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Transistor gain and voltage supply question

Discussion in 'Electronic Repair Modifications DIY' started by Guitarfreak, Jul 27, 2010.

  1. Guitarfreak

    Guitarfreak Well-Known Member

    If you have a 9v voltage supply and three transistor gain stages, does each stage see only 3v on the collector?
     
  2. MrEase

    MrEase Active Member

    I don't even know why you would think this??? Each transistor will be biased from the supply to suit its own needs. There is no magic formula that will tell you what voltages to expect on each transistor - that will depend on the biasing resistors in each stage. The only thing that you would expect to be consistent is around 0.6 - 0.7 Volts between each base and emitter but then they could be almost anywhere with respect to the supply.

    What are you trying to do? Are you fault finding - if so it seems like you are flying blind without understanding how these circuits work. I'm not trying to be rude but just responding to the question. It seems from the question you would be better off getting someone to look at this circuit for you.
     
  3. Guitarfreak

    Guitarfreak Well-Known Member

    Nope, just wondering. I mean it makes sense though. This is within the context of a simple stompbox circuit. All of the voltage comes from the same place, the source, and follows the PCB which branches off at each transistor. If the supply is putting out 9v and the first transistor pulls 9v then how could any voltage even continue down the line to the next gain stage? Everything that the supply is producing is going through the first transistor straight back to the supply again completing the circuit. The only way that I see it possible to draw 9v over three parallel gain stages is at an increase in current. So do you use more supply current from having more gain stages? That makes sense to me.
     
  4. Boswell

    Boswell Moderator Distinguished Member

    Can you post the circuit or a link to where it is?
     
  5. MrEase

    MrEase Active Member

    The full 9V is available for each stage - you could view it as them being connected in parallel on the supply. Thus, as you surmised, three identical stages would indeed draw three times the current of a single stage. In a three stage amp. though you would be very unlikely to find each stage identical and the total current would depend on the design of each stage. Note also that three transistors do not necessarily imply three gain stages. I have to say that this is a very simplistic summary though. As Boswell said a schematic would help any explanation.

    Is this something to do with the DIY stompboxes you bought recently?
     
  6. Guitarfreak

    Guitarfreak Well-Known Member

    Well, it's not for a specific project. The projects that I am building aren't complex enough to have three gain stages yet, I am just hypothesizing. So since the current draw increases threefold and the circuit resistance stays the same, according to Ohm's law the voltage draw from the power supply would increase threefold as well. Is this also true?
     
  7. Link555

    Link555 Well-Known Member

    Ohms law is V = I*R

    Three resistors in parallel will all see the same voltage across them, but will have different currents through them (unless they are exactly the same-in which case each resistor will see 1/3 the total current) .

    Each transistor in the circuit I think your talking about would be able to have 9 volts on the collector.
     
  8. MrEase

    MrEase Active Member

    Guitarfreak,

    You seem to be repeatedly making some rather wild assumptions with your theories. I have highlighted the glaring one in the above quote. If you had three identical stages and disconnected one of them, then the resulting current draw would be reduced by 1/3. How on earth you can imply the circuit resistance stays the same I do not know.... BTW you do not "draw" voltage from a supply, you draw current.
     
  9. MrEase

    MrEase Active Member

    Just another quick thought. As you are aware of Ohm's Law, I would also suggest you google Kirchoff's Law which might illuminate all this a bit more for you.
     
  10. Guitarfreak

    Guitarfreak Well-Known Member

    Will research Kirchoff's Laws, thank you.

    I built my circuit today and unfortunately only got about 30 seconds before work to plug it in and play with it. I think there may be a problem because when the circuit is active it makes my gain channel sound like I am playing on the neck pickup. I thought at first that this was because the level was overloading the input of the amp but when i turned the trimpot down it just got quieter. My assumption as of now is that the problem is the output buffer. the volume trimmer is only 15k turned up full and I think I may raise it to 100k. Any other ideas?
     
  11. RemyRAD

    RemyRAD Guest

    And what are you going to do about the difference in voltage drop between germanium and silicon transistors? Which one do you think would work best for your application? Little metal cans or black epoxy? Do you know that some transistors can become micro-phonic? Maybe you actually want to create a guitar amplifier using all thermistors? That way you can tell how hot you are playing. All you have to do is then design a speaker which can also provide a visual display of your playing temperature.

    I prefer toasted bagels with blue cheese
    Mx. Remy Ann David
     
  12. MrEase

    MrEase Active Member

    Well I think I'll ignore Remy's totally spurious response and go on to suggest that questioning a value for a trimpot without posting any form of schematic is completely unanswerable! It occurs to me that you do not have the knowledge to actually design a gain stage so I can only assume you are using a third party schematic. Does copyright prevent you from posting it?

    To be quite honest the sort of questions you are asking give so little information and display a distinct lack of knowledge of the operation of what seem to be quite simple circuits. While I am trying to help, the lack of information you are giving is beginning to test my patience. I appreciate you are trying to learn and that is the only reason I am here but there comes a stage where you really have to help yourself and stop yourself stumbling around quite so blindly.
     
  13. Guitarfreak

    Guitarfreak Well-Known Member

    I am talking about something else now. I got tired of you grilling me about not knowing about gain stages so I moved on to talking about my mail order project but didn't feel like making a new thread. The project is a BYOC confidence boost and the PDF is available for free download there with the schematic. Now go and educate yourself and return with a renewed attitude, or stop posting so I can get information and advice without any unnecessary attitude.
     
  14. MrEase

    MrEase Active Member

    If you read my posts again you should realise that there is no "attitude", only what I intended to be helpful pointers in response to the questions you asked. It is neither my fault nor my attitude that each of your questions clearly showed some fundamental lack of understanding.

    Would you be better advised if this was not pointed out to you? Probably not if I did not bother trying to point you in the right direction as well.

    Again I will say asking for suggestions regarding your circuit is hopeless without even telling us what the circuit is. What do expect me to say? Suggesting you adjust R3 from 10k to 22k is hardly possible is it? If you really want some useful help then you will need to help us to understand what it is you are trying to achieve.

    Sorry if you think this post represents continued "attitude" but please try to see what I have actually said and that I have tried more than anyone on this thread to help you.
     
  15. Guitarfreak

    Guitarfreak Well-Known Member

    R3 in that schematic is 1M, I am talking about the volume trimpot which came with it and is rated 15k. After getting a little more time to screw with it today I think that there is a problem with the circuit or how I built it. It is very gainy, and sounds almost 'fuzz' like. Under the tone there is crackling and intermittent volume swells. I checked my soldering today and it seems to be inconclusive. All joints seem solid and nothing seems to be touching or grounding out. Here are some pictures I just took.

    IMG_5045.jpg
    IMG_5046.jpg
     
  16. Boswell

    Boswell Moderator Distinguished Member

    Is this the booklet of the confidence boost unit?
    Secret

    It seems to be one of those designs that's unduly sensitive to battery supply voltage, but it's worth a go.
     
  17. MrEase

    MrEase Active Member

    Guitarfreak,

    You really don't make it easy! :<) I did ask specifically about the stompbox and you said no, then in your next post you started talking about a 15k trimpot on your circuit. How was I to know this was with the confidence booster?

    I agree with Boswell that this design is certainly not going to yield consistent results and if just for the op-amp, you will need to make sure the battery is always healthy. As to the buffer stage, bearing in mind that it seems they supply almost any transistor for this and also with transistor gain spreads, the biasing is going to be very variable. If this "confidence booster" is intended to boost your confidence in building something that works (I can see little other purpose to the circuit) then I don't think it's a great design.

    The first thing I suggest checking is the voltage at the transistor collector. This really should be sitting around 5 V or so (it's probably nearer 3V) but this is likely to vary significantly. Also, the transistor output is surely going to be loaded by the 15k trimpot, so it could be a good idea to increase this to the 100k you said. That is assuming you are taking the output to a high impedance guitar amp. Many mixing desks have relatively low input impedance so this would tend to load the output anyway.

    (Sidenote: Technically the op-amp is an input buffer here and the transistor is the gain stage.)

    The biggest downside I can see with the hugely variable collector current of the buffer is that the high frequency roll off could become significant which is maybe why it sounds as if you're on the neck pickup (less harmonics).
     
  18. MrEase

    MrEase Active Member

    BTW if the sound is fuzzy, you may well find that the collector voltage is either very low or very high. The collector needs to be a little above half the battery voltage to get the maximum ac voltage swing without clipping.
     
  19. Guitarfreak

    Guitarfreak Well-Known Member

    I did mention that the project was a BYOC confidence boost when I said the PDF is for download, but that's beside the point. Lets move on and push forward.

    Your analysis makes sense because I looked up a few op-amp configurations and the schematic matched none of them. I did think that the first resistor which goes parallel downward (the 2M2) was the buffer. The one that points upward I am not sure about, I've never seen a double resistor configuration like that. At least I think not.

    I still plan to build a pedal out of this and mod it using extention PCB's to make it better. I bought it to experiment so it allows me to do that.

    New news. Now the circuit does nothing. It gives very low very distorted tone only, with some intermittent very loud banging/popping noises. I don't even want it plugged into my rig while it's doing that or that $*^t could blow my speakers. Any idea what that sounds like?

    Maybe I'll look into a different clean boost circuit to work on?
     
  20. Guitarfreak

    Guitarfreak Well-Known Member

    Can you use stranded wire in a PCB project like this? Or should you use only solid core wire?
     

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