Measuring Amplifier AC Impedance

Knowing the impedance of an amplifier or any electronic circuit can be very useful, for lower frequency applications it’s desirable to have the source impedance be much lower than the load impedance to minimize voltage drop, in RF applications and where maximum power transfer is needed it’s better to match impedance to avoid reflections, the latter is a bit more complicated so in this article we will only be considering lower frequencies, typically below 10MHz.

Making the measurement

The only things you need to measure the impedance is a variable resistor (potentiometer), a signal generator and a multi-meter to measure resistance and AC voltage, an oscilloscope is also desirable as multi-meters generally do not measure signals of higher frequency that well.

Ensure your potentiometer is rated to handle the signal power

For measuring input impedance naturally you want the variable resistance to be at the input to your amplifier, while with output impedance you want it to be at the output, the goal is to adjust the resistance until the signal voltage drops by half the input value, the signal frequency should be the nominal expected frequency for the amplifier in question, I.E for audio you may want to start at 10kHz.

If you need to use an AC coupling capacitor make sure it’s large to avoid signal attenuation.

So for example to measure input impedance you set your frequency generator to 10kHz sine, with an output amplitude that is typical for your amplifier, let’s say 50mV RMS, you then measure the voltage on the amplifier side of the variable resistor, adjusting until you read an AC voltage of 25mV, then it’s just a simple case of measuring the resistance of the variable resistor which will give your impedance.

This works by forming a voltage divider between the signal source and the load, a voltage drop of half must mean the impedance is matched so both the variable resistor and the load will be of equal value.

This works great to give you the real component of the impedance, however it tells you nothing about the reactance, which in some situations, particularly RF is quite important to know, regardless this method is cheap and reasonably quick.

Fosi Audio DAC-Q4 Review

I’ve been having a problem with PC audio noise for quite some time, the front audio jack in particular was very susceptible to interference, primarily from my GPU, I decided to get an audio DAC to fix the problem, specifically I wanted one with an optical S/PDIF input since this provides complete electrical isolation which can itself introduce audio noise.

After looking around for a while I decided to avoid the very cheap offerings, I’m not an audiophile but I still want half decent audio quality, the Fosi Q4 appeared to be of good construction with an aluminium chassis and included bass and treble adjustment which was a nice addition.


Fosi Q4 Annotated

The Q4 appears to be a rebranded version of the Q3 Pro which was marked on the PCB, it uses the common SA9023A decoder which supports up to 24 bit 96kHz, it also uses the popular NE5532 audio op-amps, and a HT97220 125mW audio amplifier for the front headphone output.

I identified the DAC as a MS8412 which appears to be a RuiMeng 192kHz 24 bit DAC, the exact signal path isn’t clear but it appears the USB may be restricted to 96kHz while the S/PDIF and coax input are 192kHz, this may explain the mix of 192kHz and 96kHz used in advertising.

Other notable components are the UD2-3NU signal relay, ATMLH518 (possibly AT24C64D) which appears to be an EEPROM possibly for the SA9023A, a pair of fixed 3.3V LDO regulators and a B6280f which I was unable to find any clear info on, all the capacitors are 105c Lelong and Capxon brand.


Overall this is a very decent DAC and can be had for a quite reasonable price, certainly unless you’re an audiophile you’ll find nothing to complain about.

Buy Fosi DAC-Q4 (affiliate link)

Microcap 12 Review

Microcap 12 Logo

Microcap 12 is a formerly commercial circuit simulator that has been made freeware, it was developed by Spectrum Software since 1982 with the final release coming in 2020, while there will no longer be any updates Microcap 12 is fully functional and mature software.


Microcap 12 Interface
Default Interface

Microcap has a lot of functionality so as might be expected the interface is a tad cluttered, however compared with some other professional circuit simulation software it really isn’t too bad, the manual, user guide and reference manual provides plenty of help.

There is a great deal of customization options available including interface layout, keyboard shortcuts, colours and fonts which makes it very versatile.

Part Selection

The included parts library is very extensive containing over 45,000 parts, including useful macro models, while it may be a bit lacking in terms of the most modern parts, there is still enough for essentially every need, in addition in can import standard SPICE models and subcircuits.


A wide variety of simulations are available, from the standard DC, AC and transient to harmonic / intermodulation distortion, stability and optimization, for a free simulator this is by far the best I’ve seen, a number of interactive options are included as well such as sliders and animated parts which makes it very good for educational usage.

There is an extensive amount of graphing options including formulae, annotation, FFT, monte carlo and more making it a very powerful tool.

Multicap 12 Analysis


Microcap 12 is currently the best value circuit simulator that can be had, it even beats a number of commercial products in terms of functionality and component library, what I find more surprising is that Spectrum Software have decided to make this freely available rather than simply vanish like so many companies do these days.

You can get Microcap 12 for free from the Spectrum Software website.