# Signal to noise ratio on an amplifier circuit calculation

Discussion in 'Homework Help' started by Nobby Nobbs, Sep 28, 2014.

1. ### Nobby Nobbs Thread Starter New Member

Jul 31, 2014
11
0
Hi all, I wonder if you can help point me in the right direction.
I have had an assignment returned with one error to the following question about an amplifier circuit.
we are given the input voltage, the output voltage and a noise voltage.

We are required to find the Gain in Volts G=Vout/Vin this gave 3.9V
The gain in decibels = 20log10(3.9) = 11.8dB

Then we require the SNR, I worked this out as SNR= 20log10( Vout/V noise) =
in this case 20log10 (3.7/0.0023)=64dB.

I was following an example in my work book that states.
A 1.5v audio signal contains a noise component of 4.2mV. the Signal to Noise ratio is then : SNR=20log10(1.5V/4.2mV) = 51dB

This is the part I have wrong, the tutors comment are that I should calculate the noise (in dB) and add that to the signal (gain). this will give the signal to noise ratio.

For this I would work out the noise at 0.0023V to be -52dB then add this to the Gain in dB so -52+11.8dB= -42dB

Is this last part correct have I understood the tutors comments, so the SNR will be -42dB? its just a very different answer to how I thought it should be worked out.

I only need to correct this one small question but I have got myself a little confused. p.s I am a noob of the highest order so some of the equations still make my head hurt.

Any advice you can offer is very much appreciated.

2. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,500
511
Vout/Vin means that you divide volts by volts, the volts cancel each other, therefore 3.9 does not have units. The fact that you don't know/understand this... is bad.

3. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,500
511
I have a question about example that you followed.

I was following an example in my work book that states.
A 1.5v audio signal contains a noise component of 4.2mV. the Signal to Noise ratio is then : SNR=20log10(1.5V/4.2mV) = 51dB.

It says that 1.5 volt audio signal contains noise of 0.0042 volts. Is 1.5 volts input signal? Is it 1.4958 volt input signal + 0.0042 volt noise signal = 1.5 volt going into Device X?

If my assumptions are correct, then you should take your Vin add 0.0023 volt noise and use that to find SNR.

4. ### Nobby Nobbs Thread Starter New Member

Jul 31, 2014
11
0
Thanks for the reply, the example from the book does not state input volts but also does not state it passes through an amplifier so I presume it is just a 1.5 volt signal that contains 0.0042 volts of noise.

For the question I was trying to answer I have.

An amplifier built in a workshop, the following measurements were taken.
Input voltage = 0.95v
Output voltage = 3.7v
Noise = 23mv

calculate the following

Gain in volts, Done this.
Gain in decibels, Done this.
Signal to noise ratio in decibels, this is the one I am having difficulty with, I used the equation above but this is wrong.

5. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,500
511
Let us assume that my earlier assumptions are correct.

Vin is 0.95 V
Noise is 0.023 V (23 mV)
Signal is 0.95+0.023=0.973 V
Then SNR is 20*log10(0.973/0.023)=32.53 dB

6. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
It would be useful if the OP could describe the details of the measurement procedure.
Was the noise measured at the output with the input signal set to zero, for instance?
Is the required result the SNR at the input or the output?
Was the information required the Noise Figure for the amplifier?

7. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,393
1,211
Here's a perfect op amp used to mix an audio signal and white noise with the gain the OP specified.

Maybe the OP can get a better prospective of S+N/N ratio.

File size:
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8. ### Nobby Nobbs Thread Starter New Member

Jul 31, 2014
11
0
Sorry been working away for a few days.
I have no more information it is just a question worded as below no other info given, this is all of it i have to get the answers from these 3 figures.
Question 1 and 2 I have done but the SNR has me confused as to how I get it from these 3 figures.

An amplifier was built in a workshop, the following measurements were taken.
Input voltage = 0.95v
Output voltage = 3.7v
Noise = 23mv

calculate the following

Gain in volts, Done this.
Gain in decibels, Done this.
Signal to noise ratio in decibels this one I am stuck on.

9. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,393
1,211
The signal plus noise to noise ratio ... plug that ratio into the dB formula where you plugged in the Av information. Av was the Vo to Vi ratio.

10. ### Lestraveled Well-Known Member

May 19, 2014
1,957
1,217
There are some definitions that are missing.

You said, " noise = 23mv" . this statement needs more definition.

- is 23mv of noise present at the input of the amplifier?
or
- is 23mv of noise present at the output of the amplifier?