Codan NGT 2010 RF Technical & Service Manual page 117

Output stage and bias generator
Drawing
ALTERNATIVE TEXT
The Class B push-pull output stage formed by V215 and V216 is driven from T203 to
produce the nominal 100 W or 125 W PEP.
The bias voltage applied to each gate of V215 and V216 via R250 and R251 varies in
level depending on the output level.
The drive waveform from the halfwave rectifier D200 is applied to the squaring (x
function circuit comprising IC202/B, V213 and V214. The output of this circuit is then
amplified by IC202/A.
The static point of the bias is set by R240 (PA. BIAS) and the amount of waveform bias
is set by R244 (GAIN). The bias voltage is then passed to the gates of the output stage.
When the matched pair of output FETs is not perfect, the balance potentiometer R253 is
used to trim the bias voltage so that equal current is drawn by each output FET.
The balanced to unbalanced impedance-matching output transformer T204 couples the
power amplifier output to the band filters via J2 and the PTT relay K8.
Output stage and bias regulator
Drawing 04-03369, or
ALTERNATIVE TEXT
Drawing
The Class B push-pull output stage formed by V211 and V212 is driven from T203 to
produce the nominal 100 W or 125 W PEP.
The bias regulator V209 and V213 provides a voltage to the bases of V211 and V212.
The transistors V209 and V213 form a feedback voltage regulator. The output voltage is
the base–emitter voltage of V209 and is adjustable by the preset potentiometer R239.
The transistor V209 is mounted close to the output transistor and provides temperature
compensation to the bias network.
The transistor V210 modifies the output bias voltage slightly depending on the level of
signal detected by D208. The amount of control voltage is varied by R241 and is
adjusted to minimise high order intermodulation products.
The balanced to unbalanced impedance-matching output transformer T204 couples the
power amplifier output to the band filters via J2 and the PTT relay K8.
Output filters
Drawing 04-03169, or
Drawing 04-03369, or
ALTERNATIVE TEXT
Drawing
The transmit frequency range is 1.6 to 30 MHz. Seven LPFs are used to remove the
harmonics generated by the PA. The filters are operated from IC2 and IC3 and are
selected using relays K1 to K7 as required.
The output of the filter circuit passes through the RF bridge formed by T1 and associated
components to the antenna output.
NGT Transceiver System Technical Service Manual
04-03169
04-03542
04-03542
NGT—Technical description (2010)
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