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Old 14th March 2007, 08:56 AM   #1
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Default base-emitter voltage

hi,

what would be the exact voltage at base-emitter of output transistors when working at maximum stage?..is there any way to stable its voltage when it operates?..


tnx..
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Old 14th March 2007, 12:36 PM   #2
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Default The base to emitter junction stabilize the voltage around 600 milivolts

Because diodes tends to work as zener, and this keep voltage there from 500 to 700 milivolts...but the current will variate, despite voltage is controled.

The current variation will make the job..as transistor is a current amplifier, different from electron tubes that works mainly as voltage amplifiers.

And this variation is what will "order" the colector to emitter junction to let pass more or less energy...from the big supply you have connected from Colector to emitter, and this is amplification.

To hold a small energy in the input (base to emitter junction) that will control a bigger energy that is placed into the colector (colector to emitter, output junction)

When the voltage measured there, from the base to emitter junction...mean, one meter lead at base and the other at the emitter....if it is lower than, let's say, 500 milivolts (0.5 Volts) the transistor will be cutted....nothing will circulate from colector to emitter.

If you increase that DC voltage (or even audio signal voltage) into the base to emitter...the resistance from colector and emitter will be reduced tracking the input signal (if variable) or will show a fixed resistance that will drain current from colector to emitter.

The differences in "size"...."magnitude"...how big is the input signal...or how big is the DC input bias signal (bias is to put it to work with small current from colector to emitter) will result in how big will be the colector to emitter resistance.

And there is a proportion...a ratio...that is called gain...some transistors will receive 1 miliampere , from base to emitter, and will let 100 miliamperes cross the colector to emitter...this ratio, in this sittuation 100...will be the unit gain...that transistor gain.

So...transistor is transference resistor...transference resistance...and this result in "amplification"..that is a trick, as you control a bigger supply energy with a small energy placed from base to emitter.

When you have too much bias...let's say 700 miliamps...the transistor will be saturated....say...its colector to emitter resistance will be the mininum possible.......so...the current will be the biggest possible....if you force bigger voltages.... your transistor will melt...will burn.

I hope this can help...a lot of friend will help you to understand..just do not be shy and insist till things turns clear for you.

Line up will be an excelent teacher of basic electronics...not only basic electronics...but he is a natural teacher...a good teacher...i hope he will come for your help.

Maximum voltage from base to emitter will apear in the datasheet, the factory informations...go to google and type there your transistor number and them datasheet.... you will have all informs about it.

regards,

Carlos
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Old 14th March 2007, 01:18 PM   #3
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Default Not the maximum, but the good voltage from base to emitter, no signal conditions

Input shorted...goes from 590 to 630 milivolts.

But this is class AB operation, when your stand by current is small, the output is biased to drain small current, the voltage placed from base to emitter is small enougth to force the transistor into conduction.

The colector to emitter resistance will be high enougth to allow you to have some tenths of miliamps flowing.

In the Class B the transistor is cutted, the bias, the voltage that will ride the transistor will be smaller than 500 milivolts...the transistor will turn one with the signal enter and switch the base to emitter junction on.

Using class A circuits, means that current will be high, even in stand by mode, this may turn the VBE, volts from base to emitter very high....say.....680 to 720 milivolts in stand by condition...this will make colector to emitter resistance not so high and enormous current will cross the output transistors.... hundreds of miliamperes will cross.

Well.... i use to adjust my amplifiers to work with 690 milivolts, and with the VAS transistor exception, i use to bias them all to this voltage....Voltage amplification transistor, the VAS, use to be biased into class A...so, it is normal to measure bigger VBE voltages..upper than 650 milivolts.

This is DC bias voltage....a control voltage.

When audio enters( AC audio voltage going together with the DC voltage will produce a resultant voltage) , you will have sligth modifications... a voltage swing and a bigger current swing.., in the real world the input (base to emitter)will be controled because of diode junction stabilises voltage there...but will show some voltage variations ....but the bigger thing that will happens will be big current variations there...and those variations will control the Colector to Emitter junction...and the amplification will happens.

I hope i could help...if not..scream loud and call for help once again..other friends will come...the ones that wants to help will come...the others that wants to show themselves as high knowledge people will not come...as this is very basic for them.

regards,

Carlos
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Old 14th March 2007, 05:16 PM   #4
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Default Re: base-emitter voltage

Quote:
Originally posted by clovi_kate
hi,

what would be the exact voltage at base-emitter of output transistors when working at maximum stage?..is there any way to stable its voltage when it operates?..


tnx..
The base has a certain impedance. So, if you need more Ic, you must put more Ib in the base and because of this impedance, Vb (Vbe) will HAVE to rise. I don't think you want to stabilize it because that would mean that you also stabilize the Ic which makes the gain zero.

Jan Didden
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Old 14th March 2007, 11:45 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
have a look at a few datasheets.
They show the Vbe for various temperatures and Ic values as well as saturated values (very low gain).
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Old 14th March 2007, 11:49 PM   #6
CBS240 is offline CBS240  United States
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Default Re: base-emitter voltage

Quote:
Originally posted by clovi_kate
hi,

...is there any way to stable its voltage when it operates?..

for AB bias,
If by stabilizing the Vbe voltage for change in temperature, a Vbe multiplier, thermistor, or combination can be used depending on bias. Since Vbe decreases for increased temp., bias has to be adjusted for smaller Vbe by temp sensing device or .
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Old 22nd March 2007, 08:58 AM   #7
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Default hi

tnx for the advices and description guyzz...but is it advisable to put signal diode(3diodes) across the base-emitter of the transistor?..

one design i saw,he used 3diodes(in series) to control increasing voltage that may cause to burn..


tnx..
Reign..
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Old 22nd March 2007, 09:20 AM   #8
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Default Re: hi

Quote:
Originally posted by clovi_kate
tnx for the advices and description guyzz...but is it advisable to put signal diode(3diodes) across the base-emitter of the transistor?..

one design i saw,he used 3diodes(in series) to control increasing voltage that may cause to burn..


tnx..
Reign..

That may have to do with the thermal compensation mentioned. temp goes up, the 3 diode bias voltage goes down, the final Vbe goes down BUT this depends intimately on the bias and driver circuit. You can't just put on those diodes and assume it works.
In isolation, the final transistor will burn long before the 3 diodes limit Vbe. That limit will be about 3 * 0.6V while at 0.8Vbe the transistor will be fully conducting max current.

As someone said, you really need to read some basic papers & datasheets.

Jan Didden
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