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Loop Amplifier Input Impedance – Keeping It Low

loop amplifier input impedance

Popular circuits do a pretty good job of providing low loop amplifier input impedance. This means you can use these to keep a loop in short-circuit wideband mode quite easily. 

So, how well to the popular amplifiers work? Pretty well, actually. Previously, we looked at the gain provided by three popular loop amplifier designs found on the Internet. These circuits are referred to as G8CQX, Wellgood and LZ1AQ designs. You will find that lots of hams and SWL have used them to build their own wideband active loops

Using LTSpice, I estimated the input impedance or more specifically the RLOAD that each of these designs placed on my 1 meter aluminum loop. As you may recall, the amplifier is placed in short circuit with the loop terminals and converts current into voltage for the receiver. In order to maintain the wideband characteristics of the loop, the load must be at least an order of magnitude smaller than the inductive reactance of the loop.

I measured the load resistance as R = V/I, where V is the voltage at the loop terminals, and I is the current flowing through the amplifier input.

You can see that the LZ1AQ design provides the flattest loop amplifier input impedance across all frequencies up to 30 MHz. It runs at 1.5 to 3.5 Ω. The Wellgood design has better performance above 5 MHz, less so below. On the other hand, the G8CQX design while simpler and relatively flat, provides a much higher load on the antenna and would not work as well with my 1 meter single turn loop design.

If you review my previous article on the normalized loop gain from these three amplifiers, you will see how this input impedance analysis effects antenna system performance.

Loop Amplifier Input Impedance – Which Amp to Choose?

Ok, folks, enough of the LTSpice analysis paralysis. Time for me to actually build a wideband active loop. So, which of these amplifiers am I going to build and use? My answer may surprise you, but it is “none of the above.”

Not because there is something wrong with them. But because along the way I became fascinated with op amps.

I have never really done much with operational amplifiers, but now is a good place to start. I found a few monolithic transimpedance amplifier op amps that have wide enough bandwidth to cover my frequency range. And I am taken with the apparent simplicity of these circuits compared to the many inductors and transformers in the other designs.

So in January, I will turn my attention to designing a complete op amp based wideband active loop. This will be a collaborative project with my homebrew mentor Jim VE6JF and our target date is to get these running by February.


  1. Chin-leong Lim says:

    Wrt LZ1AQ, am I right to assume that you simulated the preamp without the input high pass filter (HPF)? With the HPF in place, the input impedance is only ~3 ohm at1 MHz & below. The impedance rises with frequency, reaching 385 ohm at 30 MHz. I have questioned LZ1AQ about the rising impedance but he insisted the HPF is necessary to prevent FMBC overload.

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