What Is The Maximum Useful AC Output Voltage?

Discussion in 'Analog & Mixed-Signal Design' started by elec_eng_55, Oct 28, 2018.

  1. elec_eng_55

    Thread Starter Member

    May 13, 2018
    118
    0
    Hi:

    I have computed the maximum ac output voltage as follows:

    Lesser of: Ic * (rc + re) = 1mA * (2.91K + 0) = 2.91 Vpk

    Or : Vce = 5 Vpk

    Therefore Vmax = 2.91 Vpk

    But this output is not realistic as it is badly distorted. In this
    example, Vce = 5 volts, but it can't be utilized since the
    maximum useful output seems to be approx. 215 mVpk
    per the sim.

    Why do textbooks and lecturers make these calculations?

    What is the maximum useful ac output voltage?

    Thanks,

    David
     
    Last edited: Oct 28, 2018
  2. danadak

    Well-Known Member

    Mar 10, 2018
    1,736
    354
    Here is a sim done for another poster, basically seems to work pretty
    good. Curves are input and at the load. After I posted this tried it with
    a 3904, essentially same results.

    Attached.

    Regards, Dana.
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    8,734
    2,014
    Remove the emitter capacitor and you can get about 6.5V pk-pk out.
     
  4. Bordodynov

    Well-Known Member

    May 20, 2015
    1,907
    578
    When using a cascade circuit with a common emitter and without negative feedback, you can get the limiting amplitude of the signal, but the output signal will be greatly distorted.Already at the input signal 10mV distortion will be 10%.You can get near-limit voltages by entering negative feedback.But the voltage gain will be less and sometimes much less.Discard the emitter capacitor from the circuit.The gain will be about 2.8 times.Input a signal with an amplitude of 1.1V and you will get an amplitude almost as in theory.
    See
    draft1.png
     
  5. elec_eng_55

    Thread Starter Member

    May 13, 2018
    118
    0
    I still do not have an answer to my two questions pertaining to the original
    schematic:

    Why do textbooks and lecturers make these calculations?

    What is the maximum useful ac output voltage?
     
  6. Bordodynov

    Well-Known Member

    May 20, 2015
    1,907
    578
    I believe that the calculations given by your teachers make a lot of sense. Beginners in circuit design often make mistakes. They take a kilohm resistor in the collector and load this cascade with a low-impedance load (4–8 ohms), and then wonder why there is no amplification (small output signal)! Some do not model the circuit and solder the real circuit. And they are very upset.
     
  7. danadak

    Well-Known Member

    Mar 10, 2018
    1,736
    354
    You would proceed with calculations for two cases, transistor almost
    cutoff, no current thru it, and other case close to saturation, most current
    thru it, and that will determine what the range of voltages will be seen at
    output node.

    Calculations are done to insure a circuit does what it was designed to do,
    typically over T and V and component variation.

    Regards, Dana.
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    8,734
    2,014
    That depends on how much distortion is acceptable in a particular situation. For audio work, low distortion is necessary, but if you simply wanted to use the output as a switching signal then distortion would generally be irrelevant and you could get nearly the full supply voltage out of the circuit.
     
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