# Characterization of RF Amplifier Chain

I am using a 3 Amplifiers chain for a Receiver system.

The required Output is -32dBm while the sensitivity of the system is -55 dBm, I am able to achieve this.

But the maximum I/P power for the system is 0 dBm while my amplifier chain goes into saturation after -20 dBm and If 0 dBm is fed to the amplifier chain. It will definately cross the max. I/P power rating of the third amplifier which is +15 dBm.

The maximum O/P power which is allowed is 20 dBm for the receiver. Gain of my single amplifier is between 10-13 dB.

So I need to increase my tolerance for the maximum I/P power without degrading my O/P power level and Minimum I/P power.

Looking for an easy solution? Can I use attenuators but the problem with attenuators is that when I solve the problem of max. I/P power it will decrease the sensitivity of the system above -55dBm.

• Google Automatic Gain Control. – RoyC Nov 28 '17 at 9:53
• Have my acclamation for having a short, concise flow chart of what's happening. Illustrates the question very well! – Marcus Müller Nov 28 '17 at 10:04
• Consider PIN diode attenuators. – analogsystemsrf Nov 29 '17 at 3:48

## 3 Answers

Well, that is an unsolvable problem, unless you either

• use active components that have the same gain, but higher saturation points, or
• dynamically change the gain (or dynamically add attenuation) based on your situation.

The second point is usually implemented by what is called AGC (automatic gain control), and will require you to design a control loop that is adjusted to the characteristics of your signal.

Also note that as a communications engineer, this problem is something you'll meet fairly often. A common solution is not to solve it: If the distortion caused by nonlinearity for very strong signals just makes them as good as "mediocre" signals, well, so be it, if you can still work with that. If you're in control of the power of the transmitter (and this isn't e.g. a radar system, where you effectively aren't), power control is a common thing to do (because more isn't always better).

Another option is to ignore the weak signals for now, and e.g. have multiple receivers, which, combining their receive signals, can achieve a processing gain sufficient to raise the weak signals "out of the noise floor".

But all these methods are really higher-level things that require understanding of the signals you're working with, so we can't help you with that at this point – the first thing to do when resolving receiver sensitivity/dynamic range issues is to write down a somewhat comprehensive system requirement.

Welcome to linearity hell....

What frequency are you working with, and what bandwidth?

You are only looking at 23dB of required gain, so do you really need three stages? Usually excessive front end gain is a much bigger problem then not enough, especially when dealing with fairly strong signals, -55dBm is a huge signal unless you are working with a very wide bandwidth.

What is your target noise figure?

Now your receiver dynamic range is 20dbm - -32dBm = 52dBm but your spec says you need 55dB of dynamic range (Incidentally are you sure your Rx does not have an input attenuator already +20dBm is at least 30dB more power then I would expect the usual sort of rx mixer to deal with with well), so you need some sort of variable gain stage in any event, skyworks have some fairly nice high IP3 attenuator parts that might work.

Given your evidently rather relaxed noise floor I would probably just throw the variable attenuator on in front of the amplifier chain, it hurts system noise performance by an amount equal to the insertion loss but reduces your chance of saturating the front end.

My starting point with this would be to take a long hard look at that recever design and see if I could not squeeze another 10dB of dynamic range out of it together with a more reasonable noise floor, -32dBm is not a reasonable noise floor for a receiver unless it is stupidly wide band, and 52dB between noise floor and saturation is fairly pathetic, as is needing +20dBm to the input, I would be throwing that design back and telling whoever drew it to come back with a design for a real radio.

Now the amplifier chain itself needs what is effectively a small power amplifier at the output (+20dBm is 100mW!), you cannot really do this with most mmic or such, as I say the recever spec looks improbable to me.

For my experience, please consider AGC Amplifier for the fist one, the output power after the AGC should be pined in the small power range. Well, you can choose the AGC amplifier located on the third stage too. What you need to do is set the appropriate value of the control voltage for these two AGC amplifiers to reach both your output power goal and wide range for the input signal goal.