# Power Amplifier Query

Discussion in 'Homework Help' started by VaLieN7e, Jan 25, 2013.

1. ### VaLieN7e Thread Starter New Member

Jan 25, 2013
5
0
Hi everyone , this problem has puzzled for quite some time now
it's actually about a previous exam in my college , goes as the following :

for the figure shown , determine the turn ratio of the output transformer , give that ( Vcc = 10V , RL = 8 ohm , maximum current that the transistor can handle is Ic = 2A , maximum peak value Ic' = 1.9A )

we haven't had many experiences with transformers calculation , but what i know is Ic(max) = Vcc/Rc & Ip(load) = Np/Ns Ip
i can't seem to find the right formula to connect all these aspects :\

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2. ### Audioguru New Member

Dec 20, 2007
9,411
896
You have a very old antique exam.
Power amplifiers have not used transformers since about 50 years ago.

Look at Transformer in Google to see that the impedance ratio is the turns ratio squared.

3. ### VaLieN7e Thread Starter New Member

Jan 25, 2013
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i'm aware of that
but i thought someone could help me with it :\

4. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
It has been a while, but with transformers it is the loading that matters, the rest is not as significant. You are matching impedances, which in turn determines the loading. So if you have 8Ω you need to figure out the load requirements on the other side.

5. ### VaLieN7e Thread Starter New Member

Jan 25, 2013
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so if the load requirements aren't mentioned ( as in this case )
the turn ratio can't be calculated , right ? we need at least the current or the voltage of the load , please correct me if i'm wrong ^^

6. ### #12 Expert

Nov 30, 2010
16,685
7,324
Yes. You have a serious lack of data. For instance, those transistors could power that speaker directly, no transformer needed, if the specifications were right.

The statement 2NP/NS might suggest that the number of turns in each half of the primary is equal to the number of turns in the secondary. That would suggest a 1:1 ratio if you think in terms of a half wave going through 100 turns of primary, and that is coupled to 100 turns of secondary. Then again, I could be entirely wrong. That often results from trying to guess what a cryptic label means.

7. ### VaLieN7e Thread Starter New Member

Jan 25, 2013
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totally agree with you #12
i'm starting to feel that the whole problem is absurd and unrealistic .

8. ### VaLieN7e Thread Starter New Member

Jan 25, 2013
5
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my sincere thanks to everyone ,
i was finally able to solve it and now i can sleep peacefully
vcc = ic / rc , and the relation between rc and the transformer was the key to solution .

9. ### crutschow Expert

Mar 14, 2008
13,496
3,373
Assume you want to transfer the maximum power to the load. Then for a 10V supply and 1.9A peak current, the peak available power is 19W. To transfer that to an 8 ohm load requires a peak load current of 1.54A. Thus you need a transformer turns ratio that converts 1.9A in the primary to 1.54A in the secondary.

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