# Amplifier impedance measurement

Starting with the EMRFD Fig 2.57 amplifier as a base, you can measure input returnloss using a return loss bridge. Return loss can be translated into VSWR and impedance. If the return loss is large (or the VSWR is low) the impedance is near 50 ohms. Low return loss means a higher VSWR so the impedance must be removed from 50 ohms.

**For example.**

A 9.5 dB return loss is a 2:1 VSWR.

A 2:1 VSWR could be caused by either a 100 or a 25 ohm resistance

A 2:1 VSWR could also be caused by a resistance of 40 ohms and a capacitiveor inductive reactance of 30 ohms.

If you look at a Smith Chart and draw a circle centered at the centerwith a radius (Reflection Coefficient) of .33, that is a circle of constant VSWR of 2:1 so any number of resistances and reactances couldcause an SWR of 2:1. The phase component of the impedance is difficultto measure with an return loss bridge but that does not prevent us from using the return loss bridge as an indicator of a good match in general.

See the following for returnloss to VSWR conversion: http://www.rfcafe.com/references/electrical/vswr.htm

I put its spice file with my other spice models. If you click on the spice button to the left you can see the other models.

The circuit worked as designed. I didnt use exact values of resistors as I have more of some flavors that I want to use up instead. I got 18 dB gain flat through HF with 21dBm (125mW) out.

I have been interested in trying to experimentally determine inputand output impedance of amps to see if what I see in spice really happens in the real world.

You can measure input return loss in Spice by adding a second voltagesource to the input of your spice model.

Take a look at the spice models at the bottom of this page:

Notice AC 1 and S11 from:

- S11: input reflection coefficient of 50 ohm terminated output.
- S21: forward transmission coefficient of 50 ohm terminated output.
- S12: reverse transmission coefficient of 50 ohm terminated input.
- S22: output reflection coefficient of 50 ohm terminated input.

S11 is the input reflection coefficient.

The magnitude of the reflection coefficient goes from 0 to 1 volts given the 2 and 1 volt AC sources used. If you consider the phase component of the reflection coefficient then the reflection coefficient can range from -1 to 1 which is not coincidentally the range of the horizontal axis of a Smith Chart.

If you express S11 in dB then S11 is return loss

Input and output impedance can be measured by using a return loss bridge to determine return loss which is related to VSWR and impedance.

The input return loss can be measured by the following recipe:

Attach a 50 ohm dummy load to the amplifier output Attach a siggen to the RLB “RF in” port Attach a power meter/oscope to the “Det” detector port Turn on the amplifier, siggen and power meter. Note the power reading on the power meter (“Unknown open”). Attach the RLB “Unknown” port to the amplifier input and note the power. The return loss is the difference between the two power readings.

The output return loss can be measured by the following recipe:

- Attach a 50 ohm resistor to the amplifier input -important-
- Turn on the amp. With a scope, insure that it is not oscillating.
- Attach a siggen to the RLB “RF in” port
- Attach a power meter/oscope to the “Det” detector port
- Turn on the amplifier, siggen and power meter
- Note the power reading on the power meter (“Unknown open”).
- Attach the RLB “Unknown” port to the amplifier output and note the power.
- The return loss is the difference between the two power readings.

That is it! I couldnt believe it was that easy

I understood that you could place a return loss bridge at the input of an amplifier to determine input return loss/VSWR. What I didnt understandis that you could use the same return loss bridge on the output of an ampto determine output return loss. In addition to not understanding that this was possible at all, I was concerned that I might blow up my siggenby attaching it to the amp output.

That turned out not to be an issueat the 100 mW power level. At higher power levels it may be necessary to put a 20 dB pad betwen the siggen and the RLB to protect the siggen.Same goes for the power meter. It may need to have a 20 dB pad betweenit and the detector output. I think that as long as the amp is quiet and not oscillating and puttingout more power than the siggen/powermeter/RLB can handle then you shouldbe ok directly measuring output return loss/VSWR.

It is kind of wierd to me to be applying low level power to the outputof an amplifier in order to determine “reverse SWR” but it works.

I saw 18 dB input return loss which is something like a 1.3:1 VSWRacross HF.

This is a big deal for me. To actually be able to measure outputimpedance gives me confidence that what Im doing is actually working.

Im going to do some work at this power level to get some more experience then am going to try to graduate to higher power levelsand see what happens.

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