Oscilloscope reading amplifiers question.

Discussion in 'General Electronics Chat' started by Castelat, Aug 26, 2013.

  1. Castelat

    Thread Starter New Member

    Jul 21, 2013
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    Is it possible to read AC and DC on the collector of a common-emitter amplifier?, if it does... do I get an AC reading while I put my oscilloscope probes on the collector pushing the DC button of the oscilloscope?, can someone explain it to me briefly, I'm confused. I will appreciate it, thanks!.

    [​IMG]
     
  2. sirch2

    Well-Known Member

    Jan 21, 2013
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    AC on an oscilloscope will display the AC varying around zero. DC will show the same wave form but it will be off set by the DC voltage from zero.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,507
    2,367
    A ''scope normally has a AC or DC range, on AC the display removes the DC content and you just see the varying or pulsing signal, on DC you will see the varying signal on top of the DC level.
    Max.
     
  4. #12

    Expert

    Nov 30, 2010
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    That is, your horizontal trace position will rise up above the center line and show the AC shape higher on the screen. You often have to adjust the centering and voltage range to keep the display from running off the top of the screen when using the DC coupling mode.
     
  5. Castelat

    Thread Starter New Member

    Jul 21, 2013
    12
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    I know that, but an amplifier works with both AC and DC. Let me be more specific:

    [​IMG]

    Let's assume a sine wave (AC) is feeding the circuit through the coupling capacitor (C1) in series on the left side, between R1 and R2.

    Let's also assume DC is feeding the circuit through VCC (12DCv) from the top side of the circuit.

    - If I set the scope to DC having my oscilloscope probes one on the collector (Q1) and the other on ground of the above circuit, will I have zero reading, right? (because it obviously can only be measured in AC)

    And if I set the scope to AC having one of my probes on the emitter (Q1) and the other on ground of the above circuit, will I also have zero (because it obviously can only be measured in DC) is that correct?

    What I want to know is which signal can be measured at those points, I don't have an oscilloscope.
     
    Last edited: Aug 26, 2013
  6. #12

    Expert

    Nov 30, 2010
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    The first answer is that the scope will show about 140 mv p-p of 60 Hz AC with that line centered about 11 volts above your zero reference.

    In the second part, the scope will show about 1.4 VAC p-p centered around your zero reference. If you switched the scope to DC, the line would jump to about 10 volts above the zero reference and still have the AC shape.
     
  7. Castelat

    Thread Starter New Member

    Jul 21, 2013
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    0
    Thanks #12, so I can set my scope to both AC and DC and I will still get a sine wave on the screen regardless of its reference.
     
  8. #12

    Expert

    Nov 30, 2010
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    Yes, unless you have the voltage range set too low, then the trace will jump off the top of the screen. The first example shows that setting the scope to AC will allow you to magnify the ac shape for better examination because you won't have to use 2 volts per division or 5 volts per division. You can adjust down to 50 millivolts per division and expand the AC wave without the DC component driving the trace off the screen.
     
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