power amplifier. Actually, two diodes back-to-back is probably overkill. One diode should provide
sufficient Off isolation. But during the development I was plagued by huge spurious bursts at the
instant of transmit/receive switchover – exactly as described by Pat G3VA (SK, RIP)! This was
very difficult to tame, but very educational – more on this a little later.
An additional benefit of two diodes back-to-back is that in this application, if there IS any
intermodulation distortion (IMD) it gets kind of “canceled out” to some degree, when two back-to-
back diodes are used. Again, I believe overkill in this application where the diodes are completely
On or Off. But having this extra diode in the circuit is an additional belt-and-braces, take-no-
chances, zero-risk-tolerance approach that leaves a safe and comfortable feel-good impression.
You can again see the RF-blocking diodes L3, L5 and L8 (all 47uH) to prevent RF leaking into the
bias control circuits.
During Receive, the PTT signal is low (0V) and therefore MOSFET Q5 is switched Off… this
results in the supply voltage reaching the gate of Q3 via R12, which switches Q3 on and pulls
current through L5, from the D5 cathodes to ground. Forward bias current is driven through diodes
D3 and D5’s anodes via R6 and R9 (both 1K) which delivery current via R12 (470-ohms).
Ignoring voltage drops anywhere, we can calculate the forward bias current through D3 and D5.
R6 and R9 are effectively in parallel, with a resistance of half 1K i.e. 500-ohms. This is in series
with R12 (470-ohms) so the total resistance is 960-ohms. The current through R12 will therefore
be 12 / 970 = 12mA at 12V supply and 20 / 970 = 20mA at 20V supply; this current is shared
between D3 and D5 equally. The forward bias current in each diode is therefore 6-10mA for supply
voltage 12-20V; this is plenty of forward bias current to leave the switches well turned on and
result in very low insertion loss that is negligible in the Receive signal path.
What of the OFF condition? Here the situation is a little more complex. Remember the first rule of
PIN diodes? “To switch a PIN diode off, thou shalt apply a reverse voltage that is larger than the
peak-to-peak voltage of the applied RF”? 50W of RF is 141V peak-to-peak and therefore we need
more than this, as reverse bias, to switch off the diodes! Where will we get ~150V DC for reverse
biasing these diodes? While Don W6JL had available 350V from the screen grid of the valves
(tubes) in his transmitter – we have no such luxuries available here.
The very neat solution is to “borrow” a
little energy from the PA output itself!
Then voltage double and rectify it. The
result is a nice clean DC which is always
more than the peak-peak of the signal,
and plenty enough to reverse bias the
diodes so they are always perfectly and
completely OFF.
This doubler section of the schematic is
shown here (right). C8 is connected
straight to the PA output (before even the
Low Pass Filter – so that we can pick up
any nasty spiky waveforms, warts and all).
D2/9 and D4/10 together with C10 provide the voltage doubler and rectifier. Resistor R8 (470K)
supplies this reverse bias to the diode cathodes. The current demand is incredibly small, just
leakage through diodes and MOSFETs that are OFF… so 470K, a large resistance, works just
fine.
50W QCX PA kit assembly
1.00q
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