Philips AN1651 manual Active Filters

Page 13

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

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Image 13
Page 12 Page 14
Contents AN1651 II. Detailed Description SummaryInput Stage Intermediate Amplifier and Output Stage Figure III. CharacteristicsInternal Frequency Compensation NE5234 Closed Loop Gain vs Frequency Guide Lines for Minimizing Noise IV. Noise Referred to the InputAmplified Noise = 160μVRMS VI. Multiple Stage ConsiderationsTHD vs Supply Voltage for 1VRMS Output VII. LOW Harmonic DistortionIX. LOOP-GAIN VIII. GAIN-BANDWIDTH VS Closed Loop FRE- Quency ResponseSlew Rate Response Single Supply Operation XI. ProceduresNon-Inverting Stage Biasing Strain Gauge Amplifier Instrumentation Applications ExamplesTo 20mA Current Loop Communications and Audio Active filtersActive Filters Fiber Optic Receiver for Low Frequency Data Figure NE578 ReferencesNE570/571/SA571 System Level Half Bridge Servo

AN1651 specifications

The Philips AN1651 is a highly regarded car audio speaker offering an impressive balance of sound quality and performance. Designed primarily for the car audio enthusiast, this model showcases what modern engineering can achieve in automotive sound systems.

One of the main features of the AN1651 is its dual cone structure that produces clear and rich sound across various frequencies. This design aims to enhance both low and mid-range sounds, ensuring that music is delivered with clarity and precision. Whether you're listening to rock, pop, or classical, the AN1651 is built to convey the nuances of music effectively.

The speaker boasts a peak power handling capability of up to 240 watts, which allows for immersive sound experiences without distortion, even at higher volumes. This capability means that the AN1651 can handle the punchy bass lines typical in many contemporary music genres, making it a popular choice among bass enthusiasts.

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The AN1651 also features a sensitivity rating of 91 dB, allowing it to produce powerful sound output even with moderate power input. This feature makes it an ideal choice for vehicles with smaller amplifiers or factory stereo units, as it ensures an enjoyable listening experience without requiring significant upgrades.

In terms of installation, the AN1651 is designed for versatility. Its size makes it compatible with a wide range of vehicles, and the mounting accessories included facilitate an easy and secure fit.

With a smart aesthetic that blends well with various car interiors, the Philips AN1651 speaker is more than just a functional audio component; it is a stylish addition to your vehicle.

In conclusion, the Philips AN1651 combines advanced technologies and thoughtful design to deliver a rich audio experience that car enthusiasts will appreciate. Whether for casual listening or serious audio enjoyment, this speaker meets diverse audio needs, proving to be a valuable investment for any car audio system.
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