Philips Semiconductors Product specification
SA5219Wideband variable gain amplifier
1997 Nov 07
4
AC ELECTRICAL CHARACTERISTICS
T
A
= 25
o
C, V
CC1
= V
CC2
= +5.0V, V
AGC
= 1.0V, unless otherwise specified.
LIMITS
MIN TYP MAX
BW -3dB bandwidth 700 MHz
GF Gain flatness DC - 500MHz +0.4 dB
V
IMAX
Maximum input voltage swing (single-ended) for
linear operation
1
200 mV
P-P
Maximum output voltage swing (single-ended)
R
L
= 50Ω 400 mV
P-P
OMAX
for linear operation
1
R
L
= 1kΩ 1.9 V
P-P
NF Noise figure (unmatched configuration) R
S
= 50Ω, f = 50MHz 9.3 dB
V
IN-EQ
Equivalent input noise voltage spectral density f = 100MHz 2.5
nV/√Hz
S12 Reverse isolation f = 100MHz -60 dB
∆G/∆V
CC
Gain supply sensitivity (single-ended) 0.3 dB/V
∆G/∆T Gain temperature sensitivity R
L
= 50Ω 0.013
dB/°C
C
IN
Input capacitance (single-ended) 2 pF
BW
AGC
-3dB bandwidth of gain control function 20 MHz
P
O-1dB
1dB gain compression point at output f = 100MHz -3 dBm
P
I-1dB
1dB gain compression point at input
f = 100MHz, V
AGC
=0.1V
-10 dBm
IP3
OUT
Third-order intercept point at output
f = 100MHz, V
AGC
>0.5V
+13 dBm
IP3
IN
Third-order intercept point at input
f = 100MHz, V
AGC
<0.5V
+5 dBm
∆G
AB
Gain match output A to output B f = 100MHz, V
AGC
= 1V 0.1 dB
NOTE:
1. With R
L
> 1kΩ, overload occurs at input for single-ended gain < 13dB and at output for single-ended gain > 13dB. With R
L
= 50Ω, overload
occurs at input for single-ended gain < 6dB and at output for single-ended gain > 6dB.
SA5219 APPLICATIONS
The SA5219 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 V
AGC
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 50Ω 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 SA5219 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 1kΩ. 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 1kΩ. A 4:1
balun will provide such a broadband match from a 50Ω source.
Noise performance will be optimized if the input impedance is
matched to about 200Ω. A 2:1 balun will provide such a broadband
match from a 50Ω 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 50Ω source. If the
source is terminated, the noise figure will increase to about 15dB.
All these noise figures will occur at maximum gain.
The SA5219 has an excellent noise figure vs gain relationship. With
any VGA circuit, the noise performance will degrade with decreasing
gain. The 5219 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 SA5219 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 SA5219. A
