Baisng Water Analogy

[walters]

What is a Water Analogy for Bias?

What would be a Water Analogy for baising?

Why does AC need DC offset or Biasing to not shut off?

 

[zachtheterrible]

Why does AC need DC offset or Biasing to not shut off?

Think of an AC signal. It goes from zero point to positive, back to zero point, to negative, back to zero point.

Lets use an NPN common emitter amplifier for this example, as you're probably most familiar with it. At zero point, the transistor is not even on! It starts turning on as the signal goes positive, then starts turning off as the signal goes back towards zero point. The transistor is not on during zero point, and negative.

If the transistor is biased, it is always on, and the AC signal riding on the DC will turn the transistor on a little bit more when it is positive, and turn it off a little less when it is negative, but the transistor is never completely off.

As for your other questions Im not quite sure what you mean.

 

[walters]

Thanks for the information

It starts turning on as the signal goes positive, then starts turning off as the signal goes back towards zero point. The transistor is not on during zero point, and negative.

Why does it shut off when going negative?

Why does the transistor need DC offset to make the negative cycle not shut off?

 

[checkmate]

The effect of biasing is normally shown in a graph. The attached graph is compliments from here.

The graph is that of output voltage vs input voltage for a common-emitter configuration. The slope is the gain of the amplifier. Ignore the inverting portion which is not reflected. Anyway, the output can only swing between Vcc and ground, therefore you see the gain drop to zero near these two regions. As you see, to get a proper output, you must bias it to operate in the "linear operating region" such that gain is not affected by input voltage.

Another point to note is that both input and output voltages are POSITIVE. They are made dual-polarity by the decoupling capacitors.

Hope this is clear enough.

P.S. Time to grow out of water analogy.

 

[walters]

dual-polarity by the decoupling capacitors

Where is the Decoupling Capacitor at ?

How does it Capacitor Dual Polarity a Sine wave ?

I never used a Decoupling Capacitor

 

[checkmate]

A capacitor removes the DC component, leaving a dual polarity AC.

 

[walters]

Oh yea i know what you mean but they just call it a coupling cap on the output

So if there is no DC offset the AC waveform would Cut off or Saturate
becaues the Transistor is Cutting off and Saturating ?

I thought Bias Was for the operating state so the transistor is linear

But Bias is also to not make the transistor go into Saturation or cut off
so you need DC offset to make a transistor not saturate or go into cut off fuctions

 

[checkmate]

Look at the graph again. It shows saturation/cutoff in action. Saturation and cutoffs are examples of non-linear behavior.

 

[walters]

non-linear behavior

But thats with DC offset and Biasing to create non linear behavior

I thought DC offset and Biasing was to create only linear behavior and to not have non-linear behavior

Without DC offset and Biasing you get non-linear behavior?

 

[checkmate]

Read the following sentence ten times.

"Proper biasing allows operation in the linear region to avoid non-linearities like saturation and cut-off."

I stress again, PROPER biasing.

 

[checkmate]

Just to elaborate, the graphs have shown (from left to right) the effects of under-biasing (no biasing is a form of under-biasing), proper biasing and over-biasing.

 

[walters]

under-biasing means to little DC offset needs more DC offset

proper biasing means perfect DC offset

and over-biasing means to much DC offset needs less DC offset

 

[checkmate]

Precisely, as shown on the graphs.

 

[walters]

Thanks checkmate for the information and help