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Model 415E
Section IV
Paragraphs 4-22 to 4-27
4-22. This high input impedance effectively nearly
doubles the output voltage of a source compared with an
amplifier which matches the source resistance.
It
should be emphasized that the transformer turns ratio
in the 415E is chosen for lowest noise figure rather
than to match impedances.
4-23. INPUT BIASING.
4-24.
When the 415E input switch is set to one of the
biased positions XTAL IMPED/BIASED, or 4.5 MA
or 8.7 MA), a bias source is connected in series with
INPUT connector.
An emitter follower,
A1Q1, in
this bias circuit provides bolometer protection by
limiting transients when abolometer or crystal detec¬
tor is connected or disconnected.
Three calibrated
levels of bias are available:
1 volt into 1000 ohms
(XTAL IMPED/BIASED), or 4. 5 ma and 8.7 ma into
200 ohms,
selected with the INPUT switch,
A1S1.
These bias levels are set within a specification, ±3%, by
adjusting the DC voltage potential of the positive power
supply with resistor A3R54. A single adjustment suf¬
fices since 1% resistors accurately determine the
ratios between the 3 bias levels.
The positive DC
voltage is typically 13.2 volts DC but may be as low
as +12 volts DC or as high as +14 volts DC for proper
adjustment.
4-25. POWIi? SUPPLY.
4-26.
The regulated power supplies are fed by either
an internal battery, BT1 (Option 01 instruments only),
or a conventional AC supply consisting of transformer
T2 and rectifier diodes A3CR4and A3CR5. The power
supply must provide two regulated outputs:
+13 volts
and -1.71 milliamperes offset current.
The voltage
reference diode A3CR10 (temperature compensated by
diodes A3CR7 and A3CR8) and transistor A3Q17 form
a constant current source to provide the offset current.
The voltage reference diode A3CR10 and transistor
A3Q16 form a shunt-type regulator maintaining a nomi¬
nal -7.5 volts.
4-27.
In the LINE/ON position, about 3 ma "trickle
charge" is supplied through A3R52 to the battery BT1
(Option 01 only). If the POWER switch is set to BAT-
TERY/ON position,
battery current passes through
diode A3CR6 to the regulators.
The BATTERY/
CHARGE position allows recharging of the battery by
placing A3R52 and A4R2 in parallel. About 20 ma to
30 ma then flows to the battery depending upon the
charge condition of the battery.
02152-4
4-3/4-4
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