# voltage gain in decibels

Discussion in 'Homework Help' started by djstar, Jun 29, 2008.

1. ### djstar Thread Starter Active Member

Jan 26, 2008
39
0
could some one please give me a helping hand with this question

Q2) A circuit consists of a 1.5v battery and connected to it, from the positive terminal, is a 15 Ω resistor, followed by 2 resistors in parallel, whose junction back to the battery negative. Find the voltage gain or loss in decibels across the parallel network.

what what i can understand i have to use the formula gain = 20log10 v2/v1 to get the voltage gain but where do i get the voltages from? do i have to no what the values of the other resistors are? any help would be very much apreciated. liam

2. ### Tesla00010010 Member

Jun 20, 2008
21
0
if you are talking about voltage gain in decibels,u should consider the point that u can express it on DBV units or DBu units.The v1 in the formula is the "reference"voltage used in the logaritmic scale,if u are using DBV this v1 is 1 and if you are using DBu is 0,775.Why 0.775?i don't remember well the story but i think it has to do something about the resistance of the old telephonic lines.Anyways,just get the voltage in the parallel network and use the formula either using DBu or DBV and the v1 reference for any of them.The final value will be the same because you are just using different scales to measure the same thing.

3. ### Tesla00010010 Member

Jun 20, 2008
21
0
the reference voltages are just chosen arbitrary to start measuring the scale from that point.For example,decibels are used a lot in acoustic studies and its a similar formula but using pressure levels instead of voltages,and the "reference" sound pressure level is the lowest pressure that we can feel.You can talk about dBm too that means level of power in an electric circuit.Then,for voltages as i said there are two scales and both have different references values(1 for DBV and 0.775 for DBu)

4. ### JoeJester AAC Fanatic!

Apr 26, 2005
4,074
1,776
20 Log(Vout/Vin)

It asked for the ratio, not referenced to an absolute (dBm, dB/uV, dBfW, or dBV).

5. ### djstar Thread Starter Active Member

Jan 26, 2008
39
0
thanks for the replies, im still not to sure how to work out v2 . i take it that v1= 1.5v ?

6. ### Ron H AAC Fanatic!

Apr 14, 2005
7,012
681
If you don't know the values of the other two resistors, you will have to express the answer algebraically.
V1 is apparently 1.5V.