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Optical Feedback—A Method of Stabilizing
the D. C. Conditions of Transistor Amplifiers
Ragnar Forshufvud, Olov Rydgård,
Svenska AB Philips, Stockholm
621.375.4 : 621.372.52.082.5
En vanlig princip vid stabilisering av
arbetspunkten hos transistorförstärkare är att motkoppla
långsamma variationer, medan snabba variationer
förstärks i full utsträckning. För att skilja de snabba
variationerna från de långsamma använder man
vanligen elektrolytkondensatorer. Ur
tillförlitlighetssynpunkt vore det ibland önskvärt att
elektrolytkondensatorer kunde undvikas. I uppsatsen beskrives en
metod med optisk motkoppling, som utföres med
hjälp av en glödlampa och en kadmiumsulfidcell.
Kombinationen skulle kunna betecknas som ett
lågpassfilter utan kondensator eller drossel.
Because of the low impedances of transistors
electrolytic capacitors are often used for coupling and
decoupling in transistor amplifiers. Electrolytic
capacitors, especially the midget types, have a
limited life and will increase considerably the
failure-rate of an amplifier. In d.c. coupled transistor
amplifiers no capacitors are needed, but on the other
hand the temperature stability of such an amplifier
is often much too bad for practical applications.
A change in the collector current of the first stage
will be amplified by the following stages. The
amplifier becomes inoperative as soon as anyone of the
transistors runs into cut-off or bottoming state.
A way to stabilize the d.c. conditions of an
amplifier by means of "slow" negative feedback is shown
in fig. 1. At frequencies for which the amplifier
is designed, negative feedback is suppressed by an
RC-link. Practically C has usually such a high value
that an electrolytic capacitor must be used.
Fig. 1. D.c. coupled amplifier with "slow" feedback for
stabilizing the d.c. conditions against temperature
variations.
In order to avoid use of electrolytic capacitors the
RC-link could be replaced by some active element
with a very slow reaction. Such active elements
exist, e.g. the indirectly heated thermistor. But this
component, having properties strongly dependent
on ambient temperature, is not ideal for use in
temperature stabilization circuits.
Optical degenerative feedback
Another possibility is optical degenerative feedback
by means of an incandescent lamp and a
cadmium-sulphide photoconductive cell photo-resistor. As CdS
photo-resistors of very small sizes are now available,
this solution is quite attractive.
The circuit diagram of an experimental amplifier
with optical degenerative feedback is shown in fig.
2. The bulb and the photo-resistor were mounted
Fig. 2. Amplifier with optical degenerative feedback. CdS:
Cadmium-sulphide photoconductive cell, type
Philips ORP 60. Bulb: Nominal ratings 6 V, 0.1 A.
Type Philips 8073D.
Fig. 3. Components of the amplifier in fig. 2.
ELTEKNIK 1959 1 149
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