# class C bias question

Discussion in 'Electronics Homework Help' started by foTONICS, Sep 30, 2011.

1. ### foTONICS

332
9
Sep 30, 2011
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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2. ### duke37

5,364
772
Jan 9, 2011
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

3. ### foTONICS

332
9
Sep 30, 2011
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

4. ### jackorocko

1,284
1
Apr 4, 2010
are you looking to much into this?

5. ### duke37

5,364
772
Jan 9, 2011
You seem to have the answers, why not give them here?

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,501
2,841
Jan 21, 2010
This is true. Now do the calculations based on that (they're simple )

You have determined the conditions, just follow through. the implications are unusual for an amplifier, but just see what you get.

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.

7. ### Laplace

1,252
184
Apr 4, 2010
It might improve your confidence to realize that the answer is so obvious it is almost a trick question.

8. ### foTONICS

332
9
Sep 30, 2011
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?

9. ### duke37

5,364
772
Jan 9, 2011
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.

10. ### foTONICS

332
9
Sep 30, 2011
thanks for the guidance everyone