basis of transistor amplifier

Discussion in 'Homework Help' started by swty_todd, Aug 3, 2008.

  1. swty_todd

    Thread Starter Active Member

    Aug 3, 2008
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    The transistor amplifier is biased so as to make the currents flow in particular directions during the amplification process.So when an alternating signal is applied the currents will flow in the same directions although their magnitude will be less or more…..thus the output will be D.C only …not A.C………………...pls help
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Yes you are absolutely correct in what you say - except for the last part about AC.

    What is AC except varying DC?

    If you like the output of a transistor amplifier is a mixture of DC and AC so that the base or mid level for the AC is shifted by the DC component.

    Both the input and output DC levels are not at ground potential. That is why we employ a blocking capacitor at both input and output, so only the AC can then get through.

    The input AC signal thus varies about the DC bias of the base emitter (approx .6 volts) and the output AC varies about the mean DC voltage of the collector.

    Both the input AC and the output AC are therefore limited in maximum amplitude.
    The input cannot go below ground.
    The output cannot go below ground or above the Collector supply rail Vce.
     
    Last edited: Aug 4, 2008
  3. scubasteve_911

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    Dec 27, 2007
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    You made two statements, then asked for help. I'm confused!
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    Think of a DC level that varies between 6V and 2V. There is a 4VP-P AC signal on the DC signal, and the average DC is 4V.

    AC and DC can be separated with capacitors easily.
     
  5. swty_todd

    Thread Starter Active Member

    Aug 3, 2008
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    Thnx….
    But I must b real dumb because I don’t think I am understanding u guys….
    This is what I think …pls correct me where I m wrong….
    What I m saying is that ..for the transistor to amplify the signals the emitter base junction should be forward biased and the collector base junction reverse biased. That means, the currents are always going to flow in the same particular directions irrespective of the input ac voltage’s direction ie the output current will always be from A to B (in the accompanying figure ),although this is not what we want.
    The accompanying figure is not the practical version …but then I m so confused …I decided to go with it because it’s simpler
     
  6. mik3

    Senior Member

    Feb 4, 2008
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    The current flow will be in the same direction because the ac signal is biased on a dc level but its amplitude will vary with time. Thus you can distinguish a dc and an ac component in the output voltage or current.Actually, is not an ac because the current does not change direction but it is a varying dc current.
     
  7. studiot

    AAC Fanatic!

    Nov 9, 2007
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    OK lets try to look at it from another perspective.

    Firstly you need to realise that the DC and AC conditions are different for a transistor. They need to be considered separately.

    You have to set up the DC conditions correctly first of all. This is called biasing. When you have done this the terminal voltages will be at appropriate levels and current will flow in the base, and collector circuits as you have already observed. They will flow in one direction only.

    Nothing will change until we introduce an AC signal.

    We cannot just connect an AC signal as you have shown because that would disturb the carefully set up DC voltages.

    We have to connect the AC signal through a capacitor. AC can pass through a capacitor, DC cannot.

    Contrary to what you understand, we do not want to reverse the direction of flow of the current.
    All we seek to do is to vary the magnitude of the current up and down, in sympathy with the applied signal.
    Nor do we wish to vary the voltages on the transistor.
    This is why the transistor is regarded as a current device. In order to extract our output voltage we must pass this varying current through a resistor, known as the load resistor. When we do this it appears as a varying voltage across that resistor.

    Of course there are also bias voltages across the resistor so we must also extract our output through another capacitor to avoid disturbing the DC biasing.
     
  8. swty_todd

    Thread Starter Active Member

    Aug 3, 2008
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    so an AC signal input is turned into an amplified varying DC output....right ???
     
  9. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Correct.

    On directly coupled amplifiers, the input signal can also be varying DC.
     
  10. swty_todd

    Thread Starter Active Member

    Aug 3, 2008
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    so if i am right then , the trasistor is not merely amplfying the signal as it is ,but also changing it into a dc current ie it is also acting as a current rectifier...??
     
  11. JoeJester

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    Apr 26, 2005
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    if you look on the base of the transistor, you will see a "varying dc" signal as well.

    Your AC input signal is coupled to the dc bias. If you want to say a conversion from AC to varying dc took place, it's at the base.
     
  12. studiot

    AAC Fanatic!

    Nov 9, 2007
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    No rectification takes place.

    When something is rectified alternate half cycles are blocked (in a half wave rectifier) or inverted (in a full wave rectifier). So that the output is all positive or all negative.

    A transistor shifts the level of the alternating signal so that the output is all positive but does not block any part of it.
     
    Last edited: Aug 10, 2008
  13. Audioguru

    New Member

    Dec 20, 2007
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    The transistor is an amplifier, not a rectifier.
    The input AC signal rides on the DC input bias voltage so the input current of the transistor increases and deacreases with the AC input which makes the output current increase and decrease with the AC input.

    The transistor amplifies because a low input current causes a much higher output current. If you select a suitable value for the output resistor and a suitable supply voltage for it then Ohm's Law shows its AC voltage amplification.
     
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