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Philips Semiconductors
Wide-band high-frequency amplifier
THEORY OF OPERATION
The design is based on the use of multiple feedback loops to
provide wide-band gain together with good noise figure and terminal
impedance matches. Referring to the circuit schematic in Figure 17,
the gain is set primarily by the equation:
) R
R
V
F1
E1
OUT
+
V
R
IN
E1
which is series-shunt feedback. There is also shunt-series feedback
due to R
and R
which aids in producing wideband terminal
F2
E2
impedances without the need for low value input shunting resistors
that would degrade the noise figure. For optimum noise
performance, R
and the base resistance of Q
E1
possible while R
is maximized.
F2
The noise figure is given by the following equation:
NF =
KT
) R
)
r
b
E1
2ql
1 )
10 log
R
O
where I
=5.5mA, R
=12 , r
=130 , KT/q=26mV at 25 C and
C1
E1
b
R
=50 for a 50 system and 75 for a 75 system.
0
The DC input voltage level V
can be determined by the equation:
IN
V
=V
+(I
+I
) R
IN
BE1
C1
C3
E1
1997 Nov 07
(1)
are kept as low as
1
(2)
C1
dB
V
CC
R1
650
Q3
L2
V
IN
Q4
Q1
3nH
RF1
140
RE1
12
Figure 17. Schematic Diagram
where R
=12 , V
=0.8V, I
E1
BE
at V
=6V).
CC
Under the above conditions, V
Level shifting is achieved by emitter-follower Q
provide shunt feedback to the emitter of Q
emitter-follower buffer in this feedback loop essentially eliminates
problems of shunt feedback loading on the output. The value of
R
=140 is chosen to give the desired nominal gain. The DC
F1
output voltage V
can be determined by:
OUT
V
=V
-(I
+I
)R2,(4)
OUT
CC
C2
C6
where V
=6V, R
=225 , I
CC
2
From here it can be seen that the output voltage is approximately
3.1V to give relatively equal positive and negative output swings.
Diode Q
is included for bias purposes to allow direct coupling of
5
R
to the base of Q
. The dual feedback loops stabilize the DC
F2
1
operating point of the amplifier.
The output stage is a Darlington pair (Q
the DC bias voltage on the input stage (Q
value, and also increases the feedback loop gain. Resistor R
optimizes the output VSWR (Voltage Standing Wave Ratio).
Inductors L
and L
are bondwire and lead inductances which are
1
2
roughly 3nH. These improve the high-frequency impedance
matches at input and output by partially resonating with 0.5pF of pad
and package capacitance.
R2
L2
R0
225
10
3nH
Q6
Q2
R3
140
RE2
12
Q5
RF2
200
7
Product specification
SA5205A
=5mA and I
=7mA (currents rated
C1
C3
is approximately equal to 1V.
IN
and diode Q
3
via R
. The use of an
1
F1
=8mA and I
=5mA.
C2
C6
and Q
) which increases
6
2
) to a more desirable
1
V
OUT
SR00231
which
4
0
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