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Does C3 have to be a electrolytic cap?

  1. May 30, 2016 #1
    4-transistor-class-ab-amplifier.png
     
  2. jcsd
  3. May 30, 2016 #2

    berkeman

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    Not that I can see. Sometimes folks use that same capacitor symbol for unpolarized caps, which I dislike. You should be able to use a ceramic cap there instead.
     
  4. May 30, 2016 #3
    Thanks, I sort of assumed so...but better to ask...lol
    I don't know much about transistor amp circuits so this is just an experiment.

    Billy
     
  5. May 30, 2016 #4

    berkeman

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    Do you understand why the polarities of the electrolytic caps in this circuit are the way they are? How would you determine what the polarity of the voltage is across those component locations where you use an electrolytic cap? The power supply cap is obvious, but what about the ones in series with the AC signal? :smile:
     
  6. May 30, 2016 #5
    Give me a min. to look

    The curved line on the cap symbol is positive.
    All of the caps, C1 through C5 connect their positive side to the +5 Volt rail.
    Also C1 is connected to the base of the 2222 which is positive and so is the collector.
    C1 is preventing DC from going back to the input I assume.

    Am I missing anything??

    I would measure with a DMM or a scope to ground...I assume that is what you are asking.
     
    Last edited: May 30, 2016
  7. May 30, 2016 #6
    Perhaps if you are someone has the time we could go through each component to determine just what it does and why it is there. That would be a big help for me.

    Thanks,

    Billy
     
  8. May 30, 2016 #7

    davenn

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    no, the curved side is the negative side
    Note the polarised caps ( electrolytics) are marked with a +
    the ones that are not marked with a + ARE NOT polarised :smile:

    I also echo Berkeman's comments ... that old style cap marking is a pain and a trap for young players

    yes, but can you see what is making the base side of the transistor and C1 more positive than the other (left ) side of C1 ?

    yes ... primary purposes are interstage AC (signal) coupling and DC blocking, as is C2. C4 is DC blocking as we don't want a DC offset voltage appearing across a speaker

    you could, but a bit of basic circuit analysis will tell you what points/nodes are more positive than others.
    the DMM or scope will give you the actual amount

    Dave
     
    Last edited: May 30, 2016
  9. May 30, 2016 #8
    Hi Dave...sorry..yes of course I had it backwards on the caps as is easy to see on the schematic.
    C1 is more positive because of R2...I think
     
  10. May 30, 2016 #9

    davenn

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    partly, as R2 does lift that node above the 0V rail, but that isn't the main reason, look higher up
     
  11. May 30, 2016 #10
    There is a 1K resistor between the rail and the collector and a 150K plus the 1K between the rail and the cap.
     
  12. May 30, 2016 #11
    Are the two diodes used to prevent damage from hooking up the power backwards? They also produce a voltage drop of around 1.4 v at the collector of Q4 I assume.
     
  13. May 30, 2016 #12

    davenn

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    Yes .... so overall, what process are the resistor pair R2 and R3 and the single resistor R4 doing ?
     
  14. May 30, 2016 #13
    Controlling current through Q1.
     
  15. May 30, 2016 #14

    davenn

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    no ... but we will get to that part
     
  16. May 30, 2016 #15

    davenn

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    the single resistor R4 is doing that as well as a voltage dropper

    but look at the configuration of R2 and R3 ... do you recognise that config ?
     
  17. May 30, 2016 #16
    No, I don't recognise the config but I assume the following.
    We need to get AC to the base of Q1 and R2 and R3 makes it go there.
    We need DC to get to the base of Q1 and C1 ,R2, and R3 makes it go there.

    %2F%2Fwww.circuitstoday.com%2Fwp-content%2Fuploads%2F2011%2F03%2F4-transistor-class-ab-amplifier.png
     
  18. May 30, 2016 #17

    davenn

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    OK, no prob ... .it's called a voltage divider ... sound familiar ?

    put 2 resistors between + and minus of a supply source and the voltage at the centre node is proportional to the value of the 2 resistors

    no, they don't help achieve that, only C1 passes the incoming AC signal to Q1

    Yes, but for a very specific purpose
    As said R2 and R3 create a voltage divider with the base of Q1 being at the centre node
    This biases the transistor on and keeps it within its linear range to help avoid distortion of the signal
    You also need to consider that the AC voltage at the node of C1 and Q1 base may not be high enough at
    times in its cycle to allow Q1's base to emitter path to go into conduction ... providing that bias voltage overcomes that problem

    D
     
  19. May 30, 2016 #18
    Ok...so a NPN transistors need a certain positive voltage at it's base to conduct. Correct? What would be a nominal voltage?

    As I have been only working with tubes I need to get a picture in my head of how transistors are different. The base on a transistor is like the grid on a tube and could be biased positive or negative depending on the type of transistor????

    Be back in two min.

    Back now
     
  20. May 30, 2016 #19

    davenn

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    for a standard silicon diode or transistor it is approx. 0.7 V ( + - 0.05V )

    the principle is similar ... tubes are basically voltage operating devices where transistors are current driven devices

    That sounds a contradiction in the face of my explanation of the 0.7V etc ... sometimes it's still easier to look at it that way

    for a tube, varying the voltage on the grid increases or decreases the flow of current between the cathode and the plate

    in a transistor varying the current level between the base and the emitter ( NPN transistor) and once the base-emitter path reaches that 0.7V conduction point
    that small varying current between B and E controls the larger current in the path between Collector and Emitter



    Dave
     
  21. May 30, 2016 #20
    Just to be clear, is it the voltage or the current that keeps the base to collector open?
     
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