Source voltage as 60V DC. Well – here we are applying much more than that! More like ~150V
DC! Severe abuse of the BS170!
The phenomenon seen in the ‘scope trace is known as Avalanche Breakdown. A quote from
Wikipedia
https://en.wikipedi0.org/wiki/Avalanche_breakdown
“Avalanche breakdown is a phenomenon that can occur in both insulating and semiconducting
materials. It is a form of electric current multiplication that can allow very large currents within
materials which are otherwise good insulators. It is a type of electron avalanche. The avalanche
process occurs when carriers in the transition region are accelerated by the electric field to
energies sufficient to create mobile or free electron-hole pairs via collisions with bound electrons.”
Apparently one can go quite deeply into Semiconductor Physics and Quantum Mechanics… but
let’s NOT do that. Suffice it to sweep a lot of theory under the carpet and say that the BS170
junction doesn’t like that huge violation of its specification at all and the junction “breaks”, the
insulator becomes a conductor for a short time, shorting the DC to ground and no doubt
discharging C10 instantaneously, which lowers the DC voltage and allows the BS170 to recover its
cool.
Such things would be liable to fry a transistor, but the large 470K resistor and the small size of the
rectifier capacitor C10, I think here limits the potential damage, and the BS170 transistor survives.
However, it’s unpleasant to think of such unpleasant things going on. You can even actually HEAR
it in the QCX earphones during transmit, it appears as a quiet random noise superimposed on the
sidetone during transmit. When you understand where it comes from, it’s irritating. Unforgivable,
even!
The solution is quite simple! Just use a more manly transistor for Q3. The FQPF2N60C is a 600V
2A transistor in a TO220 package. That’s more manly than manly. That’s like having the world-
champion bodybuilder come along and help you carry your weekly supermarket shopping to your
car. But… it turns out the device is low cost and easily available, and that’s good enough. I don’t
need to waste time or pay more, finding a 200V 20mA device. Sometimes the world champion
bodybuilder shows up and though you weren’t looking for him particularly, he’ll do just fine.
Now take a look at the beautiful
results… after the new transistor
substitution… again with the RF
on the top trace and the Q3
voltage on the bottom – now the
RF power shows here as 63W;
the voltage at the Q3 drain is
around 150V, and has a lot less
ripple on it too (probably due to
lower leakage of the high
impedance high voltage supply).
Note also the little 20V “pedestal”
on the right-hand side – this is
caused by the PA supply voltage
coming through D6/8 as I
mentioned, in the absence of any
RF.
A final confirmation is provided by
the now clean-sounding sidetone in the QCX earphones during transmit. Problem
comprehensively solved! Done!
50W QCX PA kit assembly
1.00q
61
