# Analogue electronics- signal to noise ratio

Discussion in 'Homework Help' started by H3isenberg, Nov 8, 2013.

1. ### H3isenberg Thread Starter New Member

Nov 8, 2013
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0
If i'm given the noise voltage at the output of an amplifier as 30mV but there is no input signal present how do I calculate the Signal to noise ratio?
I know I need to use the equation snr=log20(Vout/Vin)
but due to there being no input im confused
I attempted log20(30mV/0) however this gives a syntax error on my calculator
any help or advice on where I am going wrong would be greatly appreciated

Last edited: Nov 8, 2013
2. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
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You would need to know what the normal output of the amplifier is (or is rated for). 30mV of noise would be really annoying if the normal output level is 100mV, but would be much less annoying if the normal output is 100V.

3. ### H3isenberg Thread Starter New Member

Nov 8, 2013
5
0
okay the usual input is 100mv. And i have 3 different output values dependant on my frequency do i calculate the ratio using one of these output values?
i was unsure because the question stated no input signal was present.

Last edited: Nov 8, 2013
4. ### WBahn Moderator

Mar 31, 2012
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4,917
If you are looking for a "typical" SNR for that amplifier, then you would need to know what the "typical" input signal level is for that amplifier.

If you are looking for the SNR under those specific conditions, then the SNR is 0 (-∞ dB).

5. ### H3isenberg Thread Starter New Member

Nov 8, 2013
5
0
i probably should have included everything all at once sorry!!

Okay to recap and hopefully make clearer
i have an amplifier usually supplied with an input voltage of 100mv
dependant on frequency the output voltage varies.
the question says- The noise voltage at the output of an amplifier is measured to be 30mv when no input signal is present. calculate the signal to noise ratio in dB?
Give a reason for this noise and suggest ways of reducing it

I know i need to use 20log(Vout/Vin)
i suggested 0 as an answer to my tutor and he said it was wrong.
As for the reason i think this is caused by mains hum.

Sorry again for not being clearer i hope this is more helpful

6. ### daviddeakin Active Member

Aug 6, 2009
207
27
If it produces 30mV of noise when there is no signal present, then you may assume it is still producing 30mV of noise when there is a signal present.

Have you been told the gain of the amplifier? Work out what the output voltage will be when the 100mV input is present (it is amplified by the amplifier), then use this figure to find the SNR.

7. ### WBahn Moderator

Mar 31, 2012
18,088
4,917
In order to calculate the SNR you need the usual OUTPUT signal level, since you have the noise at the OUTPUT. Do have the nominal gain of the amplifier?

What is your basis for suspecting "mains hum" as your noise source?

8. ### H3isenberg Thread Starter New Member

Nov 8, 2013
5
0
Mains hum came to mind because it causes vibrations in the circuit and is caused by electromagnetic fields. it just seemed like a viable option at the time of typing. Unfortunately i am not given a nominal gain on the amplifier. However from the values given in an earlier section of the question i had to calculate the gain when the amplifier was set to different frequencies. for this part i was given the output voltage and the input was 100mV. so i have three values over three different frequencies all at an input of 100mV

9. ### H3isenberg Thread Starter New Member

Nov 8, 2013
5
0
I haven't been told the gain of the amplifier however in an earlier part to this question i was given 3 values of output voltage each at different frequencies and had to work out the gain using an input of 100mV. These values of gain all worked out to be similar values so should i just take this as my amplifiers gain? in doing so do i then use the output voltage when the 100mv input is present as my input when no input is actually present

10. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,507
512
Normally in situation like this, the very FIRST noise source you tell your teacher is Thermal Noise.

The basic definition of Thermal Noise==10*log(k*Te) in dBm/Hz

k is Boltzmann constant

Te is effective temperature in Kelvin. Normal Te is 290 Kelvin (at or near room temperature)

So. Suggest ways to reduce this noise. Easy! Reduce the effective temperature (Te). Instead of running your amplifier at room temperature where Te==290 Kelvin, run it at lower temperature.