AN1513 Philips, AN1513 Datasheet - Page 2

• Manufacturer Part Number: AN1513
• Description: NE5205 A Cascadable Amplifier
• Manufacturer: Philips

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Philips Semiconductors
ABSTRACT
The NE5205 is a cascadable amplifier. Two or three amplifiers can
be connected in series to produce multiple gain. The important
concerns are outlined. Some suggestions for improvement for
reliable operation in an electronically hostile environment are
presented.
INTRODUCTION
Connecting amplifiers in series to increase the over-all gain is an
attractive idea. However, there are some basic concerns which
should be addressed before one attempts to do this. Good RF layout
techniques are required in order to enjoy optimized performance.
For example, well by passed power supplies are essential. A 0.1 F
mylar in parallel with a 10 F tantalum located as close to the device
pins as possible is recommended. Generous use of ground plane on
the component side will yield good results. Back side ground plane
is desirable. Bear in mind during layout that a circuit trace which
runs along a ground plane is distributed capacitance with inductance
along its length. The frequency response of the NE5205 will allow
the amplifier to react to these minute values. Because of this,
extreme care should be taken when routing the signal carrying
circuit lines. The power supply should be placed as far away from
the signal lines as possible. The shortest lines are the best bet.
Additionally, one should use the shortest lines possible from the
circuit components as well, for these also exhibit stray capacitances
and inductances. Use of “strip line” is desirable, but not required at
these frequencies.
Short leads, therefore, are very important in RF layout. The shortest
leads which are commercially available in a packaged integrated
circuit today are in surface mount packages. Called many names,
Small Outline (SO) or Surface Mount Device (SMD), the Philips
Semiconductors designator is the “D” package. While other package
leads can be trimmed to the desired length, use of the SO package
precludes this requirement. This package is recommended for this
kind of work. In addition, use of chip components is also
recommended. Chip capacitors and resistors are commercially
available from a great variety of vendors too numerous to mention
here.
DESCRIPTION
The NE5205 is fixed 20dB gain broad band amplifier. The
recommended operating voltage is 6V; it will operate quite nicely
with a single 5V power supply. Its frequency response is flat to
+0.5dB way out to 450MHz. Its -3dB bandwidth is greater than
600MHz. It has a 6dB noise figure in a 50 environment. It draws
approximately 26mA with 6V applied. It can deliver approximately
1V peak-to-peak into a 50 load. One of the truly unique features of
this amplifier is its ability to automatically impedance match loads
from about 40 to 80 . What this means to the user is that, for 50
and 75 systems, matching is accomplished automatically. The net
result of these features is that this amplifier is very easy to use. This,
combined with low price, makes the NE5205 the preferred amplifier.
1991 Dec
NE5205: A cascadable amplifier
2
CIRCUIT DESCRIPTION
Figure [1] shows the application of the techniques discussed above.
The power supply is applied via the upper plane while the ground is
served by the lower plane. In order to protect the internal DC
quiescent operating conditions, DC blocking capacitors were used.
The value of these coupling capacitors is governed by the lowest
frequency which the system is required to pass. The coupling
capacitors are 0.1 F chip capacitors. The NE5205D is used for the
shortest leads possible. By-pass capacitors were distributed along
the length of the amplifier chain, although not shown in the
photograph. The power supply lines used are shielded coaxial cable.
The connectors were soldered directly to the ground plane, again,
with very short leads. Measuring a mere 5 by 2.5mm, the over-all
size is very small. Although not optimized for size, one can easily
see that there is room for further reduction.
The noise measured at the output was approximately 7dB. This was
predictable given the noise figure of a single amplifier.
Figure 1 shows the bandwidth of the three cascaded amplifiers. The
circuit draws about 75mA. With approximately 60dB of RF
broadband gain, the circuit is stable for its pass band. Varying the
power supply voltage throughout its recommended range does not
significantly deteriorate the performance. The automatic impedance
matching is in effect for all amplifiers. Although three amplifiers were
used here, two amplifiers can just as easily be used.
IN THE FIELD
While the laboratory performance is indicative of the possible field
performance, it is in the field that the applications become fully
useful. Electronically adverse environments are common place.
Here, shielding becomes important. The most commonly used shield
is the commercially available electronic enclosure. Many enclosures
are available and most will suit this purpose. A full shielded circuit
will perform best under all but the most hostile conditions.
Some interstage shielding may be necessary. This kind of shielding
is normally connected to the ground plane and provides further
isolation. In addition, some power supply by-passing may be
required for each shielded section. RF chokes can be used to
further isolate the environment from the circuit as well as to separate
by-pass capacitors for each section.
Typical applications where these ideas can be utilized are antenna
amplifiers, amplified splitters, signal generators, frequency counters,
oscilloscopes, signal analyzers, broadband LANs, fiber optics,
modems, mobile radio, CB radio and telecommunications.
Rewritten from a previously unpublished work by Michael Sera,
Sales Trainee, and work published in the Philips Semiconductors
LOOP number 43 by Louie Burgyan, Design Engineering Manager.
Application note
AN1513