# Transistor characteristics

Discussion in 'General Electronics Chat' started by Rohitchampion, Jun 1, 2013.

1. ### Rohitchampion Thread Starter New Member

Apr 24, 2013
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How to determine output and onput characteristics of transistor in given mode? I am confused about what versus what graph displays output characteristics of given particular transistor (same confusion about input characteristics). For example in common emitter npn transistor I$_{}C$ Vs V$_{}CE$ graph shows output characteristics, but why I$_{}C$ Vs V$_{}CE$? Please explain how to determine that for both input and output characteristics?

May 3, 2013
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3. ### ifixit Distinguished Member

Nov 20, 2008
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Hi Rohitchampion,

Transistors are current amplifiers, not voltage amplifiers. However, with the addition of resistors you can build a voltage amplifier circuit using transistors and ohms law.

A "Vce vs Ic" graph is used because the beta of a transistor varies with Vce and it also varies with Ic. This makes the output characteristics complex and that requires a graph. Input characteristics are equal to the output characteristics divided by beta.

Beta varies with temperature, and from one device to another... more graphs.

Regards,
Ifixit

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4. ### Rohitchampion Thread Starter New Member

Apr 24, 2013
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Thanks ifixit. Nice explanation. but why output characteristics depend on values that vary beta of transistor?

5. ### Rohitchampion Thread Starter New Member

Apr 24, 2013
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that is also my post..

6. ### studiot AAC Fanatic!

Nov 9, 2007
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There are two separate processes involved with designing or analysing a transistor circuit.

DC analysis

AC analysis.

I will take the second one first becuase it describes the motivation for why we do what we do.

The purpose of transistor circuits is to process signals. If the signal was already in the form we require no there would be no transistor circuit needed!

Signals are about change. A changing voltage or a changing current or both.

The graphs we draw tell us what output we will achieve if we change the input by a (small) specific amount. The slope of the graph provides a linear approximation to this. So the slope is an important parameter we can obtain from the graphs.

Secondly to DC analysis. We cannot just set up transistors in any old configuration and expect them to work. We have to set them up correctly for our specific application (processing our signal). This can also be done on the graph. It is called biasing.

You should also look up the term 'load line'.

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7. ### Rohitchampion Thread Starter New Member

Apr 24, 2013
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studiot you are giving totally different explanation. I know DC analysis, AC analysis, transient analysis, load lone and all that stuff. Just read that question once again. I just wanted to know why Ic vs Vce graph shows output characteristics of CE npn and why Ic vs Vbc shows output characteristics of CB pnp transistors etc etc.

8. ### ifixit Distinguished Member

Nov 20, 2008
639
110
Hi,

The transistor characteristics don't change just because you use the device in CE, CB, or CC configurations. They still apply, just as the specifications for a cricket bat don't change just when you use it as a rug beater.

The base is always the input to the transistor, the collector is always the output, and the emitter is what the in & out are referenced to.

E.g. In the common base (CB) configuration it just looks like the transistor input is the emitter, but its not. Think of the base as the input so you can understand and use the graphs.

Regards,
Ifixit

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9. ### studiot AAC Fanatic!

Nov 9, 2007
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If you genuinely know how to do and understand a load line analysis, you should be able to answer your own question.

Load line analysis is the graphical method equivalent to the algebraic one using 'black box' theory ie a model and box terminal parameters.

And graphical methods are plotted on --- well --- graphs.