Unusual CE Amplifier BJT response

Discussion in 'The Projects Forum' started by porkpie08, Mar 9, 2013.

  1. porkpie08

    Thread Starter New Member

    Mar 9, 2013
    Hi all,
    So I have been working on a report, going well! However during simulation and testing I decided to test the response for a signal that would drive the transistor into heavy saturation/cutoff.

    The response in heavy saturation seems to show some voltage making it to the output of the amplifier. The only explanation I can think of is that the base collector junction has become forward biased and some of the input signal is making it to the output.

    Maybe you can help me make sense of it or point me towards an article.
  2. tindel

    Well-Known Member

    Sep 16, 2012
    Yep - you're base-collector is being forward biased.

    Remember, it looks like your amplifier is has a gain of 10 (A = Rc/Re) so an input voltage of 10V is already pretty high. Try amplifying a voltage of 100mV or something like that, you will see your output is 1V or so. Much more managable for your bias settings.

    You can play some games to try to get more gain, but a gain of 10 is pretty good for this kind of amplifier, with your voltages. A gain of 20 is about as much as you can get without producing significant amounts of distortion.
  3. patricktoday


    Feb 12, 2013
    So what is your main intention with this circuit? What were your expectations that would happen at the output in response to your input signal compared to the actual results?

    Your voltage signal source is a total of 20V peak to peak and your supply is 12V. So any time your signal is at a voltage level to where it pulls the transistor's base downward to the point where it can no longer sit at at least 0.7V, the transistor will be off. When it's off the output voltage will be Vcc, there's no current flowing through the transistor. And when the signal is above that point, the emitter is going to follow the input signal 0.7 volts below it.

    You may also find it helpful to view both the emitter and the collector signals in a DC context and you'll see where they bump into each other and how the circuit reacts.
  4. porkpie08

    Thread Starter New Member

    Mar 9, 2013
    Sorry I should have been more clear, I was deliberately over-driving the circuit, just to see what would happen really. I was pretty sure the base collector junction would forward bias but wasn't sure what sort of a signal response to expect.

    Previously I was driving the circuit with a 250mVpp signal to get a frequency response, which was pretty clean throughout the mid band.

    Thanks for your replies, has helped to put my curiosity at rest.. i think! :p

    also, just noticed I posted this in the wrong part of the forums. sorry.