User's Manual

Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT835A, Rev. 1 4-31
parallel transistors that make up Q1.
The base circuit is RF bypassed by C2,
C7, C17, C28, and C29.
The collectors are impedance matched
to 12.5Ω by C22, C23, and C19, which
can be adjusted for peak output with
the best linearity. C25 provides AC
coupling and DC blocking for the output
signal to SMA RF output connector J2.
L2 and its associated circuitry form a
balun that transforms the two balanced
signals back to a single, unbalanced
50-Ω impedance output. The collector
circuit is RF bypassed by C9, C12 to
C15, C18, C21, C24, C26, C30, and
C31.
The +26 VDC needed for biasing Q1 is
applied to E1. E1 is the high side of A4-
A4, a .5-Ω/25-watt external metering
resistor that is mounted on the
heatsink next to the single stage
amplifier assembly. The metering
resistor is in the collector circuit of the
RF transistor, Q1, and provides the
main current path for Q1. The base
bias applied to Q1 is supplied through
R11, R4, R2, and R3.
The collector bias voltage drop across
the A4-A4 metering resistor is in
parallel with the branch consisting of
R10 and the green Current LED DS1;
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
action increases the voltage across DS1
and R10. This greater voltage level
increases the current flow through DS1
and R10, which increases the
brightness of DS1. The opposite occurs
when the collector current decreases.
The use of 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. This turn-on point, or
threshold, is set by the voltage-divider
network consisting 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 make 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.
4.8.4.2 Mid Band
The single stage UHF amplifier
assembly, Class A, mid band is made
from the generic single stage amplifier
board, Class A (1265-1415). The
assembly uses a single PTB20101
Ericsson device that is made up of two