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Philips Semiconductors Application note
AN1651Using the NE/SA5234 amplifier
1991 Oct
9
Slew Rate + V
P
(2p f) cos (2p f t)
Note that maximum slew rate occurs where the input sine wave
signal crosses the values of 0, π, and 2π on the radian axis. To get
a feel for what this means in regards to the typical low voltage
circuit, let us consider a 1V
RMS
sinusoidal input to a unity gain
amplifier. The peak voltage in the above equation is 1.414V. One
can then calculate the required slew rate to faithfully reproduce this
signal for various signal frequencies. Or with a given slew rate and
a required peak signal amplitude, the maximum frequency before
slew rate limiting occurs may be determined. For example using the
above amplitude of 1V
RMS
, and the slew rate of the NE5234 which
is 800,000V/sec, one determines that the highest frequency
component which may be reproduced before slew rate distortion
occurs is:
800,000 V/sec / 2π
• 1.414 volts peak = 90,090Hz. A graphical
representation of this relationship is shown in Figure 13. By using
this graph along with the information in the preceding Figure 10 and
Figure 11, which relate usable signal levels versus power supply
voltage, the dynamic behavior of a particular design may be
predicted. For instance, given a single supply configuration
operating at 2.0V, Figure 10b shows an upper limit to input
amplitude of 0.7V
RMS
, or about 1V peak for 1% THD. Using this
level with the data in Figure 13 leads to a figure of 116kHz as an
upper frequency limit for a unity gain amplifier stage operating at 2V
DC.
(EQ. 12.)
dV
S
d t
+ V
P
w cos w t
+ Slew Rate
4
R
S
V
CC
+
V
CC
2
R
S
A
1
A
4
C
S
C
L
R
R
+
–
+
A
A
+
V
CC
2
V
IN
INPUT ISOLATION
R
f
–
SL00642
Figure 14. Single Supply Biasing in Cascade
XI. PROCEDURES
Single Supply Operation
When the NE/SA5234 is used in an application where a single
supply is necessary, input common-mode biasing to half the supply
is recommended for best signal reproduction. Referring to Figure
14, a simplified inverting amplifier input stage is shown with the
simplest form of resistive divider biasing. The value of the divider
resistance R is not critical and may be increased above the 10kΩ
value shown as long as the bias current does not interfere with
accuracy due to DC loading error. However the divider junction
must be kept at a low AC impedance This is the purpose of bypass
capacitor C
S
. Its use provides transient suppression for signals
coming from the supply bus. A low cost 0.1µF ceramic disk or chip
capacitor is recommended for suppressing fast transients in the
microsecond and sub-microsecond region.
Foil capacitors are simply too inductive for any high frequency
bypass application and should be avoided. If low frequency noise
such as 60Hz or 120Hz ripple is present on the supply bus, an
electrolytic capacitor is added in parallel as shown. The
common-mode input source resistance, R
S
, should also be matched
within a reasonable tolerance for maximizing the rejection of induced
AC noise.
The output of the first stage is now fixed at the common mode bias
voltage and the amplified AC signal is referenced to this constant
value. Capacitive coupling to the inverting input is of course
required to prevent the bias voltage from being multiplied by the
stage gain. Second stage biasing may now be provided by the
output voltage of the first stage if non-inverting operation is used in
the former. For lowest noise in a high gain input stage, the
magnitude of the input source resistance is critical; low values of
resistance are preferred over high values to minimize thermally
generated noise.
Non-Inverting Stage Biasing
Non-inverting operation of an amplifier stage with single supply is
similar to the previous example but the bias resistor R
S
must now be
sufficiently high to allow the signal to pass
without significant attenuation. The input source resistance reflects
the output resistance of the preceding stage or other sourcing
device such as a bridge circuit of relatively high impedance. A
simple rule of thumb is to make the bias resistor an order of
magnitude larger than the generator resistance. Again the feed
back network must be terminated capacitively. In this case R1 and
the generator resistance should be matched and then R
S
is matched
to the feedback resistance ,R
F
.
In all cases proper bypassing of the NE5234 supply leads (Pins 4
and 11) is very important particularly in a high noise environment.
Bypass capacitors must be of ceramic construction with the shortest
possible leads to keep inductance low. Chip capacitors are superior
in this respect complimenting the increased use of surface mounted
integrated devices. Note that both the NE5234D and the automotive
grade SA5234D are available and are the surface mount versions of
the device.