Power Ratings of RF Amplifiers

[naumank]

Hi all

I am studying RF amplifiers data sheets and there is some confusion i am facing in its specifications. One such amplifier claims Power Output at 50 ohm load with 20% duty cycle to be 2 KW. However, the minimum gain it specifies is 63 dB and maximum input it claims to be 10 dBm (around 0.707 Vrms for 50 ohm ). Using this gain and input, maximum output voltage is around 990 Vrms @ 50 ohm load. Using simple power formula i.e. V^2/R, the output power comes to be around 19 KW @ 50 ohm load???.

Can any one kindly tell me where i went wrong in my calculations?

Thanks

 

[JimB]

I am studying RF amplifiers data sheets and there is some confusion i am facing in its specifications. One such amplifier claims....

The conflicting specifications may be specifying different aspects of the amplifiers performance.
All the parameters may not apply at the same time.

Can you link to the datasheet so that we can have some context?

JimB

 

[naumank]

The data sheet is attached.

 

[Les Jones]

My reading of the data sheet says for 0dBm (1mW) give a minimum od 2KW output. 60 dB is 1000000 times and 3 dB is 2 times so 63 dB is 2000000
This seems right to me.

Les.

 

[JimB]

However, the minimum gain it specifies is 63 dB and maximum input it claims to be 10 dBm (around 0.707 Vrms for 50 ohm )

This is where you are going wrong.

Towards the bottom of the specifications list there is a line which says:

RF Drive 0dBm nominal. 10dBm for no damage.

What this means is that to get the full output of the amplifier, you need an input of 0dBm.
As confirmed by Les Jones calculation.

It also means that you can over drive the amplifier input with a signal up to +10dBm, and no damage will be caused to the amplifier.
It does not mean that you will get a nice 20kW out of the amplifier, you will get a distorted about 2kW out of the amplifier.

JimB

 

[naumank]

Hi JimB and Les

Thanks for helping. Yes, you are right that 20 KW output does not seem possible from 2 KW output amplifier. However, i think we can get some output power higher than 2 KW without distortion as given in Power vs Frequency graph in the data sheet. What you say about it?

Thanks

 

[ronsimpson]

What are you going to use the amp for?
I think you are right that at low frequency it can output more power. That is typical of a wide band amp. My broadcast transmitters typically can output slightly more power at the low end of the band. (low frequency = more power)
Below 400khz you can probably get 3000 watts. I do not know at what duty cycle. Probably less than 20%.

You do know it is only a 200 watt amp at 100% of the time.

 

[naumank]

Hi ronsimpson

I want to use it to drive high frequency underwater transducers in frequency range of 380-430 KHz. As for as duty cycle is concerned, requirement is well below 20% (like 1 or 2% only). However, power requirement for my transducer is rather higher i.e. around 4.5 KW in pulsed mode.

As per my initial calculations shown above, i was wondering whether i can deliver 4.5 KW to my transducer using higher RF drive voltage rather than nominal 0dBm?

Thanks

 

[Les Jones]

It looks like an expensive piece of kit to blow up by grossly exceeding it's ratings.

Les.

 

[naumank]

Hi les

Should not the protections mentioned in data sheet prevent any serious damage to this equipment?

Regards

 

[ronsimpson]

380-430 KHz

The data sheet is lacking information. I am certain; at the highest frequency the gain is down.
Just drive it harder.

my transducer is rather higher i.e. around 4.5 KW in pulsed mode.

Your transducer does not have to have 4.5kw. It should work at 3kw just fine.

Delta Sigma Inc makes a "THEIA-H" amplifier that appears to be the same thing. It is hard to find it in the internet.
Microwave & RF, January 2018, page L49.

 

[JimB]

Looking at the graphs in the datasheet, we see this:

I have added the green lines to show the output power that could be expected at 400kHz if you give the amplifier a good spanking and over drive the input.
I make it to be about 3400 Watts.
As this is typical, what output you get from any individual amplifier could be more, or it could be less.

The manufacturers only specify a maximum of 2000 Watts, so if that is all you get from any individual amplifier, the amp meets its specification, don't go crying to them when it does not give any more.

If you must have 4500W, then you should get an amplifier which is rated to give 4500W.

JimB

 

[naumank]

Hi JimB

The same company has 4 KW amplifier but it requires no of power supplies and occupies huge space. As my frequency requirement is limited i.e. 380-430 KHz only, is there any alternate compact amplifier which can meet the 4.5 KW requirement in this band?

Thanks & Regards

 

[JimB]

is there any alternate compact amplifier which can meet the 4.5 KW requirement in this band?

Sorry, but I do not know.

JimB

 

[kubeek]

You should probably ask the manufacturer, their engineers should be able to say if the amp can stand the way you plan to use it.
Also, are your transducers 50ohm? If not you will get completely different results and likely blow the amp.

 

[naumank]

Hi Kubeek

The manufacturer of amplifier under discussion also has a 4 KW power amplifier which is recommended by company but this solution occupies large of space.

My transducer impedance is 25 ohm but have to develop 25 ohm to 50 ohm matching unit for it anyway.