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ATR4251-TKSY Datasheet(PDF) 6 Page - ATMEL Corporation |
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ATR4251-TKSY Datasheet(HTML) 6 Page - ATMEL Corporation |
6 / 20 page 6 4913E–AUDR–02/07 ATR4251 [Preliminary] 3.2 AM AGC The IC is equipped with an AM AGC capability to prevent overdriving of the amplifier in case the amplifier operates near strong antenna signal level, for example, transmitters. The AM amplifier output AMOUT1 is applied to a resistive voltage divider. This divided signal is applied to the AGC level detector input pin AGCAMIN. The rectified signal is compared against an internal reference. The threshold of the AGC can be adjusted by adjusting the divider ratio of the external voltage divider. If the threshold is reached, pin AGCAM opens an external transistor which controls PIN diode currents and limits the antenna signal and thereby prevents overdriv- ing the AM amplifier IC. 3.3 FM Amplifier The FM amplifier is realized with a single NPN transistor. This allows use of an amplifier config- uration optimized on the requirements. For low-cost applications, the common emitter configuration provides good performance at reasonable bills of materials (BOM) cost (1). For high-end applications, common base configuration with lossless transformer feedback provides a high IP3 and a low noise figure at reasonable current consumption(2). In both configurations, gain, input, and output impedance can be adjusted by modification of external components. The temperature compensated bias voltage (VREF1) for the base of the NPN transistor is derived from an integrated band gap reference. The bias current of the FM amplifier is defined by an external resistor. Notes: 1. See test circuit (Figure 8-1 on page 11) 2. See application circuit (Figure 9-1 on page 12) 3.4 FM/TV AGC The IC is equipped with an AGC capability to prevent overdriving the amplifier in cases when the amplifier is operated with strong antenna signals (for example, near transmitters). It is possible to realize an external TV antenna amplifier with integrated AGC and external RF transistor. The bandwidth of the integrated AGC circuit is 900 MHz. FM amplifier output FMOUT is connected to a capacitive voltage divider and the divided signal is applied to the AGC level detector at pin AGCIN. This level detector input is optimized for low dis- tortion. The rectified signal is compared against an internal reference. The threshold of the AGC can be adjusted by adjusting the divider ratio of the external voltage divider. If the threshold is reached, pin AGC1 opens an external transistor which controls the PIN diode current, this limits the amplifier input signal level and prevents overdriving the FM amplifier. |
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