# BJT Biasing

#### amirmg

Joined Jan 29, 2005
2
Hi!!!
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.
thanks

#### n9xv

Joined Jan 18, 2005
329
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.

#### amirmg

Joined Jan 29, 2005
2
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
amir

#### ficr

Joined Jan 5, 2005
4
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.

#### professor blue

Joined Feb 1, 2005
2
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.

Doc

Originally posted by amirmg@Jan 29 2005, 06:26 PM
Hi!!!
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.
thanks
[post=4901]Quoted post[/post]​

#### mozikluv

Joined Jan 22, 2004
1,437
hi

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.

#### vineethbs

Joined Nov 14, 2004
56
isn't the reverse active region actually there in ttl input transistors ?

#### profdan

Joined Aug 29, 2005
1
Originally posted by amirmg@Jan 30 2005, 05:08 PM
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
amir
[post=4920]Quoted post[/post]​
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.