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  1. R

    Assistance with Transistor Theory

    You can't. The base voltage has to change for Ic to change. Ratch
  2. R

    Assistance with Transistor Theory

    No, not a sustainable current. The current would stop when the uncovered charges from the diffusion process attained a high enough back-voltage. The only way you can increase the current is to increase the Vbe. That is self defining. In the case of a BJT, current is the direct result of...
  3. R

    Assistance with Transistor Theory

    Neither of those circuits show what is controlling the BJT. You have to get involved with the physics of the BJT to do that. Those two circuits are just that, two circuits. Ratch
  4. R

    Assistance with Transistor Theory

    The voltage across Re determines the current in both the emitter and collector. Ratch
  5. R

    Assistance with Transistor Theory

    It does work to apply a voltage (Vbe) to the base through a voltage source and resistor divider network. You cannot get away from the fact that Vbe controls the current in both the base and collector. A Vbe can be calculated assuming a particular Ic or Ib, but what does that get you? Some...
  6. R

    Assistance with Transistor Theory

    No, it is not the leakage that causes the nonlinearity. It is the diffusion mechanism of the BJT. It is true in other physics problems not involving a BJT that diffusion is not a linear process. The waste leakage in a BJT just means that the source has to be able to sink and source a small...
  7. R

    Assistance with Transistor Theory

    Well, Ib is not going to the collector where it can do some good. It does not control Ic, the physics of the BJT prove that. It is a nuisance that complicates the bias circuit design. So if it does not do any good and just causes complications, by definition it is a waste. Ratch
  8. R

    Assistance with Transistor Theory

    Shame on Winfield Hill for allowing that misstatement to creep into his book. That is certainly not what he is saying now. Ib is an indication of the collector current when a BJT is in the active region, not a control of Ic. When my bedside clock says 5:30 AM, the sun rises. However, I do...
  9. R

    Assistance with Transistor Theory

    BJT nonlinearity in the active region is both transconductance and distortion. If a hypothetical voltage amplifier is not linear, you would still not hesitate to call it a voltage amplifier, would you? Same with transconductance. So keep the signal input small to approximate linearity and...
  10. R

    Assistance with Transistor Theory

    The BJT in the active region always exhibits transconductance. However its transconductance is not linear. Yes, Sedra and Smith in their book Microelectric Circuits advise a maximum 20 mV P-P swing of Vbe to stay within an approximate linear region. They solve a demonstration problem where...
  11. R

    Assistance with Transistor Theory

    In a BJT operating in the active region, the Vbe is used to control the charge carriers diffusing into the thin base area where they are whisked away into the collector circuit by the collector voltage. Unfortunately, not all the charge carriers are captured. A relatively small number of...
  12. R

    Assistance with Transistor Theory

    You are correct in that a "overvoltage" of 0.7 volts has to be applied to the base in order to reach the emitter resistor. But you don't have to worry about the few millivolts that the Vbe will vary in order to control large amounts of Ic. That takes care of itself. Ratch
  13. R

    Assistance with Transistor Theory

    If a constant current source is desired, then the circuit of http://www.radio-electronics.com/info/circuits/transistor/active-constant-current-source.php would be more appropriate. A switch using a transistor is a study of making a transistor saturate or turn off. Ratch
  14. R

    Assistance with Transistor Theory

    Although the BJT collector current is controlled by Vbe, you would not be wise to use Vbe to control collector current due to its exponential response. Better to apply a constant voltage across an emitter resistance in order to obtain a constant emitter/collector current regardless of the BJT...
  15. R

    Assistance with Transistor Theory

    Arouse1973, Your question is about how to set up a current amplifier circuit. Since the BETA (short circuit current gain of the transistor) varies all over the map, any transistor replacement or component drift due to aging will require more "fiddling" with the circuit to bring it back to...
  16. R

    Assistance with Transistor Theory

    I think that calling it the Current Control Method is a misnomer. I suggest calling it the Current Indication method instead. A lot of folks are fooled into thinking that because there is a linear relationship between the Ib and Ic over a large range of the active region, the Ib is controlling...
  17. R

    Assistance with Transistor Theory

    As long as the BJT is biased to be in its active region, it will be a transconductance amplifer. The components added to the BJT determine whether the circuit is a current, voltage, or whatever amplifier. Did you not read the link given in post #38 of this thread, which quotes the...
  18. R

    Assistance with Transistor Theory

    Laplace, "The FET works as a transconductance device. That is why the manufacturer's datasheet specifies the forward transconductance or transadmittance of the device." Yes, BJT's, FET's, and tubes are all transconductance devices. The transconductance of FET's and tubes are linear, so their...
  19. R

    Assistance with Transistor Theory

    To All, This question will never get straighted out until all of you realize that when you hook up a transistor like Adam did above, you are talking about a CIRCUIT. You cannot argue about whether the circuit is a current amplifier, voltage amplifier, or whatever, and then impute that the...
  20. R

    Thevenins and nortons circuit help

    Laplace, I think you realize that the topology of the schemat in post #26 is not correct. R6 is missing. I also think the best way to solve this problem is to convert the delta resistor configuration of R2,R3, and R6 into a Y configuration. That is easy to do. Then the open circuit voltage...
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