7. Component Matching.
I’ve had a lot of questions on both the 100 W nx-Amp, and the sx-Amp about matching, and
whether or not it is important. To get the best out of the sx-Amp (and any CFA for that matter),
a bit more care is required during the build process around component matching. Since the
loop gains are lower at DC, and the requirement to use low feedback resistor values, AC
coupling of the feedback network is not a viable option, unless you are prepared of course to
use huge capacitor values. Since the sx-Amp is all DC coupled and does not use a servo, its
important that steps are taken to minimize drift and output offset. Further, this is a fully
balanced design, so if some effort match the critical components is taken, the distortion will be
The following component pairs should be matched
For Q8 through Q11, match hFE to within 5% or better, and Vbe to within 5 mV – in both cases,
tighter is better. This is easily done if you buy a bag of these devices and then sort first for hFE,
For Q7 and Q8, match hFE to within 15% and Vbe to within 10 mV - if you can, tighter of course
The Zeners are available as 1% and 2 % - I would recommend you purchase 1% types and then
match more tightly if you can. I’ve checked a batch of 10 V Zeners and gotten matches to
I do not specifically recommend that the resistors are matched. Nowadays, if you buy a
reputable brand, the matching in general is extremely good – this is as a direct consequence of
course of the improvements in processes and process control over the last 30 years or so in
passives – and semi’s are generally also very good in this regard.
Now, let me also add that if you do not match any of front end components, your sx-amplifier
will still work: matching though, really ensures that you get every last ounce of performance
You should always use the same gain grade for the front end and TIS transistors. If you use
mismatched gain grades, you will get higher distortion and greater offsets – although in this
latter case, you normally should still be able to dial it out.
out of the design.
