Philips AN1651 manual Active Filters, Stereo Bridge Amp, Using the NE/SA5234 amplifier

Philips Semiconductors	Application note
Using the NE/SA5234 amplifier	AN1651

Figure 23 shows the VOX audio circuit example. A description of its operation for voice activated transmission follows.

Audio generated by the electret microphone is fed into the non-inverting input of preamp A1 and the signal amplified by 12dB. The biasing is accomplished by the resistive divider which provides a level of half the supply voltage which is connected through a 100k resistor to the non-inverting terminal of A1. This automatically provides ratiometric common mode biasing set at VCC/2 for the device. This level is then transferred directly to the following amplifier, A2, setting its DC operating point. The DC gain of both stage A1 and A2 are unity so the cumulative DC error is not multiplied by stage gain. The peak voice level is approximately 100mVRMS at the input to A1 from the microphone and this is boosted to 400mVRMS. The feedback network gain has a low frequency corner at 160Hz and is flat up to the intersection of the closed loop gain with the open loop gain curve at nearly 500kHz. This would increase the noise bandwidth to an excessive degree unnecessary for voice channel communication. A band limiting network is, therefore, inserted across the feedback resistor to limit response to a nominal 5kHz.

Amplifier stage A2 is used to provide high level audio to the rectifier-filter stage for the rapid generation of a DC control signal for operating the voice activated switch function. Stage A2 gain is set to 20dB in order to allow activation of the voice channel on the rising edge of the first voice syllable. An attack time of 20ms is implemented by adjusting the input charging impedance (RS) between the rectifier and the A2 amplifier output. AC coupling must be used to isolate the DC common-mode voltage of the amplifier from the rectifier/storage capacitor and to allow only audio frequencies to drive the switching circuit. Amplifier A3 provides a high impedance unity gain buffer to allow a very slow decay rate to be applied to the time constant capacitor, CT. The output of the storage capacitor reaches approximately 3.2V for a 250ms duration 600Hz burst signal. Diode D1 (1N914) provides a negative clamp action which forces the full peak-to-peak voltage from A2 to charge the storage capacitor. D2 then acts to charge the capacitor to the peak input voltage minus one diode drop, 0.7V. Finally, the buffered

DC control signal is fed to A4 which acts as a threshold comparator with extremely high gain and controlled hysteresis. This provides a positive going signal for releasing the NE578 from its inhibit mode when voice input is present. The NE578 is switched from standby mode when voice input is present. The NE578 is switched from standby mode to the active state by raising the voltage on Pin 8 of the device above 2V. Shutting the audio channel off requires this pin to be driven below 100mV. This demands the extremely wide output voltage swing of the NE5234 in order to reach this near to the negative rail voltage. The voltage threshold of the comparator, A4, is adjustable by use of the sensitivity control, RS. It is used to allow the activation level to be raised or lowered depending upon the ambient audio level in the transmitter vicinity.

	Rf		+3V
	Ri
			±	VOUT
			+
VIN
	R1	R2		600Ω
			±3V
		a. VCVS Low Pass Filter
		R	R
			+5V
	C1	C2	R5
R1
VIN			+	VOUT
			±
C1		R3
	R2		R4
		C2
		b. VCVS Band Pass Filter		SL00649

Figure 21. Active Filters

			6kΩ		10kΩ	PIN
								+3V
						4	+
					X(±1)
	600Ω
				10kΩ
	2
		±	1		±
				9		8
AUDIO IN
	3	+		10	+
LEFT
				NE/SA5234
				#1
						11	±	±3V
	6	±		13	±
			7			14
	5 +			12	+	LEFT CHANNEL
						OUT

AUDIO IN RIGHT

BRIDGE AMP #2

NE5234

RIGHT CHANNEL

OUT

SL00650

Figure 22. Stereo Bridge Amp

1991 Oct 13