Elecraft KPA100 Manual - Manual (Page 69)

69
Circuit Details
Sheet 1
D11-D14, Q6, Q7, and related components form a high-isolation, full
break-in T-R switch. D12 and D14 are forward-biased in receive mode;
D11 and D13 are forward-biased in transmit mode. The "off" diodes are
reversed biased by the high-voltage bias supply (90-150 V), preventing
RF leakage across the inactive path. In QRP (bypass) mode, relays K1
and K2 route RF around the T-R switch, and both the transmit and receive
diode paths are turned off to reduce current drain.
R21-R22 and C66 form an equalization network that reduces the required
drive power on higher bands while increasing them on the lower bands.
This keeps the overall frequency response of the power amplifier
relatively flat from 160-10 meters.
Matched PA transistors Q1 and Q2 are conservatively rated at the 100-
watt power level. They are operated in push-pull, providing a high degree
of even-order harmonic suppression. U7B and Q4 supply bias. Q4 is an
emitter follower, and U7B provides closed-loop regulation of the voltage
at the emitter of Q4. Q3, attached to the heat sink, provides temperature
feedback, reducing bias at high temperatures.
R7 and U7A form a current-sensing circuit. This is used by the
microcontroller (U1, sheet 2) to check for excessive current drain.
L16, and C64 form a high-pass filter that helps ensure amplifier stability.
Its cutoff frequency is 1.5 MHz. The output of the high-pass filter is
routed to the low-pass filters (sheet 2) via D13 and C79.
Following the low-pass filters is the forward/reflected power bridge (T4,
D16, D17). Rectified DC voltage from the bridge is buffered by voltage
followers U5A and U5B. U6 further buffers the VFWD voltage (forward
power) and drives the VRFDET line, which is returned to the K2 for
purposes of closing the ALC loop (for power control).
In low-power (QRP) mode, Q10 and Q11 are turned off (high
impedance), allowing the full voltage from the bridge to reach the
associated A-to-D converter inputs on the microcontroller. In this case,
the bridge performs identically to the one in the KAT2 ATU.
In high-power (QRO) mode, Q10 and Q11 are turned on by the SCALE
line from U1. This pulls RP1 pins 2 and 3 to ground, which, in
combination with R24 and R25, divides the VFWD and VREFL voltages
by approximately 3.16 (square root of 10). Since KPA100 power output is
proportional to the square of the voltage at the VRFDET line, this in
effect scales power up by a factor of 10.
Sheet 2
Microcontroller U1 acts as a relay driver for the low-pass filter and
bypass relays, and also gathers data from the SWR, current, 12V supply,
and temperature sensors. It forwards this data to the K2 when requested,
and can also signal the K2 when any monitored condition is out of range
and may require a power reduction. This communication takes place
across the K2's 1-wire network, the AuxBus, which is connected to pin 28
of U1. U1 also controls the fan, based on heat sink temperature, and
drives the PA KEY transistor (Q9).
Unlike other AuxBus devices in the K2, which sleep during inactive
periods, U1 runs continuously monitoring KPA100 sensors. For this
reason, it uses a crystal oscillator rather than a ceramic resonator.
K3-K12 select low-pass filters. The drive lines from U1 are buffered by
U2. Note: The 40/30 m filter can be used on 60 m; it provides some
rolloff at the 2
nd harmonic, 10.8 MHz. Combined with inherent PA
balance, 60 m 2 nd harmonic suppression is typically 50 dB.
The 18.432 MHz crystal oscillator (Q8, X1, T3, etc.) is used to derive the
T-R switch bias voltage (90-150 V) as well as the (-) voltage for the RS-
232 IC. D1-D8 are used for rectification and voltage doubling.