User's Manual

Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT835A, Rev. 1 4-33
current that was fed through R11 and
Q1 to flow more through Q2, from
collector to the emitter ground, and
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.
4.8.4.3 High Band
The single stage UHF amplifier
assembly, Class A, high 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
transistors in parallel and operating
Class A, which amplify the signal by
approximately +11 dB. Bias adjust pot
R6 sets the operating current for Q1.
Q1 and Associated Circuitry
The RF input signal connects to SMA
jack J1 on the board. The RF input is
applied through AC coupling and DC
blocking capacitor C1 to L1 and
associated circuitry. This forms a balun
that converts the input signal from a
50-Ω unbalanced impedance to a 12.5-
Ω balanced impedance configuration
with the two outputs. The two outputs
are applied to the bases of Q1 and are
180° out of phase with each other. C6
and C5, which are adjusted for peak
output, are for impedance matching to
the input of the 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 C16, C22, C23, and C19,
which are 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 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,
which, in turn. increases the voltage
across DS1 and R10. This greater
voltage level increases the current flow
through DS1 and R10 and 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