HF PACKER
AMP
V4R6
Page 49
relationship. Each output is fed to one input of the Push-Pull
Linear Amplifier.
T2
DC powered is supplied to Q1 and Q2 through a phase
reversal dual choke designated T2. This is a very effective
method to provide power to Q1 and Q2 while presenting a high
impedance to the RF signal over a broad range of frequencies.
The drain chokes for Q1 and Q2 are wound on the same core,
and the phase of one of the chokes is reversed.
T1
The drains of Q1 and Q2 are connected to the primary of
output transformer T1, where the two signals are recombined
in phase to produce a single output. T1 also provides
impedance transformation from the low output impedance of
the MOSFETs to the 50Ω output connector, J1. C5 is
important since it increases the bandwidth of impedance
transformation provided by T1, especially at 21 MHz. It is
responsible for low distortion at higher RF levels while the bias
provides low distortion at low RF levels.
BIAS
The 5 V bias supply voltage is derived from a 78L05 regulator.
Bypass capacitors C4, C6, C8 and C1 remove RF voltages
from the bias supply voltage. Gate bias for Q1 and Q2 is
controlled independently. VR1 adjusts Q1's gate
‐
bias voltage
via R1 and L1. VR2 works similarly for Q2 via R2 and L3. At
low frequencies, the amplifier's input impedance is essentially
equal to the series value of R1 and R2. L1 and L3 improve the
input
‐
impedance match at higher frequencies. The low value
of series resistance provided by R1 and R2 also reduces the
Q.
BIAS Adjustment
The bias adjustment puts the MOSFETs slightly into
conduction for each half cycle. The amount of bias was
chosen to eliminate low level cross over distortion that would
exist otherwise. It is not necessary to increase or decrease the
bias from the nominal 100mA specification. Once you have
removed the distortion, it is pointless to increase the bias
further. To do so only increases self heating of the MOSFET.
You cannot adjust the power level of the AMP by adjusting the
bias. If you want to change the power level, which is best done
by changing the DC volts on the drains of the MOSFETs. The
bias level is set by first turning both pots (VR1 and VR2)
completely CCW (counter clock wise). Monitor the DC input
current. The pots are 25-turn and there is no mechanical stop
or audible noise. So count the turns. You can measure ohms
from the wiper to ground for zero ohms as a secondary means
of determining you are fully CCW. Next, jumper the TEST pins
of H3 to activate the PSU. Note the current and turn VR1 until
you increase the current by 100mA. Now turn VR2 until you
increase the current an additional 100mA. You may find the
setting sensitive at the 100mA set point. Make tiny pot
adjustments to raise/lower the current.
Heat Sinking the MOSFETs
The MOSFETs are thermally attached to while electrically
isolated from the heat sink. SIL-PADs are used for a neat
clean thermal solution. The little fiber washers traditionally
used with the TO220 case is not used due to their possible
failure to maintain isolation. In its place we use a 6-32 x 5/16
inch Nylon Hex Head screw to thermally couple the MOSFET
to the heat sink. A hex wrench is provided with the project to
aid in the assembly process. The wrench provides sufficient
torque to hold the MOSFET is intimate contact with the SIL-
PAD below. Things to consider are the cleanliness of the SIL-
PAD installation (no burrs) and using alcohol to remove oily
Summary of Contents for PackerAmp V4
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