NE5205AD Philips, NE5205AD Datasheet - Page 7

• Manufacturer Part Number: NE5205AD
• Description: Wide-band high-frequency amplifier
• Manufacturer: Philips

Philips Semiconductors
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:
V
which is series-shunt feedback. There is also shunt-series feedback
due to R
impedances without the need for low value input shunting resistors
that would degrade the noise figure. For optimum noise
performance, R
possible while R
The noise figure is given by the following equation:
NF =
10 log
where I
R
The DC input voltage level V
1992 Feb 24
V
OUT
0
V
IN
=50 for a 50 system and 75 for a 75 system.
Wide-band high-frequency amplifier
IN
=V
C1
BE1
F2
1
=5.5mA, R
R
and R
+(I
F1
C1
r
R
E1
b
F2
+I
E1
E2
and the base resistance of Q
is maximized.
C3
R
which aids in producing wideband terminal
E1
E1
) R
R
=12 , r
E1
R
E1
O
IN
2ql
b
KT
can be determined by the equation:
=130 , KT/q=26mV at 25 C and
C1
V
IN
dB
3nH
L2
1
are kept as low as
Q1
RE1
650
R1
12
Figure 17. Schematic Diagram
140
RF1
Q3
V
(2)
CC
(1)
Q4
RF2
200
7
where R
at V
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
output voltage V
where V
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
R
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
roughly 3nH. These improve the high-frequency impedance
matches at input and output by partially resonating with 0.5pF of pad
and package capacitance.
F1
F2
V
=140 is chosen to give the desired nominal gain. The DC
OUT
CC
to the base of Q
=6V).
=V
CC
5
E1
is included for bias purposes to allow direct coupling of
Q6
CC
=12 , V
=6V, R
1
R3
140
225
R2
and L
-(I
C2
OUT
Q5
+I
2
R0
10
Q2
2
RE2
12
=225 , I
BE
C6
are bondwire and lead inductances which are
1
can be determined by:
=0.8V, I
. The dual feedback loops stabilize the DC
)R2,(4)
3nH
L2
C2
C1
=8mA and I
IN
=5mA and I
V
is approximately equal to 1V.
OUT
NE/SA/SE5205A
6
and Q
C6
1
1
) to a more desirable
C3
via R
=5mA.
=7mA (currents rated
3
2
Product specification
and diode Q
) which increases
F1
. The use of an
SR00231
4
0
which