effect of cascading LNAs
 

I am wondering what's the bad effect of cascading a lot of low noise
amplifiers? It seems that if I devide 90dB into 4 LNAs I will have much
less noise figure. Can I use 5 LNAs in series? Any disadvantage about
this way? Thanks.

effect of cascading LNAs
 

wrote in message
ups.com...
I am wondering what's the bad effect of cascading a lot of low noise
amplifiers? It seems that if I devide 90dB into 4 LNAs I will have much
less noise figure. Can I use 5 LNAs in series? Any disadvantage about
this way? Thanks.

If your 1st LNA has the lowest noise figure and sufficient gain, it will set
the noise floor.
You can work the cascaded amplifier equations and see what you will need.
90dB seems like a LOT of front end gain- unless this is very narrow band, I
would be very concerned about overload resulting in all kinds of in band
garbage.

Dale W4OP

effect of cascading LNAs
 

What do you mean by overload? How come there is too much garbage as
long as my NF is low?

I actually need 143dB gain. What kind of problem I will have if I just
use 6 stages of amplifiers, like 1stage(20dB, NF2.4dB) 2nd stage(30dB,
NF3db), 3rd stage(43dB, NF 6.2dB), 4th stage(43dB NF6.2dB). It seems my
NFtotal is pretty low. What's my problem?

Thanks.

Dale Parfitt wrote:
wrote in message
ups.com...
I am wondering what's the bad effect of cascading a lot of low noise
amplifiers? It seems that if I devide 90dB into 4 LNAs I will have much
less noise figure. Can I use 5 LNAs in series? Any disadvantage about
this way? Thanks.

If your 1st LNA has the lowest noise figure and sufficient gain, it will set
the noise floor.
You can work the cascaded amplifier equations and see what you will need.
90dB seems like a LOT of front end gain- unless this is very narrow band, I
would be very concerned about overload resulting in all kinds of in band
garbage.

Dale W4OP

effect of cascading LNAs
 

On 11 Oct 2006 11:38:03 -0700, wrote:

What do you mean by overload? How come there is too much garbage as
long as my NF is low?

I actually need 143dB gain. What kind of problem I will have if I just
use 6 stages of amplifiers, like 1stage(20dB, NF2.4dB) 2nd stage(30dB,
NF3db), 3rd stage(43dB, NF 6.2dB), 4th stage(43dB NF6.2dB). It seems my
NFtotal is pretty low. What's my problem?

Your problem is you don't know what you're doing.

What frequency is this oscillator going to run at?

effect of cascading LNAs
 

I asked what I don't know. I am expecting a helpful answer.

The frequency is 70MHz.


Wes Stewart wrote:
On 11 Oct 2006 11:38:03 -0700, wrote:

What do you mean by overload? How come there is too much garbage as
long as my NF is low?

I actually need 143dB gain. What kind of problem I will have if I just
use 6 stages of amplifiers, like 1stage(20dB, NF2.4dB) 2nd stage(30dB,
NF3db), 3rd stage(43dB, NF 6.2dB), 4th stage(43dB NF6.2dB). It seems my
NFtotal is pretty low. What's my problem?

Your problem is you don't know what you're doing.

What frequency is this oscillator going to run at?

effect of cascading LNAs
 

I asked what I don't know. I am expecting a helpful answer.

The frequency is 70MHz.


As Dale gave you an answer that is correct, you might reason with him, I
recognize that the last answer wasn't very helpful.

Basically, with 20 dB of gain, your noise figure will be well established
with the NF of that first stage. It will actually deteriorate a tenth of a
dB or so for the further stages down the line, but not very much.

If you need additional gain, it's best obtained after some selectivity. 140
plus dB of gain on a single frequency is going to be VERY difficult to tame
and keep from oscillation in it's own right.

W4ZCB

effect of cascading LNAs
 

I asked what I don't know. I am expecting a helpful answer.

The frequency is 70MHz.


As Dale gave you an answer that is correct, you might reason with him, I
recognize that the last answer wasn't very helpful.

Basically, with 20 dB of gain, your noise figure will be well established
with the NF of that first stage of 2.4 dB. It will actually deteriorate a
tenth of a
dB or so for the further stages down the line, but not very much. The second
stage could have as great a noise figure as 10 dB and the total would only
deteriorate to 2.62 dB

If you need additional gain, it's best obtained after some selectivity. 140
plus dB of gain on a single frequency is going to be VERY difficult to tame
and keep from oscillation in it's own right.

W4ZCB

effect of cascading LNAs
 

It seems that I know the Friis equation.

I am asking because it seems difficult, and maybe impossible. I've
never heard of this high gain before, but I dont' know why I cann't
stack amplifiers and I am looking for somebody who can tell me what
exactly the problem is: you see the NF is nice, and I can get the SNR
if the band is narrow enough.

It also seems that I "don't know what I'm doing". So please explain a
little if you want me to understand. Thanks.

Harold E. Johnson wrote:
I asked what I don't know. I am expecting a helpful answer.

The frequency is 70MHz.


As Dale gave you an answer that is correct, you might reason with him, I
recognize that the last answer wasn't very helpful.

Basically, with 20 dB of gain, your noise figure will be well established
with the NF of that first stage. It will actually deteriorate a tenth of a
dB or so for the further stages down the line, but not very much.

If you need additional gain, it's best obtained after some selectivity. 140
plus dB of gain on a single frequency is going to be VERY difficult to tame
and keep from oscillation in it's own right.

W4ZCB

effect of cascading LNAs
 

wrote in message
ps.com...
It seems that I know the Friis equation.

I am asking because it seems difficult, and maybe impossible. I've
never heard of this high gain before, but I dont' know why I cann't
stack amplifiers and I am looking for somebody who can tell me what
exactly the problem is: you see the NF is nice, and I can get the SNR
if the band is narrow enough.

It also seems that I "don't know what I'm doing". So please explain a
little if you want me to understand. Thanks.

Well, how about a little additional information? What is it that you're
trying to accomplish? A gain of 140 dB would amplify a signal of strength 8
x 10^-10 Watts (About the equivalent of a received signal of 10 dB over
"S"-9) to a kiloWatt. I don't think you're trying to do that, so what
convinces you that you NEED 140 dB of gain?

Regards
W4ZCB

effect of cascading LNAs
 

I'm dealing with a DSSS current signal at 70MHz. Before I start
designing anything, I am calculating the signal amplitude. It's going
to be 5nArms. If I have a 50ohm resistor, it's -153dBm. I'd be happy to
know any solid proof that it's not doable because it's my job now.

In theory DSSS signal can work under noise level, but can it be so
much?


Harold E. Johnson wrote:
wrote in message
ps.com...
It seems that I know the Friis equation.

I am asking because it seems difficult, and maybe impossible. I've
never heard of this high gain before, but I dont' know why I cann't
stack amplifiers and I am looking for somebody who can tell me what
exactly the problem is: you see the NF is nice, and I can get the SNR
if the band is narrow enough.

It also seems that I "don't know what I'm doing". So please explain a
little if you want me to understand. Thanks.

Well, how about a little additional information? What is it that you're
trying to accomplish? A gain of 140 dB would amplify a signal of strength 8
x 10^-10 Watts (About the equivalent of a received signal of 10 dB over
"S"-9) to a kiloWatt. I don't think you're trying to do that, so what
convinces you that you NEED 140 dB of gain?

Regards
W4ZCB