Is RF-amplifier output grounded?

Hi, recently I found a confusing phenomenon in lab. The output of linear rf amplifier and its ground was measured to be a 0 ohm resistance, which means the output of the amplifier is grounded. The same phenomenon was seen on 180 degree power splitter, which also has its output grounded.

I know maybe this is a common phenomenon in rf field. But still it couldn't stop me thinking about if it's true. Do I missed any important knowledge here?

Thanks very much.

Aye, it *is* 'grounded'... at DC

Whilst certainly not universal, it isn't uncommon for RF components to have DC input/output resistances that appear to be near zero ohms. The key thing is at the intended operating frequency the input/output impedance is a specified value, with 50 and 75 ohms being typical.

The scenario you observed is often a consequence of transformer coupled input/outputs, where the DC winding resistance is very low. Since inductive coupling is an AC-only property of a transformer, the coupled stage impedance only becomes apparent at RF.

You can easily observe this property with mains power transformers too - the DC winding resistance suggests it should burst into flames when you plug it in, but thankfully they don't for the same reason!

Afterthought: There are many other RF structures which would also yield low-DC resistance values - it's not the exclusive domain of transformers. Link coupling (common in filters), baluns, quarter wave stubs etc will also do the same.

This remember me a question that a PhD guy asked me years ago. And he was not joking.
-Why when measure inductors (with various values) using the ohm-meter, always gets almost zero ohms resistance?

try this simple experiment. Get an "8 ohm" speaker from your music system and measure its impedance with your ohm meter. Does it read 8 ohms? why not???

The reactance 2pi f L will increase impedance .

The power splitter uses 2R internally across the 2 ports for balancing impedance, while the transformer has a low ESR but high impedance at RF.

In the case of speakers, the L is large but shunted by the equivalent Power resistance of moving air, whereas at DC the ESR is very low but never zero.

In reality, unless carefully selected, you can never neglect the ESR of an inductor, splitter, speaker or reactor when high power or current is involved. .... Unless core losses are far greater than conduction loss.

Thanks very much.

So now I understand what I am measuring is actually a DC resistance of an inductor. When inputting RF signal, the impedance will not stay zero.

At the first glance, the phenomenon do surprise me (counter-intuitively). Since the outer electrode was connected to an instrument (GND), it seems to me that the input and instrument shell was connected.

:smile: