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class C bias question

foTONICS

Sep 30, 2011
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I was asked to calculate DC voltages VB, VE, and VC.

I am a little stuck on this CLASS C design since there is no input voltage for me to work with.

I'm just looking for a little guidance as where to begin. I was given no beta values. ive attached the photo below, thank you all for your help

P.S. sorry for the blurry-ness
 

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duke37

Jan 9, 2011
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You are asked for the DC conditions.
What is the voltage on the base?
What is the current into the base?
What is the resultant collector current?
What is the voltage drop across R3? Subtract this from 12V to get Vc.

Remember Ohms law, V = I * R

Duke
 

foTONICS

Sep 30, 2011
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no current/voltage at base

There is no base current and the transistor is biased into cutoff. this is why I am so confused.

This lab has me calculate the voltages without a Vin. is there a way to do this or should I be speaking with my professor?
-Thanks again
 

duke37

Jan 9, 2011
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You seem to have the answers, why not give them here?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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There is no base current and the transistor is biased into cutoff

This is true. Now do the calculations based on that (they're simple ;))

this is why I am so confused.
You have determined the conditions, just follow through. the implications are unusual for an amplifier, but just see what you get.

This lab has me calculate the voltages without a Vin. is there a way to do this or should I be speaking with my professor?
You've almost done it.

Do the calculations and then tell us if you think you're right, and why you're confused.

Once you do the calculations we can help you by telling you if you're on the right track or if you've made a mistake. Perhaps we can also reassure you about the operaing conditions of a class C amplifier.
 

Laplace

Apr 4, 2010
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There is no base current and the transistor is biased into cutoff. this is why I am so confused.

It might improve your confidence to realize that the answer is so obvious it is almost a trick question.
 

foTONICS

Sep 30, 2011
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well if its in cutoff then the transistor should act like an open, making VE zero

if its in cutoff then VB should also be zero

so if I measure VC should it get the entire 12V, or is there a voltage divider between the tank cct and R3?
 

duke37

Jan 9, 2011
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Well done, you have it.
There is no resistance specified for the IF circuit and in any case, the current is zero as you determined.
 
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