transistor VEBO help

Thread Starter

nickolas

Joined Oct 10, 2009
4
I did a search for VEBO and only found one thread and it didnt tell me anything. If the max VEBO for a transistor is 5VDC and I bias it for a class ab amp so it is just off then the max is a hair over 4.3VDC that can still be applied to the base. Does that mean the ac voltage that can be applied to the base cant go over this voltage of 4.3V or am I missing something here? I will have an ac signal higher than that, and each stage will be higher than the last so do I need to have max VEBO higher than my ac signal? I am trying to learn how transistors work, and I am a novice so any help will be appreciated.
 

MikeML

Joined Oct 2, 2009
5,444
Transistors amplify current into the base, not voltage at the base. If you put a resistor between your input voltage source and the base of the transistor, the resistor effectively converts voltage into the base current.

Veb0 for an NPN transistor refers to how far negative the base can go with respect to the emitter. In the positive direction, the base voltage is purely a function of the base current.
 

Thread Starter

nickolas

Joined Oct 10, 2009
4
So base dc voltage and ac voltage combine through a resistor and are converted into a current? And the current is amplified through the transistor and applied to the collector resistor and converted to a voltage in a common emitter amp for the output? So I dont need to look at VEBO then?
 
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MikeML

Joined Oct 2, 2009
5,444
If I want to bias it a hair under being on at around 0.7 volts and it doesnt do anything with the voltage just current, how do I know what the base current needs to be then? If VEBO is of no use then why do they list it?
Base current is collector current / hfe. Veb0 is very important because if you exceed it, you might damage the transistor, or clip the input signal.

Stare at the attachment. Everything is plotted vs the input voltage as it swings from -6V to 2V. Note what the base voltage does. At -6V in, the Veb0 is exceeded.
 

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Thread Starter

nickolas

Joined Oct 10, 2009
4
I dont know what I am supposed to be looking for all I see are a bunch of colored lines. What is the bad part that I should notice? I am only amplifying the positive half with an NPN and the negative half with a PNP.


Transistors amplify current into the base, not voltage at the base.

I thought that voltage didnt matter just current, so why does the input voltage of -6v to 2v matter if it being changed to a current?
 
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Audioguru

Joined Dec 20, 2007
11,251
I wish you would attach your schematic so we can show you how it works.
The class-AB output has an NPN emitter-follower and a PNP emitter-follower. They are both turned on a little to avoid crossover distortion. Their input is a signal voltage at a very low current.

Since the transistors are turned on their Vbe is about 0.7V. They do not have a Veb voltage which is backwards and turns off the transistors.

Some oscillator circuits accidently apply a reverse voltage to the base and they must be careful to avoid exceeding the Veb maximum allowed voltage.
 

Thread Starter

nickolas

Joined Oct 10, 2009
4
I dont have a schematic, I am just trying to learn how a transistor does what it does. I have no problem with the math part, just the concept of the transistor. What I have been told I guess is that a voltage with no current applied to the base will do nothing so VBE0 should be a current measurement then since no voltage gets in the base? I need this explained more please. If I had an easy way to draw a schematic I could throw one together and post it, but I cant find a program that I like, and I dont want to spend money on a program I may not use.
 
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