User's Manual

300-Watt Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT830A, Rev. 1 4-31
the branch consisting of R10 and DS1, a
green LED current indicator; as a result,
it has the same voltage across it. The
collector bias voltage drop biases on the
green LED DS1. The current flow through
R10 and DS1 gives a visual indication of
the current draw of Q1 by the relative
brightness of DS1. The higher the
collector current of the transistor, the
larger the voltage drop across the
metering resistor. This, in turn, increases
the voltage across DS1 and R10. This
greater voltage level increases the
current flow through them and increases
the brightness of DS1. The opposite
occurs when the collector current
decreases.
The use of an opto-isolator (U1) allows
no direct connection between the base
and collector-biasing circuits, other than
R11, the 200-Ω/5-watt control resistor
that determines the actual base current
flowing in Q1. R11 provides the primary
current path from the collector circuit to
the base of Q1. If there is no current
flowing initially through Q1, R11 provides
a substantial amount of base drive. When
the collector current of Q1 increases to
the desired operating level, the
opto-isolator LED, which is across U1, is
turned on. The turn-on point, or
threshold, is set by the voltage-divider
network that consists of R7, R9, and
adjustable resistor R6. R6 is adjusted to
set up the operating current at 5 amps.
When the opto-isolator LED turns on, it
causes the transistor portion to also turn
on. When the transistor portion turns on,
it biases on Q2, which acts as a shunt
regulator for the base current of Q1.
Negative feedback for the circuit is preset
so that if Q1 draws more than the
desired amount of collector current, the
voltage drop across A4-A4, the metering
resistor, becomes greater than normal.
This increase causes the voltage across
the opto-isolator to increase; as a result,
the opto-isolator LED is brighter than
normal and forward biases Q2 even
more. This action tends to shunt the
available current that was fed through
R11 and Q1 to flowing more through Q2,
from collector to the emitter ground,
setting up the negative feedback path.
The negative feedback tends to stabilize
the current flow through Q1. Capacitor
C8 provides an active filtering action for
this current source, which provides an
improved response in Q1 during the
vertical interval average current changes
that take place.
4.3.13 Generic Single Stage
Amplifier Board, Class A (1265-
1415; Appendix D)
The generic single stage UHF amplifier
board, class A, is used as the basic board
in building the low-band single stage
amplifier assembly, class A (1265-1418);
the mid-band single stage amplifier
assembly, class A (1265-1416); and the
high-band single stage amplifier
assembly, class A (1265-1417), an
aluminum enclosure in which the dual
stage amplifier board (1227-1501) is
mounted. The assembly provides RFI and
EMI protection for the amplifier board.
Mounted on the generic board are the
components that have no frequency-
determining factors. The low-band, mid-
band, and high-band assemblies are
produced by adding the specific
frequency-related components to the
generic board to make the specific
frequency assembly.
4.3.14 (A2) Variable Gain/Phase
Board Enclosure (1265-1426;
Appendix D)
The variable gain/phase enclosure
assembly provides EMI and RFI
protection for the variable gain/phase
board (1265-1425) that is mounted
inside of the assembly.
The RF input to the assembly is at SMA
jack J1 and the RF output is at SMA jack
J2. There are two control inputs that
connect to the assembly: the attenuator
bias input and the phase control input.
The attenuator bias input from the