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參數(shù)資料
| 型號： | 935030700623 |
| 廠商： | NXP SEMICONDUCTORS |
| 元件分類： | 放大器 |
| 英文描述： | RF/MICROWAVE WIDE BAND LOW POWER AMPLIFIER |
| 封裝： | SOT109-1, MS-012AC, SO-16 |
| 文件頁數(shù)： | 16/20頁 |
| 文件大小： | 272K |
| 代理商： | 935030700623 |
Philips Semiconductors
Product specification
SA5209
Wideband variable gain amplifier
1997 Nov 07
5
SA5209 APPLICATIONS
The SA5209 is a wideband variable gain amplifier (VGA) circuit
which finds many applications in the RF, IF and video signal
processing areas. This application note describes the operation of
the circuit and several applications of the VGA. The simplified
equivalent schematic of the VGA is shown in Figure 2. Transistors
Q1-Q6 form the wideband Gilbert multiplier input stage which is
biased by current source I1. The top differential pairs are biased
from a buffered and level-shifted signal derived from the VAGC input
and the RF input appears at the lower differential pair. The circuit
topology and layout offer low input noise and wide bandwidth. The
second stage is a differential transimpedance stage with current
feedback which maintains the wide bandwidth of the input stage.
The output stage is a pair of emitter followers with 50
output
impedance. There is also an on-chip bandgap reference with
buffered output at 1.3V, which can be used to derive the gain control
voltage.
Both the inputs and outputs should be capacitor coupled or DC
isolated from the signal sources and loads. Furthermore, the two
inputs should be DC isolated from each other and the two outputs
should likewise be DC isolated from each other. The SA5209 was
designed to provide optimum performance from a 5V power source.
However, there is some range around this value (4.5 - 7V) that can
be used.
The input impedance is about 1k
. The main advantage to a
differential input configuration is to provide the balun function.
Otherwise, there is an advantage to common mode rejection, a
specification that is not normally important to RF designs. The
source impedance can be chosen for two different performance
characteristics: Gain, or noise performance. Gain optimization will
be realized if the input impedance is matched to about 1k
. A 4:1
balun will provide such a broadband match from a 50
source.
Noise performance will be optimized if the input impedance is
matched to about 200
. A 2:1 balun will provide such a broadband
match from a 50
source. The minimum noise figure can then be
expected to be about 7dB. Maximum gain will be about 23dB for a
single-ended output. If the differential output is used and properly
matched, nearly 30dB can be realized. With gain optimization, the
noise figure will degrade to about 8dB. With no matching unit at the
input, a 9dB noise figure can be expected from a 50
source. If the
source is terminated, the noise figure will increase to about 15dB.
All these noise figures will occur at maximum gain.
The SA5209 has an excellent noise figure vs gain relationship. With
any VGA circuit, the noise performance will degrade with decreasing
gain.
The 5209 has about a 1.2dB noise figure degradation for
each 2dB gain reduction. With the input matched for optimum gain,
the 8dB noise figure at 23dB gain will degrade to about a 20dB
noise figure at 0dB gain.
The SA5209 also displays excellent linearity between voltage gain
and control voltage. Indeed, the relationship is of sufficient linearity
that high fidelity AM modulation is possible using the SA5209. A
maximum control voltage frequency of about 20MHz permits video
baseband sources for AM.
A stabilized bandgap reference voltage is made available on the
SA5209 (Pin 7). For fixed gain applications this voltage can be
resistor divided, and then fed to the gain control terminal (Pin 8).
Using the bandgap voltage reference for gain control produces very
stable gain characteristics over wide temperature ranges. The gain
setting resistors are not part of the RF signal path, and thus stray
capacitance here is not important.
The wide bandwidth and excellent gain control linearity make the
SA5209 VGA ideally suited for the automatic gain control (AGC)
function in RF and IF processing in cellular radio base stations,
Direct Broadcast Satellite (DBS) decoders, cable TV systems, fiber
optic receivers for wideband data and video, and other radio
communication applications. A typical AGC configuration using the
SA5209 is shown in Figure 3. Three SA5209s are cascaded with
appropriate AC coupling capacitors. The output of the final stage
drives the full-wave rectifier composed of two UHF Schottky diodes
BAT17 as shown. The diodes are biased by R1 and R2 to VCC such
that a quiescent current of about 2mA in each leg is achieved. An
SA5230 low voltage op amp is used as an integrator which drives
the VAGC pin on all three SA5209s. R3 and C3 filter the high
frequency ripple from the full-wave rectified signal. A voltage
divider is used to generate the reference for the non-inverting input
of the op amp at about 1.7V. Keeping D3 the same type as D1 and
D2 will provide a first order compensation for the change in Schottky
voltage over the operating temperature range and improve the AGC
performance. R6 is a variable resistor for adjustments to the op
amp reference voltage. In low cost and large volume applications
this could be replaced with a fixed resistor, which would result in a
slight loss of the AGC dynamic range. Cascading three SA5209s
will give a dynamic range in excess of 60dB.
The SA5209 is a very user-friendly part and will not oscillate in most
applications. However, in an application such as with gains in
excess of 60dB and bandwidth beyond 100MHz, good PC board
layout with proper supply decoupling is strongly recommended.
Q1
Q2
VCC
Q3
Q4
Q5
Q6
VBG
BANDGAP
REFERENCE
OUTA
OUTB
Q8
Q7
A1
VAGC
0–1V
I1
I2
I3
50
50
R1
R2
R3
R4
INA
INB
+
–
SR00238
Figure 2. Equivalent Schematic of the VGA
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