Philips AN1651 manual XI. Procedures, Single Supply Operation, Non-Inverting Stage Biasing

Philips Semiconductors	Application note
Using the NE/SA5234 amplifier	AN1651

Slew Rate + VP (2pf) cos (2pf 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 1VRMS 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 1VRMS, 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 0.7VRMS, 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.

dVS	+ VP wcos wt	(EQ. 12.)
d t

+Slew Rate

A

+

CS

		+	VCC
R	4	VCC
RS			2	+
	+		A	A4
	A1
R		VCC		±
	±
RS		2
CL	Rf
VIN INPUT ISOLATION

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 CS. 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, RS, 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 RS 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 RS is matched to the feedback resistance ,RF.

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.

1991 Oct 9