AS 4ST GEC PLESSEY [SEMICONDUCTORS | SL6270 ? GAIN CONTROLLED MICROPHONE PREAMPLIFIER/VOGAD Tne SL6270 Is a silicon integrated circuit combining the functions of audio amplifier and voice operated gain adjusting device (VOGAD). It 1s designed to accept signals from a low sensitivity microphone and to provide an essentially constant output signal for a 50dB range of input The dynamic range, attack and decay times are controlled by external components FEATURES mm Constant Output Signal m Fast Attack Mm Low Power Consumption gm Simple Circuitry APPLICATIONS m Audio AGC Systems W Transmitter Overmodulation Protection Mi Tape Recorders QUICK REFERENCE DATA ME Supply Voltage: 4.5V to 10V Mi Voltage Gain. 52dB ABSOLUTE MAXIMUM RATINGS Supply voltage: 12V Storage temperature: 55C to + 125C ORDERING INFORMATION SL6270 C CM $L6270 C DP SL6270 CB CM SL6270 NA MP 1-146 MAIN AMP OUTPUT 1 MAL MAIN AMP INPUT \ 7 bbe TIME CONST PREAMP INPUT \ vee PREAMP tNPUT CM8 Fig.1 Pin connections, SL6270 - CM (bottom view) acc Time const [}1 8{] main amp OUTPUT preamp ourPut (2 7 MAIN AMP INPUT Vee (}3 61) OV PREAMP INPUT [14 {| PREAMP INPUT DP8, MP8 Fig 2 Pin cannectons, SL6270 - OP (top view) MAIN AMP 8 OUTPUT | | AGC | DETECTOR | I Fig 3 SL6270 block dragramELECTRICAL CHARACTERISTICS Test conditions (unless otherwise stated) : Supply voltage Vcc: 6V Input signal frequency : 1kHz Ambient temperature: 30C to +85C Test circuit shown in Fig. 4 SL6270 Characteristic Value Units Conditions Min. Typ. Max. Supply current 5 10 mA Input impedance 150 Q Pin4or5 Differential input impedance 300 Q Voltage gain 40 52 dB 72uV rms input pin 4 Output level 55 90 140 mV rms] 4mvV rms input pin 4 THO 2 5 % 9O0mV rms input pin 4 Equivalent noise input voltage 1 LN 3002 source, 400Hz to 25kHz bandwidth 4a7n J ourpur 10p (tka MIN LOAO) 47p ae 100n \f input not AC coupled the resistance between pins 4 and 5 must be less than 10 ohms. INPUT INPUT N.B. EXTERNAL &7n 10k -C_ st EXTERNAL 680 22y Mn bos 2 7 Vol 10k oltage gain = 6800) Upper frequency response 10kQ/4.7nF = 3kHz Lower frequency response 6800/2.2pF = 300Hz Fig.4 SL6270 test and application circuit APPLICATION NOTES Voltage gain The input to the SL6270 may be single ended or differential but must be capacitor coupled. In the single-ended mode the signal can be applied to either input, the remaining input being decoupled to ground. Input signals of less than a few hundred microvolts rms are amplified normally but as the input level 1s increased the AGC begins to take effect and the output ts held almost constant at 90MV rms over an input range of 50dB. The dynamic range and sensitivity can be reduced by reducing the main amplifier voltage gain. The connection of a 1k resistor between pins 7 and 8 will reduce both by approximately 20dB. Values less than 680Q are not advised. Frequency response . The low frequency response of the $L6270 1s determined by the input, output and coupling capacitors. Normally the coupling capacitor between pins 2 and 7 is chosen to give a -3dB point at 300Hz, corresponding to 2 2yF, and the other capacitors are chosen to give a response to 100Hz or less Fig.5 SL6270 frequency response The S$L6270 has an open loop upper frequency response of a few MHz and a capacitor should be connected between pins 7 and 8 to give the required bandwidth. Attack and delay times Normally the SL6270 is required to respond quickly by holding the output level almost constant as the input |S increased This attack time, the time taken for the output to return to within 10% of the original level following a 20d8 increase in input level, will be approximately 20ms with the circuit of Fig.4. It is determined by the value of the capacitor connected between pin 1 and ground and can be calculated approximately from the formula Attack time = 0.4ms/uF The decay time ts determined by the discharge rate of the capacitor and the recommended circuit gives a decay rate of 20dB/second Other values of resistance between pin 1 and ground can be used to obtain different resuits 1-1473L6270 OUTPUT, PIN 8 {mv RMS) 20 10pV 100pV | Vg = s6v Tas 625C PeikHz | 1 im 10mV 100mv INPUT (RMS) Fig 6 Voltage gain (single ended input) (typical) SECONO HARMONIC OUTPUT DISTORTION PRODUCTS (dB RELATIVE TO WANTED OUTPUT} THIRO HAAMONIC 1 10 Vg 2 OV, fo tkHz 1090 SINGLE ENDED INPUT [mV RMS} Fig 7 Overload characteristics (typical) THIRO ORDER OUTPUT INTERMODULATION PRODUCTS (dB RELATIVE TO EACH TONE} 1 10 100 SINGLE ENDED INPUT (mV RMS) . Fig & Typical intermodulation distortion (1 55 and 1 85kHz tones) 1-148 & rr Qa VOLTAGE GAIN (dB) tkHz 1OkHz 100kHz taMz 10 MHz FREQUENCY Fig 9 Open loop frequency response {typical}