Connect with us

Common Emitter Amplifier - Class B

Discussion in 'General Electronics Discussion' started by Chr1s, Feb 9, 2020.

Scroll to continue with content
  1. Chr1s

    Chr1s

    27
    2
    Dec 20, 2016
    Hi All,

    I am designing a Class-B amplfier in tina using a BC109 transistor with a calculated gain of 5. I have attached a screenshot of the the AC transfer characteristic, however i dont really understand what it means? Can someone help me to understand what is going on?
     

    Attached Files:

  2. bertus

    bertus

    256
    82
    Nov 8, 2019
    Hello,

    Do you have a complete schematic?
    The behaviour could come from the capacitors used in the circuit.

    Bertus
     
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    9,546
    1,971
    Nov 17, 2011
    How about a schematic diagram?
    Our crystal balls have collectively gone on holidays;)
     
  4. Chr1s

    Chr1s

    27
    2
    Dec 20, 2016
    Sorry Guys. Attached is the circuit in Tina 9
     

    Attached Files:

  5. bertus

    bertus

    256
    82
    Nov 8, 2019
  6. Chr1s

    Chr1s

    27
    2
    Dec 20, 2016
  7. Audioguru

    Audioguru

    2,694
    610
    Sep 24, 2016
    Without base bias current, the transistor is turned off and does not conduct unless the input voltage is fairly high. Look at the current, it is very very close to zero. With a fairly high input voltage then a signal at the output would have severe distortion.
    Also, if it is biased then it would be class-A, not class-B. Look in Google for class-B that uses two transistors and both have no bias.
     
  8. BobK

    BobK

    7,671
    1,681
    Jan 5, 2010
    A class B amplifier requires 2 transistors, 1 for the positive half and one for the negative half of the signal. One a PNP and the other an NPN.

    Bob
     
  9. Ylli

    Ylli

    259
    59
    Jun 19, 2018
    Class B is defined as an operating point such that the active device conducts over 180° of the input signal. If you take your circuit and add a bias network that holds the bias at a point where the transistor is *just* starting to conduct, you will have a class B amplifier. In the single ended configuration, it is not very linear and the output looks more like a half wave rectifier than the input.
    Annotation 2020-02-10 194415.png
     
    bertus likes this.
  10. BobK

    BobK

    7,671
    1,681
    Jan 5, 2010
    Not much of an amplifier if it ignores half of the signal. Maybe it's a class B half-amplifier.

    Bob
     
  11. bertus

    bertus

    256
    82
    Nov 8, 2019
  12. Ylli

    Ylli

    259
    59
    Jun 19, 2018
    Works fine in the proper application. 10 MHz class B.
    Annotation 2020-02-11 102533.png
     
    bertus likes this.
  13. WHONOES

    WHONOES

    756
    162
    May 20, 2017
    If the amp is biased to have a dc current flowing through it, it is class A.
     
  14. Ylli

    Ylli

    259
    59
    Jun 19, 2018
    And if it is biased so that there is no current flowing, then it is likely in class C. Class B means 180° conduction angle. Impossible to maintain in real life. What I posted has 3 uA of idle current. About as close to Class B as you are going to get.
     
  15. WHONOES

    WHONOES

    756
    162
    May 20, 2017
    It's still class A though.
     
  16. Ylli

    Ylli

    259
    59
    Jun 19, 2018
    Class A is defined as the active device conducting over 360° of the input waveform. Here is the sim showing the input voltage and the transistor Ic.
    Annotation 2020-02-12 123330.png Annotation 2020-02-12 124117.png
    Are you really going to tell me you see the transistor conducting over the full 360° of the input waveform?
     
  17. Harald Kapp

    Harald Kapp Moderator Moderator

    9,546
    1,971
    Nov 17, 2011
    @Ylli : Understanding these waveforms would be greatly simplified if you had labeled the nodes with "speaking" names (F4 key in LTSpice) or at least indicated which node is n002 :confused:.

    Having the transistor not correctly biased and thus not being conductive over the full 180 ° doesn't mean this is not a class A amplifier.
    To be classified as class B the second transistor is missing which would fill in during the non-conducting phase of Q1.
     
  18. Ylli

    Ylli

    259
    59
    Jun 19, 2018
    point taken
    Yes it does. By definition a class A amplifier conducts over the full 360°. If it does not conduct over the full 360° it is not class A.
    The device *is* correctly biased for operation as a class B amplifier.


    There are no requirement under the definition of 'Class B' that more than one active device needs to be used. From Wiki:

    "In a class-B amplifier, the active device conducts for 180 degrees of the cycle. This would cause intolerable distortion if there were only one device, so two devices are usually used, especially at audio frequencies. Each conducts for one half (180°) of the signal cycle, and the device currents are combined so that the load current is continuous.

    At radio frequency, if the coupling to the load is via a tuned circuit, a single device operating in class B can be used because the stored energy in the tuned circuit supplies the "missing" half of the waveform. Devices operating in Class B are used in linear amplifiers, so called because the radio frequency output power is proportional to the square of the input excitation voltage. This characteristic prevents distortion of amplitude-modulated or frequency-modulated signals passing through the amplifier. Such amplifiers have an efficiency around 60%."

    Now go back and look at my reply's #9 and #12. A single ended class B amp is not useful at audio frequencies, but as an RF amplifier, with a tuned network as the load, it works just fine. The posted circuit is properly biased and operating as a class B RF amplifier.
     
    Last edited: Feb 13, 2020
    bertus likes this.
  19. BobK

    BobK

    7,671
    1,681
    Jan 5, 2010
    That definition is not very useful. Consider a symmetric square wave with 90% of the cycle positive and 10% negative. Would a class B amplifier conduct over half the cycle?

    Bob
     
  20. Ylli

    Ylli

    259
    59
    Jun 19, 2018
    Useful to you or not, that *is* the definition.
    Apples and oranges. The amplifier classes are defined with sine wave inputs. For what you describe, you are not using an amplifier, you are using a switch.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-