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Connecting a signal generator to an amplifier

Discussion in 'General Electronics Discussion' started by Bluepoint SEO, Jul 28, 2016.

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  1. Bluepoint SEO

    Bluepoint SEO

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    Jul 28, 2016
    Hi all, I'm new here but I have this trouble when I'm building circuits. The problem is and please don't laugh :) say I have a signal generator - like from a circuit I made out of one of the 2n2222 transistors and a capacitor - a relaxation oscillator I believe it's called. Now I have two wires one positive and one negative. If I connect them to a speaker I can hear the frequency and make the frequency higher or lower using a variable resistor. But what I really want to do is connect it to a basic single transistor amplifier and hear it through my head phones.

    What I don't know what to do is how to connect the outputs (negative and positive) to the single transistor amplifier. I know or I think, the positive goes to the base of the transistor (BS478) but where does the negative wire go? To which Earth? I use two separate breadboards you see. Or do I have to connect both grounds to each other on the breadboards?

    Does that make sense sorry?
     
  2. Gryd3

    Gryd3

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    875
    Jun 25, 2014
    That makes sense, but I have to ask you about your signal generator.
    Do you have a schematic to share?

    In theory, you can mix and match the positive and negative wires from an AC signal like audio.
    In practice, one of the two wires is usually 'common' (ie, tied to ground, or the negative rail of the power supply... sometimes even the positive rail!)
    So, I would like to see what you have built to determine which 'commons' are in use for both circuits.

    I would suggest you connect the negative signal generator output to the negative power supply rail on the amplifier, and connect the positive to the base (Gate) of the BS478.


    I'm operating on an assumption here... That the signal generator 'negative' terminal is also connected to the rest of the circuit as a 'virtual ground', or to the battery - terminal.
    If connected to the battery - terminal, I would hope a decoupling capacitor is in-place to remove any potential DC offset from the signal generator output.

    I would very much like to see the circuits you have built though.
     
  3. Bluepoint SEO

    Bluepoint SEO

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    Jul 28, 2016
    Thank you for your help Gryd3, I think I will find some online software to show you the schematics. You see, now this is the forum for me :) I'm already learning and now I need to draw the schematic! Good stuff.
     
    Arouse1973 likes this.
  4. Bluepoint SEO

    Bluepoint SEO

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    Jul 28, 2016
    On a side not I did the math to find the gain and it was 170 - I pretty much designed the amplifier circuit with what little knowledge I have from tutorials online, a book and of course YouTube vids.
     
    Gryd3 likes this.
  5. Bluepoint SEO

    Bluepoint SEO

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    Jul 28, 2016
    Hi there I sketched these of the circuits I noticed I chose the wrong value for the resitor at 47K!!! Should have been closer to 500ohm. Please see the attached.
     

    Attached Files:

  6. AnalogKid

    AnalogKid

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    Jun 10, 2015
    My guess is that the amp and oscillator are powered by two separate batteries. In this case. there are several connection configurations that will work. but before that, there is an error in the amp schematic. You have the headphones connected directly across the battery, not to the output of the transistor circuit. One side of the phones to GND, one side to the collector. The resistor pulls the output up, and the transistor pulls it down. Note that this is a very basic, and very inefficient amp circuit, and the transistor will get hot.

    ak
     
    Gryd3 likes this.
  7. Bluepoint SEO

    Bluepoint SEO

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    Jul 28, 2016
    Hi AnalogKid thanks for the tips I'm impressed. They are powered separately; the relaxation oscillator runs from my bench supply which is 14v. The amp (or attempt at an amp) is powered by 3 x 1.5v batteries and yes the transistor did get very hot.

    I think I can see what you're saying about it being connected directly across the battery. So I should put the resistor on the other side of the headphones? Can you point me in the direction of some good reading as I don't understand about it pulling up and pulling down.

    Are the phones not on the collector side? I thought that was the collector and the emitter is the one that goes straight to ground? I'll check up now.

    I know it's basic however I just want to learn everything from the ground up (ha get it?).

    Anyway thank you for your help and if you have any more to say please do!
     
    Gryd3 likes this.
  8. davenn

    davenn Moderator

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    Sep 5, 2009
    here's a couple of amp ideas I found ... worth experimenting with .....

    [​IMG]



    [​IMG]



    Dave
     
  9. Bluepoint SEO

    Bluepoint SEO

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    Jul 28, 2016
    Hi there Davenn thanks very much. Still though if I made these where would the negative lead go. I've two bread boards one with the noise maker (A) and the other with an amp (B). Where would I put the negative cable of (A) on the output? Cheers haha you're all great so far in this forum I like it.
     
  10. davenn

    davenn Moderator

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    1,882
    Sep 5, 2009
    for your oscillator cct, the output should be across the collector and emitter, not in the collector cct as you have shown

    1k to 1.5 k for the resistor would be good
     
  11. Bluepoint SEO

    Bluepoint SEO

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    Jul 28, 2016
    Hi Davenn I just tried to do that - out puts across the emitter and the collector and it wouldn't work. Any chance of a schematic please? I'm learning so much already!
     
  12. Bluepoint SEO

    Bluepoint SEO

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    Jul 28, 2016
    My set up.
     

    Attached Files:

  13. Gryd3

    Gryd3

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    Jun 25, 2014
    Consider this:

    The relaxation circuit you have built requires a 'load' to be present across the 'output'.
    What happens is that the headphones you use are a low enough resistance to allow current flow.
    When you connect the 'positive' and 'negative' wires from your oscillator to your amplifier, you no longer have that low resistance. You now have a much higher resistance path for current to flow, or in the case where a decoupling capacitor is used on the input of the amplifier... no DC current path exists.
    This is a requirement for the oscillator to work... when the capacitor charges to a sufficient level, the voltage causes the transistor to break-down and begin to conduct which then drains the capacitor allowing the transistor to turn off again. Once off, the capacitor begins to charge and the cycle repeats itself...
    Well... if the capacitor can no longer discharge 'through' something (at least at a reasonable rate) then your oscillator will basically just charge the capacitor and stay *stuck* in that state.

    You can confirm if you have a multi-meter. Measure the voltage across the capacitor and you will most likely find the 14V-DC or close to. When operating properly, your meter may change values or rest at a lower voltage when measuring DC.

    I would suggest you make the following change:
    - Swap the location of your 'output' with the location of the 2N2222 transistor.

    This will provide you with a better 'common' for the output of your oscillator. The negative terminal on your power supply will now be 'common', and anything on your circuit directly connected will be as well.
    The Collector of the transistor will now be the 'positive' output.

    Now... to fix this 'current path' problem stopping your oscillator from functioning...
    Connect a resistor across the output of your adjusted oscillator circuit.
    I would try 100Ω, but may move to something smaller. The Collector of the transistor is still going to be the 'positive' output of your oscillator, but it will also be connected to ground through a resistor. This way, you can ensure that there is a discharge path for the capacitor.

    Now... The 'common' of your oscillator can be connected to ground of the amplifier, and the positive output (which is connected to the resistor and Collector of the 2N2222) can be connected to the input of the amplifier.

    You need a little work on the amplifier. Try the ones Davenn had posted, or try to correct yours.
    It would be a very good idea to use a capacitor on the amplifier input like the example amplifiers. You can also leave your headphones connected to the output of the oscillator before and after you connect it to the amplifier. This should give you an audible indication of which of the two circuits may not be operating as expected.
     
  14. Gryd3

    Gryd3

    4,098
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    Jun 25, 2014
    One of the reasons I had suggested correcting the 'common' of your oscillator is mainly for the fact that if you ever decide to run both circuits from the same power supply.
    If left as-is... connecting the same power supply to both the oscillator and amplifier would accidentally result in you bypassing the 2N2222 transistor. Luckily, this should not cause any damage, but doing so would cause at most, a constant 14mA draw through the 1kΩ on the oscillator circuit.
    I've recently damaged a bluetooth stereo headset... I had the wise idea of modifying it with a 3.5mm jack for a DIY bluetooth receiver in my car. Needless to say, I did not do my due diligence and during the testing phase plugged it into my computer speakers. There was a current path I didn't account for and smoke began to raise from my headset. The project ended, I need another donor...
     
  15. Bluepoint SEO

    Bluepoint SEO

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    Jul 28, 2016
    Wow thank you for that it's a lot to take in but I'll absorb it. At the moment I've done an all nighter and have been awake 24 hours+ so I'm very tired. But thank you for that massive contribution to my knowledge.
     
  16. Bluepoint SEO

    Bluepoint SEO

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    Jul 28, 2016
    Oh no I've burnt so many things out experimenting. Tell me this can my 30 vault bench machine kill me?
     
  17. Bluepoint SEO

    Bluepoint SEO

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    Jul 28, 2016
    Okay I can't get the amp in the first image to work... nightmare guys. Any tips on debugging it? Thanks
     
  18. Bluepoint SEO

    Bluepoint SEO

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    Jul 28, 2016
    I tell lies! I just litterally swapped the positive and negative around from the signal generator to the amp and it worked hooray!!! Thank you so much my very first Amplifier :) progress
     
    Arouse1973 likes this.
  19. Gryd3

    Gryd3

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    Jun 25, 2014
    Good to hear!
    Do you have a capacitor between the signal generator and the transistor of the amplifier?

    Yes and No...
    Not normally directly, but indirectly yes.
    Your body and skin has electrical properties just like any component you put on the board. Human skin varies in resistance but when wet or damaged the resistance drops quite a bit! The resistance of your body and the voltage it is exposed to directly relate to how much current is going to pass through your body. This is the dangerous part. Current is what can cause your heart to stop pumping.
    Take a read at this wikipedia article as you continue to read, or after you have finished reading this post.
    https://en.wikipedia.org/wiki/Electric_shock#Body_resistance

    5% of the human population has a resistance from hand-to-hand of 1,750Ω @ 25V. This would allow 14mA to travel through the body. According to the graph on the same article, this could cause muscle contractions in the body. This would not be pleasant. If your hands happen to be wet or sweaty, that resistance will go down, and according to the same graph, allowing 30mA from hand-to-hand will present a chance of fibrillation which may lead to death if not immediately or very quickly attended to.
    So... your 30V power supply, when cranked up to it's maximum voltage has a chance of causing bodily harm. The chance is very low, and relies on being applied to wet or broken skin, but the chance is there. The information I provided from the article is based on hand-to-hand contact which will travel across your chest. Please note! The heart can be kicked out of it's natural rhythm by *much* less current if directly applied. Additionally, the closer the wires are together on your body, the lower the resistance will be! It's possible to cause burn marks or damage to the body with your power supply if you put the the positive and negative leads close enough to each other on your body.
    If someone told you to touch both ends of a pair of jumper cables to your body, I'm sure you wouldn't do it. Your power supply will provide almost 3 times the voltage of a car battery. Treat it with respect.

    Now, I don't mean to be a fear monger. I'm telling you what is 'possible', but the probability of you hurting yourself with is is pretty low unless you are incredibly unlucky, and ignorant of safety precautions. I quoted worst case values.

    It's *always* good practice to work on your circuit with power turned off when possible. If you are working on a live circuit, avoid touching any conductors with your live skin. Try even harder to avoid touching two conductors at a time, and try your hardest to never touch a conductor with each hand.

    Use your best judgement when dealing with electronics and you'll be fine. Keep your eyes out for capacitors, because they are *very* high energy storage tanks that can unload in a fraction of a second... Make note of the voltage they are rated for. I bring these up because cameras often operate on between 1.5V to 6V, but often contain over 300V inside! Just because a circuit uses a low voltage for a source, does not mean it's voltages inside are also low.

    *Now that I've said all that... if your power supply is set to a lower voltage many of these concerns would be addressed right then and there... additionally, if the power supply has a 'current set' feature, you could limit the current output as well which would help. I would not let my son touch 30V, but I would certainly let him play with 12V and below. If he was safe about it, I'd let him work on higher.

    Of course, if you have any specific questions. Please don't hesitate to let us know.
    Many of us have had our fair share with electricity and have most likely touched mains voltage or higher.
    It's not pleasant... but electricity will flow from point A to point B. The golden rule is to *never* place your heart between those points.

    Many others can add to this as well.
     
  20. Bluepoint SEO

    Bluepoint SEO

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    Jul 28, 2016
    Hi Gryd3, yes the first thing the signal encounters is a 10μF capacitor. I used the first schematic provided by Davenn. Thanks Davenn. As for your post about electrocution - thank you very much for the detailed information. I will play safe!

    About your other post making recommendations for improvement of the relaxation oscillation circuit; I can't visualise what you mean I'm afraid.

    I really need to learn more! I know capacitors allow AC and block DC, and inductors block something and allow something lol. High Pass Filter: cap and resistor combo? Can't remember need to go through my book again.

    My friend give me Crocodile Clips today so I'm learning to use that.
    I'm going to start a new thread as I have a question about transformers. Thanks everyone for all your help :)
     
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