High amlification RF amplifier chips

[buenos]

I always wondered how these 2 things seem to contradict:
-all or almost all RF amplifier chips have a 10-20dB Gain
-Many equipment like cell phones or anything running from an antenna need to reliably use -80dBm...-120dBm signals.
I don't think they have 5 LNA in a chain, but then, how do they amplify 120dB?

I am building some test equipment that has to pick up a -110dBm 100MHz signal that comes from Mega Ohm impedance and have to amplify it to maybe 1V. Where should I start? Most high frequency amplifiers have on-chip 50R termination that makes it useless for my application.

 

[godfreyl]

I am building some test equipment that has to pick up a -110dBm 100MHz signal that comes from Mega Ohm impedance...

That Mega Ohm source impedance doesn't sound right. Just 1pF of capacitance has an impedance 600 times lower than that at 100MHz.

 

[BigBoss]

1Vrms signal level =13.01 dBm so amplification should be minimum 120+13.01=133dB !!!
This amplification is impossible with a classical amplifier chain. ( Multiple LNAs)
Instead, you can amplify this signal to certain level with 2-3 stages of LNA then you will able to use High Gain OPAMPs at 100MHz.
For instance..
Your signal level=-120dB
First 3 stages Gain=45dB
So, OpAmps Input Level will be around -75dBm that is also equal to 39.76uV@50Ohm Input Impedance ( You should do this math with real OpAmp/similar devices).
This level can be amplified with-for instance-Instrumentation Amplifiers.
My calculation is based on 50 Ohm terminations but it will be of course different in according with your Instrumentation Amplifiers which you intend to use..

 

[buenos]

Does such thing exists as "high input impedance LNA"? The only ones I found have on chip 50R termination resistance.
How do they do this with mobile and GPS applications where they also deal with -120dBm. I know these have low impedance antennas, not Mega Ohm sources, but still it is a lot of amplification.

 

[BigBoss]

How do they do this with mobile and GPS applications where they also deal with -120dBm.

Those systems convert RF signal to IF signal for easier amplification..

 

[buenos]

Those systems convert RF signal to IF signal for easier amplification..

They do the down conversion at a low signal level? How low?
I use 100MHz in this project, which might be considered IF in some systems.

 

[jiripolivka]

The secret of low RF input is in the SYSTEM bandwidth. This is done in the IF where also the high gain amplifiers are used.

LNAs are most designed for 50 Ohm impedance as all test equipment runs with 50 Ohms. Why would one want to use a high impedance at 100 MHz? If needed, use RF transformers. Look at MIni Circuits, they offer a nice assortment.

You can read some old textbooks where vacuum tubes were used, most had high impedance in/out.

 

[FvM]

Some 1000 ohms impedance level at 100 MHz can be achievd in discrete electronic circruits, higher impedances only in micro circuits and on a chip level.

For the time being, I assume that the "Mega ohm impedance" source is the result of calculation errors or unrealistic assumptions.

 

[buenos]

Actually it comes from a photodiode. I have considered transimpedance amplifiers too, but they might not perform very well in this particular application. I have tried. The best signal that I could get out of it was with a resistor bias (4.7MR) and an AC coupled OPAMP. Basically the 100MHz signal is received with a lot of ambient light that can be dark (night) or bright (day).

 

[thylacine1975]

When it comes to high bandwidth photodetectors, a friend of mine produced a paper a few years ago that used exactly what you feared in your OP... a cascade (of 4 MMIC gain blocks to achieve wideband amplification of low level optical signals. His paper is here: https://seit.unsw.adfa.edu.au/research/quantelec/pdf/RevSciInst_77_114701.pdf. It works exceptionally well, but fabricating the design such that it didn't want to oscillate was quite a challenge!

If the optical signal you are trying to detect is coherently related to another with greater power (thus practically implying it is derived from a laser), you might be able to exploit homo/heterodyne detection techniques to improve your sensitivity?

 

[FvM]

To achieve high bandwidth with a photodiode, it has to be operated as current source with low impedance load, e.g. a transimpedance amplifier. In some cases (e.g. GHz bandwidth requirement), a 50 ohm amplifier can serve the purpose, as suggested by thylacine1975. In so far the original problem description is misleading.

For medium bandwidth photodiode amplifiers (e.g. 100 MHz), the noise current density of usual 50 ohm matched LNAs is probably too high.

 

[buenos]

In the article they have a 50R resistor in series with the PD. In my tests with average ambient light I get VCC/2 on the divider point if I use a 4.7MR resistor. I think that is needed to maximize the AC signal that we ge out of this. So far I found that I could make a first stage amplifier like one of these:
-use a fiber optic TIA like PHY1095
-use a VGA with high impedance input like LMH6505
-use an opamp like THS4275 in voltage follower (G=1) mode.