Transistor (BJT) bias compensation

[harii07]

Dear all
I need to understand BJT bias compensation by diode, thermistor , ..etc.
To start with diode compensation of BJT. Will any one discuss on this or suggest any book which discuss about this.
( do not worry i will not ask any ebook which u suggest, rather i will buy it)

Please help me in this.

 

[dick_freebird]

Are you asking about maintaining a constant bias point
(Ic) across temp, supply? Or are you asking about base
current cancellation (compensation)?

I think I've seen some reference to it in older analog
design texts, back when bipolar devices were still
dominant in analog design - Blue-cover Gray & Meyer,
etc.) - but a lot of the art was hand-me-down between
people actually working on product design. Not that
there was -that- much art to it.

 

[goldsmith]

Dear all
I need to understand BJT bias compensation by diode, thermistor , ..etc.
To start with diode compensation of BJT. Will any one discuss on this or suggest any book which discuss about this.
( do not worry i will not ask any ebook which u suggest, rather i will buy it)

Please help me in this.

Hi harii07

If i could understand your question as well you are asking about temperature bias stabilizer in transistors . right ? it is pretty simple . you will use a resistor in order to prepare a good DC bias for your BJT right ? ( in class A for instance )
if temperature being changed then thermal voltage or in fact VT will be changed . if we use a diode exactly like the BE diode or like that , then temperature won't have many of the bad effects on quiescent point and beta . how ? simply ! if forward diode being increased according to the temperature situation ( being decreased ) then Vdiode will be decreased so the Base current will be decreased and vice versa .
i.e : the diode will be used in series with the biasing network . if it is hard to understand tell me , please i can draw something in order to make better understanding for you .
Best Wishes
Goldsmith

 

[harii07]

Dear dick and goldsmith
Thanks a lot for your reply.

Mr. Dick
Yes, it is correct, i am asking that to keep q point to be stable in variation with temperature.

Mr.Goldsmith
I have gone through the material saying that a diode in series with base resistor (pull down) in voltage divider bias will stabilize the biasing point.
Is this only for voltage divider biasing not for any other biasing? ( shall i have derivation for this?)
How to stabilize the bias point with thermistor and sensistor?

 

[goldsmith]

I have gone through the material saying that a diode in series with base resistor (pull down) in voltage divider bias will stabilize the biasing point.
Is this only for voltage divider biasing not for any other biasing? ( shall i have derivation for this?)
How to stabilize the bias point with thermistor and sensistor?

Hi Harii07 !
it can be used for all kind of biasing networks in BJTs but sometimes with some simple changes . but the format is approximately the same .
Thermistors can be easily used ! if it is PTC use it as pull up and if it is NTC in pull down path . but thermistors are dealing with some sort of non linearity so they won't be as good as enough in cases which precision is so important .
Best Wishes
Goldsmith

 

[dick_freebird]

Many devices have a "magic" point where a zero-drain-
current-tempco Vgs or a zero-collector-current-tempco
Vbe can be set. It may not get you the -value- you want,
but stability girst, gain second, right?

Now of course this all presumes that the source of said
"magic" voltage is itself stable. And power supply variation
(less temperature variable offsets in active devices) makes
"magic" hard to come by no matter how cute the concept.

If (say) you plotted Id vs Vgs as a family of curves
parametric in temperature, you would (in a relatively
ideal world) see a point where all lines intersect. That's
your "magic" point; then your bias network has simply
(heh) to provide that.