more n00b questions

Discussion in 'General Electronics Chat' started by disantlor, Jul 9, 2006.

  1. disantlor

    Thread Starter Member

    Jun 21, 2006
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    0
    http://www.passdiy.com/pdf/zenamp.pdf

    I'm looking at this circuit (on page 3) trying my hardest to figure out what is going on with the constant current source in the power supply. Specifically in the explanation it says that Q3 is being biased through the capacitor and 2 resistors. It says the capacitor is used to reduce ripple but it would appear to me that because it's in series that it would block the DC bias.

    Additionally, it says R3 and R4 are used for stability and to prevent oscillations, but how does it accomplish that.

    It's very frustraiting looking at these circuits and not being able to visualize how the current is flowing to the different branches. I have a pretty good understanding of how to do all the math involved with this stuff, but I can't just look a circuit like this one and picture whats happening on an intuitive level. It feels like I'm missing like one piece of information and once I figure out what that is, the lightbulb is going to go off!

    Anyway, enough rambling, thanks for any insight you can offer. It is MUCH appreciated. Hopefully once I get a handle on some of this stuff I will be able to better contribute to the community.
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Hi disantlor.

    I made several attempts to exercise the link you included in your post. I was unable to get it to work.

    Would you mind checking it to see if it works for you?

    NEVER MIND. I GOT IT TO WORK ON A SUBSEQUENT TRY.

    hgmjr
     
  3. disantlor

    Thread Starter Member

    Jun 21, 2006
    20
    0
    well while I'm at it, I may as well ask this one.

    On page 2, Figure 1, it shows a very simplified version of circuit, and I can understand all of that well enough except that I don't see why the other end of the speaker is returned back to the drain of the transistor. Is it some sort of feedback loop? Could they not just return it to ground and avoid using the blocking capacitor all together?
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Hi disantlor,

    These questions are fairly easy to answer so I'll go ahead and tackle them first.

    The speaker is connected to ground because this design is a single-ended amplifier. That means that the input signal and the output signal share the same reference point which is ground.

    As for your question about why the blocking capacitors are being used rather than connecting the speaker directly between ground and the drain of mosfet Q1, the answer lies in the fact that the quiescent DC present at the drain of Q1 is probably on the order of +15 volts when there is no signal applied to the amplifiers input. You can see that without the DC blocking capacitors the speaker would quickly burn out.

    One way to analyze the amplifer is to consider what happens immediately after you turn on the power switch.

    The +34VDC power supply buss begins to rise from 0V toward its final value of +34V. Initially Q2 and Q3 are both in their off state. As the power supply continues to rise in voltage the mosfet Q2 begins to turn on and as it does the current in R1 increases. Q3 stays in the off state until the current in R1 increases to around 2 amps. With 2 amps flowing in R1 the voltage drop across R1 reaches a point that it begins to turn BJT PNP Q3 on (around 0.66V). As Q3 turns on the voltage at its collector begins to rise. As the collector becomes more positive it begins to turn off Q2. As Q2 turns off an equilibrium condition develops. This equilibrium condition involves a balancing act between Q2 and Q3. As you can see, Q2 has to conduct at least 2 amps to maintain the 0.66V drop across R1. If Q2 conducts more than 2 amps, Q3 turns on harder which drives the gate of Q2 in a direction that adjusts Q2 so that the current through R1 is decreased. The opposite action occurs when the current flowing in R1 drops slightly below 2 amps. In this instance, Q3 begins to turn off which causes the voltage at its collector to drop. As the collector voltage drops, Q2 begins to turn on which brings the current back up to the target 2 amps.

    Thus a delicate balance is maintained between Q2 and Q3 resulting in the 2 amp constant current source.

    hgmjr
     
  5. disantlor

    Thread Starter Member

    Jun 21, 2006
    20
    0
    sweet thanks man! I wasn't thinking about it as it was powering on, I began thinking with the bus at 34V, so it makes plenty of sense now.

    one thing is still bothering me but I think I just need to go back and make sure I understand FETs. My mind wants to see the current source connected to the source of the FET and the speaker connected to the drain. I picture a larger current sitting at the source waiting to be controlled by the gate and passed through the drain to the speaker. However in Fig. 1 the current source is going into the drain and the output is also coming out of the drain. Anyway just something I need to mull over a little bit.

    Thanks so much for taking the time to help me out!
     
  6. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Hello disantlor,

    Sometimes all it takes is to open the door to understanding is to have someone decribe how they would tackle the problem. I am glad that I succeeded in making the current source portion of the circuit a little less mysterious.

    You may have access to material of your own that can help you clarify the operation of a mosfet but just in case you don't, the link below takes you to just one document I snagged from the Internet that presents mosfet theory of operation. It looks like it is aimed at the intermediate level of understanding.

    http://www.techlearner.com/Apps/MOSFETBasics.pdf

    There are many more sites just a google search away that present mosfet theory on a variety of different levels.

    hgmjr
     
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