The question is what is a stage gain and the answer is different from each reference. Can they all be correct? How can a gain of 0 equal a gain of 1 and equal a gain 8db? I guess the first step is to determine what gain we may be looking at. This link leads to a site with a calculator and some explanation of terms.
The first thing you see there is this.
OK so 1mw at 600 ohms is 774.6mv. The only question here is why 1mw?
Now we see 1mw at 50 ohms =223.6mv. Again at 1mw why?
The standard is 0dbm which is 1 mw with a 600 ohm load. Or as stated above 774.6 mv drop across a 600 ohm load. If all our measurements were across a 600 ohm load it would be simple to use a meter and read dbm direct. When the load changes the game changes. Now let's look at the stage characteristics.
They say current gain is infinite! Assuming the input current was zero it would be. In the real world we have some current so we will look at that more later.
The voltage gain is very near 1. So we are told the current gain is limited by the load current at a voltage level close to the input. Now let's look at the circuit.
The chart states Zin = infinity. As you can see there is a couple of resistors in the circuit. You could do some math and figure all the resistors in parallel to find the Zin. I'm just going to use 300k which is a nice high value. We must assume an input voltage so I go with 1mv. Now back to the site linked above to see what db level 1mv at 300k would be.
So 1mv at 300k is -85dbm. Now assuming a voltage gain of 1 what would the output be in dbm with a 200 ohm load?
Remember our reference is 0dbm so we are down 53 dbm on the output and were down 85 dbm on the input. The reference quoted said the circuit had a gain of 0. That was that guys way of saying it gave no increase or had a unity gain. The voltage gain is 1. What is the power gain? 85-53=32 so our circuit has a gain of 32dbm ( the m references power gain). A voltage gain of 1 is a 0 db gain. Hey! maybe the guy in the reference meant 0dbv gain which is also correct?
What if the output was 50 ohm?
Now it is 47dbm with the 85dbm input we would have a gain of 38dbm.
So what have I learned from this exercise? Let's look at a circuit with dual output and see.
In this sample circuit I am using the JFET as an electronic transformer to match the load. My load can be either 60 ohm earbud or 2k ohm headset. What would the output be?
The sim shows a -62 dbm output to an earbud.
The sim shows a -51 dbm output to a 2k headphone.
I could optimize the circuit and make it more efficient for either purpose but it would drive either an earbud or headphone as is. (The sim is using -80dbm drive)
Hopefully that makes since and you can see the terms are relative and you need to know the point of reference to use the numbers.