A simplistic impedance/phase measuring board Modification.

[Qmavam]

Years ago I worked measuring peizo transducers using a simple board with a few resistors and some curly Qs to hold the scope probes. The concept is to measure current by measure the input voltage and measure the current by measuring the Voltage across a current sense resistor. The sense resistor can be 1Ω, 10Ω, or 100Ω, depending on the load you are measuring. By knowing the input voltage and calculating the current from the voltage across the sense resistor, you can calculate the impedance of the load. The transducers were capacitive, so while watching the scope we would adjust a variable inductor until the phase angle between the voltage and the current was 0*. Take the voltage and current in the 0* phase to know the R of the transducer, then short the load and and measure the impedance of the inductor to find the impedance of the capacitance of the transducer. But, it can be used to measure all impedances. Depending on how well you build the board to eliminate parasitics the higher the frequency of accuracy. Note; accuracy is limited to scope readings,  it is usable, and I have for 30+ years.  I'll post a schematic, for others to ponder, note, in my picture I show a cap to adjust phase it can be swapped for an inductor.
 Now the question, I was measuring the input impedance of an amplifier at 1MHz, I wanted to keep the 50Ω drive impedance from the signal generator, so I was using a 1Ω current sense resistor, with an 8mV input, my current was too low to read on the 2mV scope scale. Leading me to want a (bypass 1X) , 10X, and 30X gain amplifier to raise the signal across the 1Ω resistor. Anyone have a amp flat from 20Hz to 2MHz that will run off either 3V, 7.4V or a 9V battery.
 I have always wanted to build Wenzels low noise Altoids box amp, and it may be a good choice if I can make the gain adjustable.
http://techlib.com/electronics/audioamps.html#Curious%20New%20Amp  (page down to the Altoids box picture)
 It has a gain of 100, Is there a way to make the gain of Wenzels amp switchable other than an attenuator, i.e make the gain switchable from 10X or 30X?
 Thanks, Mikek