LNA..how it works

Hi All,
         I have few questions.

1. How does LNA helps in reducing the noise

LNA adds a gain K
Vout = K*Vin

Few answers on above equation (please confirm)
1. Vin shoud be composed of both signal voltage + noise voltage
2. the Noise figure of the LNA is very small that it almost acts as a
passive device. By the way, LNA is a passive device or active device.
3. The K is so high that it takes care of Noise added to Vout in later/
subsequent stages (stages after LNA).

Please explain how exactly the LNA handles the noise..

Thanks
Particlereddy

Hi,

LNA stands for "low noise amplifier". It is an analog circuit.

It amplifies the signal, so that an additive noise contribution from later
stages looks smaller. 

It does not "reduce" the noise that is already present at the input, it
gets amplified in the same way as the input signal.

You'd model it as
signal = signal + (input-referred LNA noise contribution)
signal = signal * (LNA gain)
signal = signal + (noise contribution from the rest of the receiver)

The higher the LNA gain, the smaller the impact of the latter noise term.

mnentwig <markus.nentwig@n_o_s_p_a_m.renesasmobile.com> wrote:

> LNA stands for "low noise amplifier". It is an analog circuit.

This I believe is true.

> It amplifies the signal, so that an additive noise contribution 
> from later stages looks smaller. 

OK, but does that really explain why it is called low noise?
Presumably no-one will use an HNA or even an ONA (ordinary
noise amplifier), so why the need to emphasize the low noise?

> It does not "reduce" the noise that is already present at the
> input, it gets amplified in the same way as the input signal.

My understanding from when I first heard about them was that it
was related to thermal noise.  That a low-noise amplifier has less
noise than a resistor with the same impedance as the load, and with
the resistor at ambient temperature.  (That is, the same temperature
as the important parts of the amplifier.)

That is as far as I remember it, though.

-- glen

>mnentwig <markus.nentwig@n_o_s_p_a_m.renesasmobile.com> wrote:
>
>> LNA stands for "low noise amplifier". It is an analog circuit.
>
>This I believe is true.
>
>> It amplifies the signal, so that an additive noise contribution 
>> from later stages looks smaller. 
>
>OK, but does that really explain why it is called low noise?
>Presumably no-one will use an HNA or even an ONA (ordinary
>noise amplifier), so why the need to emphasize the low noise?
>
>> It does not "reduce" the noise that is already present at the
>> input, it gets amplified in the same way as the input signal.
>
>My understanding from when I first heard about them was that it
>was related to thermal noise.  That a low-noise amplifier has less
>noise than a resistor with the same impedance as the load, and with
>the resistor at ambient temperature.  (That is, the same temperature
>as the important parts of the amplifier.)
>
>That is as far as I remember it, though.

People mostly do use ordinary noise amps, because LNAs cost more. At the
front end, with a rather weak signal, people are generally prepared to pay
the premium. However, the most visible feature of most LNAs is they sit
very close to the receiving antenna, so they amplify the signal before it
has any opportunity to be attenuated by the cabling. You can't beat kTB,
but you can do your best to amplify where the signal is well above it.

Steve

I don't think there can be any clear-cut definition what performance is
required to qualify as "low" noise amplifier: On a hot day, my LNA will
unavoidably become more noisy, but it doesn't stop being "the LNA".
Instead, it hints at its role: "LNA" => first amplifier after the antenna.

Simply treat "LNA" as a noun without worrying about the words. 

PS: no such rule without exceptions, although I can't think of any good one
at the moment.

On Mon, 02 Jan 2012 22:30:57 -0800, aizza ahmed wrote:

> Hi All,
>          I have few questions.
> 
> 1. How does LNA helps in reducing the noise
> 
> LNA adds a gain K
> Vout = K*Vin

Here is where you are in error.  If V_in is the voltage you want to 
amplify, then

V_out = K * (V_in + V_an),

where V_an is the noise from the amplifier, referred to the amplifier 
input.  V_an varies with the amplifier design and the devices used -- an 
LNA is designed (and uses parts) specifically to hold V_an down.

> Few answers on above equation (please confirm)

> 1. Vin shoud be composed of both signal voltage + noise voltage

Yes, but you need to distinguish between the ambient noise that's "out 
there" and the noise of the amplifier itself.

> 2. the Noise figure of the LNA is very small that it almost acts as 
> a passive device.

That makes no sense.  Passive devices have noise figures, too.

> By the way, LNA is a passive device or active device.

If you mean it _can_ be passive or active, then no -- amplifiers are 
active.  If you mean that it _is_ one of either the set of all passive 
devices or the set of all active devices -- well, yes, but you can say 
that about any device.

> 3. The K is so high that it takes care of Noise added to Vout in
> later/ subsequent stages (stages after LNA).

Only if you've designed your system correctly.

> Please explain how exactly the LNA handles the noise..

It has low noise itself, and it amplifies enough to significantly reduce 
the noise contribution of following stages.  It doesn't "handle" noise in 
any way, shape, or form -- it just shouts down noise from later stages.

-- 
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

>> 2. the Noise figure of the LNA is very small that it almost acts as 
>> a passive device.
>
>That makes no sense.  Passive devices have noise figures, too.

Very much so. The noise from a passive source can dominate in many
electronics application. For example, a studio mic produces more AWGN than
the studio pre-amp it is connected to.

Steve

On Jan 3, 9:29=A0am, "mnentwig"
<markus.nentwig@n_o_s_p_a_m.renesasmobile.com> wrote:

> Simply treat "LNA" as a noun without worrying about the words.


Alas, my DirecTV antenna contains LNBs where
LNB =3D Low Noise Block amplifier because these
amplify the signal for a large number (block) of
TV channels (i.e. the signal being amplified is broadband)
as opposed to an LNA in an ordinary TV set which
might be amplifying the signal for just one TV channel.
Thus, kTB as someone (steveu?) said is something
that one cannot get away from is even more of
an issue here.

>> for a large number (block) of TV channels 

Both approaches are rather common.

A typical cellphone receiver in a handset amplifies the whole band with
minimum selectivity before the first amplifier. 

A cavity filter that can isolate a single GSM channel before the LNA with
tolerable losses has about the size of a watermelon. That's the way it's
done in some types of cellular basestations.

To address the OP's question:  An LNA just has been designed with the
idea of optimizing the Noise Figure of the device.   This is needed in
systems where gain is needed and good SNR (for sensitivity, whatever)
is needed.   Since later gain stages will further amplify whatever
noise comes out of the first amplifier, the very first amp in a
receiver will be optimized for low noise and good gain.   Hence, an
LNA.

On Tue, 3 Jan 2012 09:24:08 -0800 (PST), dvsarwate
<dvsarwate@yahoo.com> wrote:

>On Jan 3, 9:29=A0am, "mnentwig"
><markus.nentwig@n_o_s_p_a_m.renesasmobile.com> wrote:
>
>> Simply treat "LNA" as a noun without worrying about the words.
>
>
>Alas, my DirecTV antenna contains LNBs where
>LNB =3D Low Noise Block amplifier because these
>amplify the signal for a large number (block) of
>TV channels (i.e. the signal being amplified is broadband)
>as opposed to an LNA in an ordinary TV set which
>might be amplifying the signal for just one TV channel.
>Thus, kTB as someone (steveu?) said is something
>that one cannot get away from is even more of
>an issue here.

Usually LNB stands for Low Noise Block converter, where the C for
'converter' is just dropped.   In other words, it's an LNA and a
downconverter integrated together.    The idea was that it sits right
at the antenna so that does the LNA function, and it downconverts the
entire tuning block to a lower frequency that suffers less loss
through the cable to the receiver.   This also lowers the cost of the
receiver and makes the receiver a bit more generic, as LNBs can be
made for multiple air interface bands that convert down to a common
IF.

Most LNAs are fairly broadband to support tuning over a wide range.


Eric Jacobsen
Anchor Hill Communications
www.anchorhill.com