Why do we amplify signals at the receiver side?

Hi,

Why do we amplify the received signal at the receiver side? I mean, the noise will be amplified too. I mean, SNR will be the same before and after amplification right?

Thanks

You amplify signals, because your A-to-D converter doesn't have enough resolution to "see" a typical, SMALL incoming signal, say around -90 dBm (which is about 20 microvolts, pk to pk). Most receivers have a conversion gain on the order of 50-100 dB, so that brings your signal up to a range of -40 to +10 dBm (6.3 mVpp to 2.0 Vpp). These voltages are much more "receiver-friendly", in terms of amplitude.

For a single amplification stage, yes... you will retain the same SNR using an ideal amplifier. However, amps are hardly ideal and will induce their own noise, thus degrading the SNR from input to output. This ratio is called the noise figure, or NF, of a device. To improve the SNR of your received signal, you can amplify and mix your signal to a fixed IF, then run it through a hard bandpass filter to drop the noise floor back down to near ambient conditions. That will improve your output SNR, compared to the original incoming signal.

enjunear said:

You amplify signals, because your A-to-D converter doesn't have enough resolution to "see" a typical, SMALL incoming signal, say around -90 dBm (which is about 20 microvolts, pk to pk). Most receivers have a conversion gain on the order of 50-100 dB, so that brings your signal up to a range of -40 to +10 dBm (6.3 mVpp to 2.0 Vpp). These voltages are much more "receiver-friendly", in terms of amplitude.

For a single amplification stage, yes... you will retain the same SNR using an ideal amplifier. However, amps are hardly ideal and will induce their own noise, thus degrading the SNR from input to output. This ratio is called the noise figure, or NF, of a device. To improve the SNR of your received signal, you can amplify and mix your signal to a fixed IF, then run it through a hard bandpass filter to drop the noise floor back down to near ambient conditions. That will improve your output SNR, compared to the original incoming signal.

Click to expand...

Thank you for replying. We can also enhance the SNR by introducing pre-amplifier, where the composite noise figure is low compared with the amplifier alone, right?

Someone asked me, why we do not use pre-amplifier at the transmitter side, and I am not sure, but I guess because there is no noise at the transmitter to enhance the SNR by introducing a pre-amplifier, since the noise is injected at the receiver end beginning from the receive antenna. Is this answer true?

Thanks in advance

David83 said:

Thank you for replying. We can also enhance the SNR by introducing pre-amplifier, where the composite noise figure is low compared with the amplifier alone, right?

Someone asked me, why we do not use pre-amplifier at the transmitter side, and I am not sure, but I guess because there is no noise at the transmitter to enhance the SNR by introducing a pre-amplifier, since the noise is injected at the receiver end beginning from the receive antenna. Is this answer true?

Thanks in advance

Click to expand...

The initial receive amplifier, by itself, will only degrade your SNR. If the amplifier has 20 dB of gain, it'll increase the desired signal by 20 dB, and it will add 20 dB to the noise (which are simply random signals that make up the "noise floor"). Additionally, a real pre-amp will add some noise that it self-generates, thus raising the noise floor even more, which degrades the SNR. So the desired signal goes up by 20 dB, but the noise floor comes up 22 dB.... a 2 dB reduction to your output SNR. You can't improve SNR with JUST an amplifier, physics says you can't get better than 0 dB of SNR "improvement".

Introducing an LNA and filter will allow you to improve the SNR of a signal as it flows through your receiver chain, since you increase the signal level, then filter off the broadband noise.

SNR doesn't mean much on the TX side. Transmitter specs typically call out things like broadband noise level (shall be no greater than -XX dBm at > YY MHz from the carrier), and adjacent channel power (shall be no greater than XX dB below the carrier signal level). This is commonly how transmitters are specified, in regard to the noise they produce. Generally, transmitter specs define how much power you are going to be dumping into the neighboring channels... that makes the most sense if you think about a multi-user environment.

enjunear said:

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SNR doesn't mean much on the TX side. Transmitter specs typically call out things like broadband noise level (shall be no greater than -XX dBm at > YY MHz from the carrier), and adjacent channel power (shall be no greater than XX dB below the carrier signal level). This is commonly how transmitters are specified, in regard to the noise they produce. Generally, transmitter specs define how much power you are going to be dumping into the neighboring channels... that makes the most sense if you think about a multi-user environment.

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I am sorry is this confirming my answer or oppose it? I mean can we use a pre-amplifier at the transmitter side?

Thanks

David83 said:

I am sorry is this confirming my answer or oppose it? I mean can we use a pre-amplifier at the transmitter side?

Thanks

Click to expand...

SNR is a measurement that is useful when talking about receivers, but it's nearly useless on transmitters because of the other noise specs that one typically has to adhere to. Those other specs drive the output RF signal characteristics much more so than simply measuring an SNR value.

An amplifier is an amplifier; there is no such thing as a "pre-amplifier"... it's simply another amplification stage before your finals.

enjunear said:

SNR is a measurement that is useful when talking about receivers, but it's nearly useless on transmitters because of the other noise specs that one typically has to adhere to. Those other specs drive the output RF signal characteristics much more so than simply measuring an SNR value.

An amplifier is an amplifier; there is no such thing as a "pre-amplifier"... it's simply another amplification stage before your finals.

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yes you are right, a pre-amplifier is an amplifier before (pre-) an amplifier. So we can say that there is an amplifier at Tx that amplifies the signal to meet some specifications. Right?

Thanks

David83 said:

yes you are right, a pre-amplifier is an amplifier before (pre-) an amplifier. So we can say that there is an amplifier at Tx that amplifies the signal to meet some specifications. Right?

Thanks

Click to expand...

In the vast majority of radios I've ever seen, yes, there is an amplification stage on the transmit side of the system. Typical specs of concern on the transmitter are gain, output power, linearity, distortion, broadband noise, and adjacent channel power. There are others, but those are the key ones you'll see over and over, in various forms.

Now the question is: why not to use one stage amplifier with good noise figure instead of multiple stages at the receiver side?

David83 said:

Now the question is: why not to use one stage amplifier with good noise figure instead of multiple stages at the receiver side?

Click to expand...

That really depends on the architecture of your RF system... the amount of receive gain you need, how much AGC, selectivity, noise figure, etc. Typically, you can't satisfy all of these needs with one device, so you use a couple gain stages, a filter, a mixer, another filter, another gain stage, etc. All cascaded together, the chain of devices can meet a broad variety of specs, but it's pretty much impossible to do with a single device (again, it depends on the performance needs of the system).

That was very helpful. Thanks a lot.