Чтение общенаучной литературы. Кытманова О.А. - 11 стр.

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12
Transistor Radio Frequency Amplifiers
Like valve amplifiers a transistor r. f. amplifier may be of the tuned or of the
band-pass variety. On long, medium and short waves, the transistor is usually
connected into a common emitter circuit while in the
VHP and UHF bands use is sometimes made of the
common base arrangement.Transistor amplifiers differ
from valve amplifiers in interstage coupling.
Operation of a transistor amplifier is affected by the
output resistance of the transistor, which is much
lower than the output resistance of an amplifier valve.
This is why transformer and tapped-coil coupling is used extensively in r. f.
transistor amplifiers.
The stability cf the operation of a transistor amplifier largely depends on the
position of the quiescent operating point. To stabilize this point the circuit employs
negative direct-current feedback provided by R8 (Fig. 17) connected in the emitter
circuit. Such an arrangement is similar to the current feedback arrangement in
valve circuits.
To eliminate a. c. feedback R3 ,is bypassed to earth by C6. Should the operating
point shift due to temperature changes it will be restored by the feedback voltage
built up across R3 and applied to the transistor base. It should be noted however
that in both configurations the tuned circuit may be connected to the collector
circuit directly, provided the output resistance of the transistor is sufficiently high.
Radio Transmitters
General Considerations. A radio transmitter is known to be essentially a device
for producing radio-frequency energy that is controlled by the intelligence to be
transmitted. A transmitter accordingly represents a combination of oscillator,
power amplifiers, harmonic generators, modulator, power-supply systems, etc.,
which will best achieve the desired result.
Commercial transmitting equipment is ordinarily mounted on a framework of
structural-steel members fronted by a vertical metal panel containing the controls
and meters necessary for adjusting and monitoring the transmitter. All equipment
appearing on the panel is at ground potential, instruments which must be observed
during adjustment or operation and which are not at ground potential being located
behind the panel and viewed through windows. The steel frame is normally
enclosed with wire mesh of some sort and is provided with doors that cut off the
transmitter power when opened. This type of construction requires a minimum of
floor space in proportion to the amount of apparatus involved, makes the
transmitter accessible for inspection and repairing, and eliminates all hazard to
persons.
The design of most radio transmitters, particularly those intended for broadcast
and short-wave transmission, is dominated by the need of maintaining the
transmitted frequency as nearly constant as possible over long periods of time.

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