Transmitter Power Amplifier - SEA 222 Operator's Manual

5.4.12
5.4.13
the common emitter configuration and is transformer coupled to the
push-pull driver stage. Bias for Qll is provided by the base resistor
network with R41 used to adjust the idling (no signal) current in the
device to 60
mA.
(0.30 Volts across R43) R43 is used, along with the
press on heat sink, to provide thermal stability.
TRANSMITTER DRIVER
Transistors Q9 and QlO are small plastic RF power devices, connected
as push-pull common emitter amplifier. Transformer Tll provides
push-pull base drive from the collector of Qll, while transformers T9
and TlO provide collector to load impedance matching and DC power
decoupling, respectively. Gain/bandwidth compensation is provided by
the collector/base feedback networks and the various peaking capacitors
and terminating resistors. Temperature tracking bias is provided for
Q9 and QlO by bonding a silicon di ode, CR9, to the PC mounted heat
sinks for Q9 and QlO. Idling current (no signal) for the driver
devices is approximately 130 mA and is adjusted by selecting the
correct value for FS3 and/or FS4.
As part of the gain/bandwidth compensation circuitry for the total
power amplifier, the output from the transmitter driver is connected to
the output power amplifier through a 3 dB pad consisting of R30, R31
and R32.
TRANSMITTER POWER AMPLIFIER
The power amplifier in the SEA 222 is made up of two 75 Watt power
amplifier modules, a power splitter and a power combiner. Each ampli-
fier module is a push-pull common emitter design, each is independently
fused and each has its own temperature stabi
1
i zed bi as source. Each
amplifier, like the push-pull driver, has the collector voltage present
at all times. The amplifiers are activated by turning on the various
bias supplies when in the transmit mode. The power amplifier bias is
provided from the +10 Volt Tx buss across two silicon diodes, one diode
of which is bonded to the heat transfer plate used to heat sink the
power devices. This is CR 8. CR7, in series with CR8 is used to
compensate for the junction vo
1
tage drop of the two amp
1
if i er bi as
emitter followers, Q7 and Q8. Each amplifier is normally adjusted to
500
mA
idling current (no signal). R26 adjusts the idling current for
Q3 and 04, while R27 adjusts the idling current for QS and Q6.
At the input to the power amp
1
ifi er, TB is provided as a wide band
hybrid which splits the driver power output into two equal parts.
Each of these s i gna
1
s is then independently amp
1
ifi ed by a two tran-
sistor push-pull power amplifier module. The amplifier outputs are
then recombined in hybrid Tl. Resistors are provided to terminate each
hybrid in the event of failure of one of the power amplifier modules.
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