Circuit, Amps & Attenuations

[sarah90]

Hi I have a question but am finding it difficult to answer:

A circuit consists of two amplifiers with gain 6.6 dB and 15.5 dB and two filters with attenuations of -10.4 dB and -2.8 dB. If the output voltage is 800 mV, the input voltage is.

I know I have to add 6.6 + 15.5 then subtract 10.4 - 2.8 = 8.9

but from then on Im lost.

 

[ericgibbs]

Hi I have a question but am finding it difficult to answer:

A circuit consists of two amplifiers with gain 6.6 dB and 15.5 dB and two filters with attenuations of -10.4 dB and -2.8 dB. If the output voltage is 800 mV, the input voltage is.

I know I have to add 6.6 + 15.5 then subtract 10.4 - 2.8 = 8.9

but from then on Im lost.

hi,
Wouldn't subtracting 2.8 from 10.4 mean that the 2.8dB filter had a gain of 2.8dB....?

 

[ronsimpson]

+6.6+15.5-10.4-2.8=8.9

 

[ericgibbs]

+6.6+15.5-10.4-2.8=8.9

hi Ron,
Its Homework..

Eric

 

[crutschow]

A circuit consists of two amplifiers with gain 6.6 dB and 15.5 dB and two filters with attenuations of -10.4 dB and -2.8 dB. If the output voltage is 800 mV, the input voltage is.

I know I have to add 6.6 + 15.5 then subtract 10.4 - 2.8 = 8.9

but from then on Im lost.

So do you know how to calculate a gain of 8.9dB? The dB to voltage-gain (ratio) conversion is 10^(dB/20).

 

[sarah90]

Hi im still confused as to how you get 2.8dB as gain

 

[ronsimpson]

hi,
Wouldn't subtracting 2.8 from 10.4 mean that the 2.8dB filter had a gain of 2.8dB....?

Look at what you said "subtract +10.4 and subtract -2.8) Subtracting -2.8 gives you +2.8. +2.8=-(-2.8)

 

[ericgibbs]

Look at what you said "subtract +10.4 and subtract -2.8) Subtracting -2.8 gives you +2.8. +2.8=-(-2.8)

This is what the OP said
I know I have to add 6.6 + 15.5 then subtract 10.4 - 2.8 = 8.9

This is what I said.
Wouldn't subtracting 2.8 from 10.4 mean that the 2.8dB filter had a gain of 2.8dB....?

No mention of a minus sign in my text., the HINT I was trying to give was that the two filter attenuations have to be added to give -13.2dB.
Then this 13.2dB is subtracted from the sum of the 6.6dB and the 15.5dB = 22.1dB - 13.2dB = 8.9dB

I was trying to get the OP to figure it out rather than just give the answer, which will not help her to solve future similar problems.

 

[KeepItSimpleStupid]

Yipes!

The idea someone was trying to get across is that two attenuators of -10.4 db and -2.8 db can't have an answer of 8.9 in the OP post #1. Two attenuators can't have gain meaning a mistake was made. The sign of that answer must be negative.

It doesn't matter whether what order you add and subtract, you will end up with some gain or loss in db. When you are adding in db, you are multiplying or dividing the Gain = A of each stage together.

At this point you know the output voltage and the total db gain.