The opamp is not properly biased. The inverting input is returned to ground, while the non-inverting input is sitting at V+ (because the reverse leakage through the transistor is tiny. This will cause the opamp output to rail positive.
What is the supply voltage?
Even if it worked, the gain of the opamp is only 11, which is no where near high enough. I'm guessing you need a gain of >1000.
The LM324 is not a low-noise opamp. Question is, will you be hearing the opamp noise or the transistor noise?
f(c) = 1 / 2ΠRC√2
I'm an experienced electronics tinkerer with many gaps in my knowledge of the subject, and am currently trying to educate myself properly. Part of this involves experimenting with actual circuits. I'm hoping to get some help here from folks more knowledgeable than I.
Current project involves fooling around with a simple noise generator. I'm using a reverse-biased NPN transistor, as shown in lots of circuits I've seen. I'm trying to use an op amp to amplify the output. At this point, I'm just trying to get the damn thing to work at all; I've constructed the circuit shown below, with the output connected to a small audio amplifier, in order to hear if it works.
**broken link removed**
Well, it ain't working. The noise generator seems to generate nothing. When I remove the wire connecting it to the op amp noninverting input, I hear no difference in the sound. The op amp definitely works; if I touch the input lead, I get very loud hum. Very frustrating; I've tried a bunch of stuff to no avail. Tried a bunch of different transistors, all tested good. Put in a coupling capacitor between the noise generator and the op amp input. Removed the feedback resistors from the op amp. Put in a load resistor across the op amp input. The only noise I get out of this is the noise of the op amp itself.
So apart from the actual application here (I have some fun things involving random noise generation in mind here), I'm trying to teach myself how to use op amps, and also just basic circuit design principles: computing op amp gain, different op amp configurations and feedback, impedance matching, etc.
My assumption (which could be wrong, of course!) is that since the noise generator produces such a small signal (in the millivolt range), I need a fairly high-impedance input so as not to overload it. But now I'm wondering if there's not enough of a load; should I use a load resistor? Part of my problem, it seems, is an impedance mismatch somewhere.
Also, it seems that the circuit here should give me plenty enough gain to at least hear something through a regular line-level audio input.
Any help will be very much appreciated! Ask me about anything I may have neglected to mention that may be relevant.
But, as Mike pointed out, the op amp will saturate at the positive rail unless a cap is added in series with the 10k resistor.Hello there,
You didnt mention how high your V+ supply was. You may have to go as high as +20 volts to get it going.
You can also try heating the transistor a little to see if it will ever work at all.
Zener noise sources may be a little better. If you use zeners of say 12v you should get a nice flat band of white noise (lower voltage zeners tend more toward tunneling rather than avalanche) as long as the op amps dont filter the noise and create their own. I dont know how good your noise has to be (pure white or anything goes) as you can also use logic gates biased into linear operation (lots of gain) if you are not too picky about the generated spectrum.
The noise from both those sources is likely fairly white with a similar spectrum since they are both generated by a similar breakdown process.Well, since you mentioned zeners, can you tell me what the difference is between zener noise and reverse-biased transistor (E-B) noise, in terms of the noise spectrum? I'm guessing if there is any difference that it's pretty subtle.
The noise from both those sources is likely fairly white with a similar spectrum since they are both generated by a similar breakdown process.
I would think the noise likely continues to a very low frequency. Make sure the output capacitor resistive load does not roll-off the frequencies you want. (For example a 10kΩ load will give a low frequency roll-off of anything below 0.6Hz with the 25µF output cap.)OK, thanks for that helpful reply. Now, can you tell me if it's possible for me to extract any usable amount of subsonic (say, < 10 Hz) components from such a signal, using one or more low-pass filters? Or am I just going on a fool's errand here?
Your cutoff frequency is about 3.2Hz. You probably slipped a decimal point in your calculations.
The rolloff is not 40dB/octave. It's 40dB/decade. I'm guessing that was a typo.
Are you currently getting noise below 3.2Hz?Not a typo, a brain fart. Thanks. Yes, it's 40 dB/decade.
So I'm wondering if adding another low-pass filter stage like this one (probably after a gain stage) would make any difference. Or do I need a higher-order (that is, steeper-rolloff) filter? (Warning to self: getting into deep waters here, considering my lack of formal training in such matters.)
Are you currently getting noise below 3.2Hz?
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