Thursday, April 18, 2019

measuring an amps input Z

The question of Z matching comes up now and then. Sadly this is a topic that can become confusing because we have rules that apply to different cases and are sometimes misapplied. If I make a transmitter I want the output Z to be small with respect to the load. The current in a series circuit is the same  thru all components and Power = I squared * R. If we have R1 and R2 (Zgen and Zload) equal they share half the power. If R1 (Zgen) = 0 the load would receive all the power and if R1 (Zgen) = infinity no power can be produced. So for max power transfer we have Zgen = Zload. That is the fly in the ointment. The relationship is developed to show that we ONLY want Zgen = Zload when we are willing to lose half our power. When would that be?
What if I am at work in my 400 megawatt powerhouse and I set the generator equal the distribution system Z? Developing 200 megawatts within the generator would not only be inefficient it would melt the generator in short order. I saw a generator go into 400% overload. It was about 5 seconds and the wire feeding the armature melted and killed power. Something like the fuse link in your car but about a big as my little finger. Ideally I want Zgen = 0 Ohms.
When adjusting the load on a vacuum tube we sometimes see the plates glowing red. Is this good? In this case we need to adjust the load to take more power from the amp. I did a couple of screen shots to show
1. how to measure the Zin for an amp.
2 how the load effect the signal level.
Maybe it will be food for thought?
 First what would we expect to find? We are looking into a forwarded biased junction. The emitter resistance is non linear as is any semiconductor junction. The resistance varies with current so we would need to know the current to determine the resistance. I want to just measure the value and avoid the possible error of guessing or assuming the device parameters.

I put R6 in series with the amp input and measured each side of it to ground. The difference between these two reading is the voltage drop across R6. Finding the current is simple enough I=E/R.
Now R=E*I. We measured E and calculated I so multiply and find the input is 136 Ohms.
This is the junction forward bias resistance and the substrate resistance both of which are not easily determined. Yes it can be done in a different manner but the one just demonstrated is the simplest I know.
Now to test the result.
The generator is set to 50 Ohm.
The generator set to 400 Ohm.
As you can see the generator is dropping signal internally.
Now back to the project of building a RF Amp. My antenna will be a "long wire" with a Z of more than 50 Ohms.
 Time to build the circuit and  RF transformer.

*EDIT: My radio is for 80 meters (4Mhz). The sim show about 6db loss with the higher Z source.

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