose intended in Chrisvasi's original post. I think you would be amazed at how much military and Fortune 500 equipment the 741 wound up being designed around. Would I use it today for non critical applications? You betcha!

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Bill -

Laziness is not exactly how I would characterize it. I think hermetic sealing and reliability may have just had a little something to do with it :-)

For military applications, and the occasional harsh industrial environment, but even there, using the 741 is generally a cop-out. These days there are always better amplifiers for any specific job.

------ Bill Sloman, Nijmegen

Well, the LM324 for example isn't.

Thomas

Opps. I was thinking of another op-amp. The 741 probably isn't so bad.