Author Topic: Please help with understanding RF amplifier (tekbox TBMDA3/B) - constant Pout  (Read 839 times)

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Offline WarhawkTopic starter

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Hello everyone,
I have a reasonably strong background in electronics but RF is new to me. I am re-doing some tests at work for per-compliance radiated immunity test. I use a TEM cell and TBMDA3 from TekBox as my predecessor did. It is a great learning exercise. I mainly do comparative measurements.

First, I wanted to make sure that the signal from the RF power amplifier matched what I expected. I used a spectrum analyzer for measuring the output power. (through 30dB atenuator). I somehow expected that the TBMDA3 amplifier (link) would take whatever is on the input and add +37dBm to it. Of course, within some reasonable range.

Pin=-10dBm, Pout = 27dBm,
Pin=0dBm, Pout = 37dBm

However, the output stays around 37dBm without regards to the input power change. This matches the datasheet for the amplifier. Only when I go for very low input power, the output power starts dropping. To me it seems that the amplifier is designed with variable gain/constant output power in mind.

Q1: What is the name of such amplifier? Is there a special keyword?
Q2: What is the typical construction? Is it a common emitter type of an amplifier where impedance in the emitter path generates the negative feedback?

To my surprise, there is also a successor called TBMDA3B . This unit acts differently. At least when reading the datasheet. Reducing the input power by 5dBm results in decrease of the output power by ~5dBm.

There is a chance that couple colleagues took measurements with the constant output power when just blindly expecting the output power changes too... However, more likely, I am a fool and do not understand how it works. That's why I kindly ask for education on this subject :-)

PS: When I connect the signal generator (some R&S model) directly to the spectrum analyzer (MSX?) through various atenuator I am getting reasonable and repeatable results when it comes to reading power.

Regards, Jiri
« Last Edit: May 15, 2024, 07:06:49 pm by Warhawk »

Offline BigBoss

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This phenomena is called as "Compression".Typical Compression curve is as follows.

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