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E. CHOPPER AMPLIFIER
(Fig. 24)
The chopper amplifier consists of:
1. the 400 Hz oscillator
2. the chopper
3. the amplifier
4. the demodulator.
The oscillator is an inductively fed-back push-pull
oscillator which is tuned to 400 Hz by fixed capaci-
tor C3 13 and the adjustable inductance of trans-
former L302. lts output drives the chopper directly,
and the demodulator via a phase-shifting network.
A DC voltage caused by the drift is applied to termi-
nal I of the chopper. The latter switches the input
of the amplifier alternately to this voltage and earth
(at terminal 3) so the input receives a square wave
voltage. In order to avoid hum, the earthing of the
chopper is carried out at a single point near input
cathode-follower B 101.
The square-wave input-signal is amplified about
l2ox in stage TS301
TS302
TS303, the AC-
amplification being determined mainly by the relation
R3 I 1 : R309. The DC-amplification is determined
by
the
relation
R3 1 I : (R309 + R308)
which
amounts to about 1 : 1.
The next stage consisting of TS304 - T5306 gives a
further 20x amplification for AC, but a 2 : I reduc-
tion for DC; so DC-variations have hardly any
influence on the stability of the control circuit.
Via emitter-fo,llower T5306, the chopped signal is
fed to the phase-sensitive detector which contains
the symmetrical NPN transistors TS307
TS308
T S3 0 9 - TS 3r 1.
The driving vo'ltage of the detector is generated at
two separate windings of transforÍner L301 which
receives its primary voltage from the chopper driving
source.
15
By the action of the collector-to-base diode of the
transistors a biasing voltage builts up across capaci-
tors C308 and C309, the charge of which can leak
away across resistors R322 and R323. During the
crests of the sinusoidal driving voltage the charge
is supplemented until the biasing voltage equals the
driving vo,ltage, so during those times a base-current
exists, which makes the transistors conductive.
In this w&y, there is a change-over switch which
operates synchronous with the chopper; network
R332
C3I7 provides the necessary phase-shift to
be sure that the contact is made when the top of the
square-wave is steady.
In the simplified diagram (Fig. 12) the demodulator
is represented by a switch which connects capacito,r
C307 to capacitor C31 1 or earth. The switch makes
a momentary contact under influence of the earlier
mentioned biasing voltage.
From Fig. 12 it is clear that the polarity of the
voltage over C3 1 I will be inverse to that of the input
voltage, due to the phase reversal of the amplifier.
The amplitude virtually equals the peak to peak
value of the amplifier output being insensitive to
variations of the mean value, which could be caused
by variations of the chopper duty cycle.
As the output may be either positive or negative,
symmetrical transistors are used for the demodulator
as they allow a bidirectional current flow as long as
base-current is present. The AC earthing of the
demodulator is carried out by capacitor C164. Po-
tentiometer R187 allows the balance adjustment of
the differential (first) stage of the DC amplifier. Once
correctly adjused, the balance will be maintained by
the control-circuit.
The interconnection of the 1.1 1 Mohm resistors can
be seen as a virtual earth and, as a consequence, the
input resistance of the DC-amplifier will not change
due to the action of the chopper amplifier.
Vc=Vcr
##
R10 2
?
Ri03
R3 00
+
R 30 1
Operation of the demodulator
PEM 3'r41
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