Collector feedback bias varies too much?

Discussion in 'The Projects Forum' started by rougie, Oct 15, 2012.

  1. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Hello,

    Using a pn2222 transistor (see attachment), I have done a ramping circuit with a new gain value which is:

    2.0 -> 3.3VDC for Vin
    and
    1.0 -> 0.5VDC for Vout

    Most of the transistors I tried resulted in a pretty precise ramp. But other transistors with different betas resulted in imprecise values such as:

    2.0 -> 3.3VDC for Vin
    and
    0.9 -> 0.33VDC for Vout

    Since Vc of this circuit is fed back into the base of another transistor which requires an approximate voltage of 0.55 to 0.6VDC to begin its linear region activity, there is too much of an offset for the 0.33VDC value. I will be losing approximately 34% of my linear region because of a premature vout value!!!! That's quite unacceptable.

    So what do we do in a case like this.... Do we have to test every transistor and make sure its a certain suitable beta for my circuit.... or perhaps provide different Vs values... for example dip switch #1 makes Vs 1.7VDC, dip switch #2 makes Vs 1.5VDC and so on.....

    Also, I never experimented with any transistors other than the pn2222. Is there a transisitor that does exactly what the the PN2222 does, but with a much more precise beta range?

    All feedback appreciated!
    r
     
    Last edited: Oct 15, 2012
  2. ramancini8

    Member

    Jul 18, 2012
    442
    118
    If I understand your data you can't get what you want from that circuit. Vcc is 3.3V, so the max output voltage is 3.3V. I assume Vin ranges from 2V to 3.3V, and you want a corresponding Vout of 1V to 5V. If you made a mistake in the data and 5V is really 0.5V then a solution is possible.
    I believe that you need an inverting transfer function to obtain the results you want. I recommend that you work out the desired transfer function prior to circuit design.
     
  3. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    arrghh!!!

    I am sorry Ramancini8, it's 0.5 and not 5.0 !!!

    darn typo!!!

    r
     
    Last edited: Oct 16, 2012
  4. ramancini8

    Member

    Jul 18, 2012
    442
    118
    If you solve simultaneous equations with the given data (Vin, Vout) You get the equation Vo = -Vin/2.6 +1.769. This result lends itself more to an op amp than a transistor, but it can be done with discrete components. .
     
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