BJT Biasing

Discussion in 'Homework Help' started by amirmg, Jan 29, 2005.

  1. amirmg

    Thread Starter New Member

    Jan 29, 2005
    Can you tell me how do you test if a BJT is working in active,saturation or cut off region,of course I wanna know teory and also in practice.
  2. n9xv

    Senior Member

    Jan 18, 2005
    Without actually connecting or measuring anything you can view a data sheet which will show the relationship between VCE & IC. VCE is the voltage on the collector with respect to the emitter. IC is the current flowing from collector to emitter for a given base current. They either tabulate the VCE Vs IC or shoe it in graphical form. The graph is whats known as a "load line". It shows the operating characteristics of the transistor, i.e. cutoff region at the bottom of the load line, active region at the middle of the load line, and saturaton at the top of the load line. The basic theory is that for a given supply voltage; an increase in base current causes a proportional increase in collector current. The base current Vs collector relationship refferes to a term called "beta" or "hfe". The higher the value of beta, the greater the change in collector current for a given base current.

    As a formula:

    IC is Collector current & IB is Base current.

    IC = BETA X IB or BETA = IC / IB

    You can find data sheets on the web and I think Motorola will send you an entire data book library for free.
  3. amirmg

    Thread Starter New Member

    Jan 29, 2005
    Hi n9xv
    Thank you so much for for reply but will you tell me more how can I test a BJT in a problem,I know if a transistor's emiter forward bias and collector reverse is working in active region but I'm curious to know if there's other ways to test.
    with best
  4. ficr

    New Member

    Jan 5, 2005
    Depending on the information given, simply calculate the various voltage and currents through base, emiter and collector. There is no easy answer. You have to do the work and understand if the values make sense or not.
  5. professor blue

    New Member

    Feb 1, 2005
    There are several ways to tell the mode of transistor operation. For troubleshooting purposes, the best way is to use voltages only.

    A way to describe the modes would be:

    cutoff - transistor non-conducting. CB junction is reverse-biased, EB junction either has reverse bias OR no forward bias.

    linear (active) - transistor conducting less than maximum. CB junction is reverse-biased, EB junction is forward-biased.

    saturation - transistor has max Ic. BOTH junctions are forward-biased.

    How to tell by voltage readings taken to circuit common:

    (Note: NPN is used for reference)

    CUTOFF: There is less than 0.5 volts difference between Vb and Ve or Vb is negative. Vc will be equal to Vcc.

    LINEAR: Vb is more positive than Ve by at least .6 volts and Vc is between about 1 volt and Vcc. For a class A amp of course, this would be near .5Vcc.

    SAT: Same as linear, except Vc will measure almost zero, typically about .1 to .3 volts.

    Hope this helps.


  6. mozikluv

    AAC Fanatic!

    Jan 22, 2004

    let me add this bit of info on the mode of operation of BJTs, what has already been mentioned like the forward active, saturation and cut-off. but i believe there is still another one, the reverse active. frankly i have not seen this kind of operation, i guess it is rarely used or not use at all. :)
  7. vineethbs

    Well-Known Member

    Nov 14, 2004
    isn't the reverse active region actually there in ttl input transistors ?
  8. profdan

    New Member

    Aug 29, 2005
    With your circuit "live" measure the DC voltage from base to emitter. If the BJT is biased properly and fully functional you should measure between 0.6vdc to 0.7vdc.
    Any other value means the biasing circuit has a problem or the transistor does.
    Next measure the voltage from Collector to Emitter, if you measure source or nearly source voltage your transistor is open, or in cutoff. If you measure near 0 volts your transistor is shorted or in Saturation.