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2-(2) RF Stage Consisting Cascode-Connected
FETs in Parallel
Fig. 9 shows the cascode connected FETs with the source of the input FET (Tr1) grounded and the gate of the output
FET
(Tr2) grounded. Connected
in this way they act as an impedance transformer. This type of cascode connection of
FETs is made possible using MOS FETs; called ''dual-gate MOS FETs."' Junction FETs will have improved characteristics
if used in cascode connection.
D
Features of cascode connection are:
(1)
High input impedance
(2)
Low noise factor
(3)
Excellent intermodulation and spurions response characteristics
(4) High stability and no need of neutralization
Fig. 3
s
Neutralization is not required because the feedback capacity (Crss) of cascode connected FETs is very low (about 0.1 pF).
Since reverse transmission admittance is very low, undesirable oscillations are largely suppressed. Cascode FETs with these
desirable characteristics can be connected in parallel to give greatly improved sensitivity.
In the MC head amplifier in audio circuits, parallel connected bipolar transistors are used to reduce rbb while
single-gate
J-FETs are used to increase gm. The degree of improvement is given by 20log1//n dB where ''n" is the number of units
connected
in parallel. In an RF amplifier, however, the feedback capacity Crss mentioned above and feedback through the
air must be taken into consideration. Crss and gm will improved by parallel connection, but response will be degraded
because the input will be reduced with too many units connected in parallel. For this reason, two units are used connected
in parallel in the T-X5.
ci05
eP
x101.131
2SKIG8(E.F)
To mixer
Antenna
terminals
e-------~--~----J
Fig. 4
Parallel connection of cascode FETs
2-(3) Quieting Slope Control Circuit
Weak broadcast stereo signals cannot normally be reproduced with high quality because of noise. The reason is that the
signal-to-noise
ratio
is reduced
by a theoretical
value of 21.7 dB during stereo reception when compared
with mono
reception.
This problem is solved by using
a Hl BLEND
MPX filter which reduces the noise level when stereo broadcast signals are
weak. This filter remains in the signal path regardless of intensity of the signal unless the HI BLEND
switch is OFF and
causes the crosstalk to deteriorate. The newly developed Quieting Slope Control (QSC) circuit automatically varies the
amounts
of the signals blended, according to the input signal level, to improve the -50 dB quieting sensitivity by 6 dB
when the AUTO
position is selected. The QSC circuit will be automatically cut off when the AUTO
position is selected
if the signal strength indicator exceeds about ''3", i.e. in areas where the signal strength is sufficient. This facility makes
it possible to always optimize reception by simply selecting the AUTO
position.
Block diagram
Comparison of characteristics
LPF
¢301
a
4
Output
Signal
OF
"301
4
an
L-channet signal
°
3
Approx. 21.7 dB
2-20]
from pin 9 (signal indicator
e
A
6 dB
.
e
FOX.
aire terminal) of FM-IF @
Quieting slope
5°
rs
IC
HA4
asc "OFF"
control (QSC) circuit
com
Stereo noise level during
From
+B
"|
conventional reception
ye
QSC switch turned OFF)
60}
6302
Stereo noise level when QSC
LPF
"00
ee
switch
is set at AUTO
oe
t#—o
:
Fig. 6
R-channel signal
Stereo
noise
*
"saturation level
ez:
Stereo noise
Fig. 5
"20;
=
ration level
Monaural noise!
T-X5
—~4 —
"eee
we
Antenna inputiever
No. 2468
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