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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 =
r
R
b
E1
10 log
1
R
O
where I
=5.5mA, R
=12Ω, r
C1
E1
R
=50 for a 50Ω system and 75 for a 75Ω system.
0
The DC input voltage level V
V
=V
+(I
+I
) R
IN
BE1
C1
C3
E1
1992 Feb 24
are kept as low as
1
KT
2ql
C1
dB
=130Ω, KT/q=26mV at 25 C and
b
can be determined by the equation:
IN
R1
650
L2
V
IN
Q1
3nH
RF1
140
RE1
12
Figure 17. Schematic Diagram
where R
=12Ω, V
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
(1)
problems of shunt feedback loading on the output. The value of
R
=140Ω is chosen to give the desired nominal gain. The DC
F1
output voltage V
OUT
V
=V
-(I
+I
OUT
CC
C2
where V
=6V, R
=225Ω, 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
F2
operating point of the amplifier.
(2)
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
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.
V
CC
R2
R0
225
Q3
10
Q6
Q2
Q4
R3
140
RE2
12
Q5
RF2
200
7
Product specification
NE/SA/SE5205A
=0.8V, I
=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
can be determined by:
)R2,(4)
C6
=8mA and I
=5mA.
C2
C6
. The dual feedback loops stabilize the DC
1
and Q
) which increases
6
2
) to a more desirable
1
are bondwire and lead inductances which are
L2
V
OUT
3nH
which
4
0
SR00231
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