Philips AN1651 manual Active filters, Communications and Audio, Stereo Bridge Amplifier

Philips Semiconductors	Application note
Using the NE/SA5234 amplifier	AN1651

		20kΩ
	+
	A2			+4.5VDC
	±
10kΩ		4	10Ω
IC-1				0.1μF
NE5234
VR1			0.1μF		10Ω
VCC				BDX45
					+
+	+	150		150
A1	A3				A4
					±
	±
±
		IC-2
100kΩ		NE5234
VR2	+	150		150	+	VR2
10kΩ	A5				A6
RS	±				±
CL				BDX42
VREF						PM
CS
						MOTOR
						100
						1
VR1-3 = 1.4V
						SL00648

Figure 20. Full Bridge Motor Drive

Active filters

The NE5234 is easily adapted to use in a variety of active filter applications. Its high open-loop gain and excellent unity gain stability make it ideal for high-pass,

band-pass and low-pass configurations operated with low voltage single supplies. Its low output impedance also makes it capable of obtaining low noise operation without resorting to separate high current buffers.

Figure 21a shows the circuit for a VCVS low-pass filter with dual supply biasing and 600Ω output termination. Figure 21b is a band-pass filter with AC coupled gain network for single supply operation.

Communications and Audio

Stereo Bridge Amplifier

Figure 22 shows two NE5234 ICs in a bridge amplifier application. The choice of split supplies allows DC coupling, both from the input signal source and to the load. The gain is set to a nominal 20dB.

Either inverting or non-inverting operation is available. The inverting input impedance is chosen as 600Ω in order to match standard

audio impedance lines within a system. The use of two such amplifiers will provide stereo operation to +10dBm for a 600Ω load.

Voice Operated Microphone

The processing of voice transmissions for communications channels is generally coupled with the need for keeping the signal-to-noise ratio high and the intelligibility optimized for a given channel bandwidth. In addition, when a circuit is battery operated and portable, the requirement to obtain maximum battery life becomes important. The circuit example shown here is aimed at filling the need for a portable voice operated transmitter, cordless phone, or tape recorder. It utilizes the Philips Semiconductors NE5234 quad op amp in conjunction with the new low-voltage NE578 compandor to create an audio processor capable of operating in just such an environment. Both devices are operational to a low battery voltage of 2.0V. In addition the design further conserves current by automatically shifting the NE578 compandor to standby during the period when no transmissions are being made. Total current consumption at 3.0V is 2.8mA for the NE5234. In the active mode the NE578 draws 1.4mA and this drops to 170μA in the standby mode. This amounts to reducing the supply current demand by approximately 25% in the `listen mode'.

1991 Oct 12