amplifier question

[gjpollitt]

I am studying amplifiers from a book and at the end of the chapter are questions. One that I am stuck on is

If an amplifier has a power gain of 25 and input and output resistance of 600ohms what is the input voltage if the output voltage is 10v?

Now the answer in the book is 2v but I cannot locate anywhere how to calculate this correctly.

Power gain is pVout * pIout / pVin * pIin so I thought that by using ohms law for the output I could do

pIout=Vout / Rout which is 600ohms = 10/600 = 17mA

The formula for current gain is Iout / Iin, I know the Iout (17mA) but cant calculate Iin as I only know the 600ohm input resistance and not the voltage.

Am I going the correct way about this and if so what is next please?

Thanks
Graham

 

[bloki]

Answer is:
Pout=Vout^2/600
Pin=Pout/A
Vin=sqrt(Pin*600)
-----------------------
Pout=10^2/600=0.166666666W
Pin=0.1666666666/25=0.0066666666W
Vin=sqrt(0.00666666*600)=sqrt(4)=2V

 

[gjpollitt]

thanks