Jump to content
Electronics-Lab.com Community

reliablity of hFE


shekhar_dandya

Recommended Posts


I know that a transistor is biased a it's mid current value. From there the power must be decided on and that will determine the voltage. I suppose half power is reasonable. The real test is to determine the beta and the changes in VCE that will keep the beta constant. I would like to tell of an easy way to find that beta and and voltage. I think there does exist a Q point.

Link to comment
Share on other sites

Shekhar,
If you are talking about biasing the base of a common-emitter transistor, then you won't know what the value of hFE will be.
See the data sheet for the 2N3904 transistor here:
http://www.fairchildsemi.com/ds/2N/2N3904.pdf

1) Page 2 shows that at room temperature, the hFE can be from 30 to more than 300, depending on the transistor's tolerance and amount of collector current.
2) Page 3 shows that the hFE varies with temperature and amount of collector current.
3) Page 3 shows that the Vbe also changes with the temperature and amount of collector current.
So if you bias the base of a common-emitter transistor with simply a single resistor to its base from the supply voltage, then its Q-point could be anywhere.
To overcome the obstacles above, it is recommended to use negative-feedback, in the form of an emitter resistor, and a constant-voltage voltage divider to the base from the supply voltage (with the current through the voltage divider about 10 times the highest specified base current), and/or a feedback voltage divider to the base from the collector of the transistor.
Then the Q-point of the transistor will be very close to your plan.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
  • Create New...