common emitter BJT amplifier question

Thread Starter

xmadmaxx

Joined Apr 22, 2007
6
Hi, I'm new here so I hope you answer quickly as I have to hand this up tomorrow. I'm planning to visit this site regulary beacuse it's got almost all I need for uni...

I have a problem with this question: If you vary Vcc (in common-emitter BJT amplifier using voltage-divider bias circuit) how does that affect input and output signals and hence voltage gain Av?

Thanks in advance
 

Thread Starter

xmadmaxx

Joined Apr 22, 2007
6
ok, sorry about that, didn't see it and I wanted to be as quick as possible...


basically I figured out that varying Vcc doesnt change Vi at all, but it is proportional to Vo. And beacuse Av=Vo/Vi, Av is also proportional to Vcc. Is that right?

And now I have read somewhere on the site that DC bias has to be sufficient enough so decreasing Vcc below some value I guess will cause the Vo to clip?
 

Thread Starter

xmadmaxx

Joined Apr 22, 2007
6
thanks! I was hoping thats right...

And I got one more query thats very similar to the other one...

If now, input signal Vi is slowly increased (Vcc stays the same), I know the Vo will get clipped on top and bottom (on the graph), but I don't know which side will get clipped first, top or bottom?

I'm guessing that top Vo will get clipped first and then soon after the bottom part will... I guess its beacuse at the top of Vo Vi is down the bottom and hence Ib is down the bottom (close to zero) so it's gotta be cut off,

and when Vi is on top, Vo is on bottom, and beacuse Ib is high and Ic also that means its saturation...

hehe, hope you undestood what I meant...
 

kubeek

Joined Sep 20, 2005
5,794
That depens on the bias.
If it is too low it will clip at the bottom, if too high it will clip on the high side.
 

Thread Starter

xmadmaxx

Joined Apr 22, 2007
6
And what does it mean low bias, Q-point closer to y-axis(saturation mode)?

So basically, If I know that it clipped at the bottom first this means transistor was working in saturation mode?
 

kubeek

Joined Sep 20, 2005
5,794
If it clips at the bottom, than it means that the DC component of the signal is too low, and because it cannot go lower than ground it clips at the bottom.

Common emitter inverts the signal, so if you clip at the bottom, the bias is a little higher than it should be. The right bias will clip high signal at both sides simetrically.
 

Thread Starter

xmadmaxx

Joined Apr 22, 2007
6
I dont quite understand this, if increasing Vi, clips Vo at the bottom first, for the common emitter,the DC voltage bias at the input is a little higher than it should be?
 
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