Philips Semiconductors RF Communications Products
Product specification
Programmable analog compandor
NE/SA572
V
T
I
n
1
I
)
2
G
1
I
2
O
I
S
V
IN
R
1
*
V
T
I
n
1
I
*
2
G
1
I
2
O
V+
I
S
where I
IN
+
R
1
= 6.8kΩ
I
1
= 140µA
I
2
= 280µA
V
T
I
n
I
1
)
I
S
I
IN
*
V
T
I
n
I
2
*
I
1
*
I
S
I
IN
1
I
)
2
G
1
I
2
O
(2)
A1
+
Q
3
–
I
1
140µA
where I
IN
V
IN
+
R
1
I
O
R
1
= 6.8kΩ
I
1
= 140µA
I
2
= 280µA
I
O
is the differential output current of the gain
cell and I
G
is the gain control current of the
gain cell.
If all transistors Q
1
through Q
4
are of the
same size, equation (2) can be simplified to:
2
@
I
2
1
I
*
I
2 2
Q
4
Q
1
Q
2
R1
6.8k
I
G
THD
TRIM
V
REF
I
2
280µA
V
IN
I
O
+
I
IN
@
I
G
*
2I
1
@
I
G
(3)
Figure 1. Basic Gain Cell Schematic
The internal bias scheme limits the maximum
output current I
R
to be around 300µA. Within a
±1dB
error band the input range of the rectifier
is about 52dB.
The first term of Equation 3 shows the
multiplier relationship of a linearized two
quadrant transconductance amplifier. The
second term is the gain control feedthrough
due to the mismatch of devices. In the
design, this has been minimized by large
matched devices and careful layout. Offset
voltage is caused by the device mismatch
and it leads to even harmonic distortion. The
offset voltage can be trimmed out by feeding
a current source within
±25µA
into the THD
trim pin.
The residual distortion is third harmonic
distortion and is caused by gain control
ripple. In a compandor system, available
control of fast attack and slow recovery
improve ripple distortion significantly. At the
unity gain level of 100mV, the gain cell gives
THD (total harmonic distortion) of 0.17% typ.
Output noise with no input signals is only 6µV
in the audio spectrum (10Hz-20kHz). The
output current I
O
must feed the virtual ground
input of an operational amplifier with a
resistor from output to inverting input. The
non-inverting input of the operational
amplifier has to be biased at V
REF
if the
output current I
O
is DC coupled.
Rectifier
The rectifier is a full-wave design as shown in
Figure 2. The input voltage is converted to
current through the input resistor R
2
and
turns on either Q
5
or Q
6
depending on the
signal polarity. Deadband of the voltage to
current converter is reduced by the loop gain
of the gain block A
2
. If AC coupling is used,
the rectifier error comes only from input bias
current of gain block A
2
. The input bias
current is typically about 70nA. Frequency
response of the gain block A
2
also causes
second-order error at high frequency. The
collector current of Q
6
is mirrored and
summed at the collector of Q
5
to form the full
wave rectified output current I
R
. The rectifier
transfer function is
I
R
+
V
IN
*
R
2
V
REF
(4)
If V
IN
is AC-coupled, then the equation will be
reduced to:
I
RAC
+
V
IN
(
AVG
)
R
2
October 7, 1987
5
PHILIPS [ NXP SEMICONDUCTORS ]
