5

9154AS–AUDR–09/09

ATR4252 [Preliminary]

The AM buffer amplifier has a very low input capacitance of typically 2.45 pF and can also be

connected directly to the car antenna if no additional gain is required. Due to the low output

impedance of 8

Ω, the buffer amplifier is perfectly suited to drive the capacitive load of long

antenna cables. The voltage gain of this amplifier is close to 1 (0 dB), but the insertion gain that

is achieved when the buffer amplifier is inserted between antenna output and antenna cable

may be much higher (up to 35 dB). The actual value, of course, depends on antenna and cable

capacitances.

The input of the buffer amplifier is connected by an external 4.7 M

Ω resistor to the bias voltage in

order to maintain high input impedance and low noise voltage.

AM tuners in car radios usually use PIN diode attenuators at their input. These PIN diode atten-

uators attenuate the signal by reducing the input impedance of the tuner. Therefore, a series

resistor is used at the AM amplifier output in the standard application. This series resistor guar-

antees well-defined source impedance for the radio tuner and protects the output of the AM

amplifier from short circuit by the PIN diode attenuator in the car radio.

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 signal sources, e.g., transmitters.

The AM amplifier output AMOUT is applied to a resistive voltage divider. This divided signal

feeds the AGC level detector input pin AMDET. The rectified signal is compared against an inter-

nal reference. The threshold of the AGC can be adjusted by modification of the divider ratio of

the external voltage divider. If the threshold is reached ,the pin AMPD opens an internal transis-

tor, which controls the pin diode current and limits the antenna signal to prevent an overdriving

of the AM amplifier.

As the AM AGC has to react very slowly, large capacitors are usually needed for this time delay.

To reduce the cost of the external components, a current control for the time delay is integrated,

so that only small external capacitor values are needed.

The necessary driver for the external pin diode is already incorporated in the ATR4252 IC, which

reduces the BOM cost and the application size.

3.3

FM Amplifier

The FM amplifier is realized with a high performance single NPN transistor. This allows the use

of an amplifier configuration, which is optimized for the desired requirements. For low cost appli-

cation, the common emitter configuration provides good performance at reasonable BOM cost.

For high end application, common base configuration with lossless transformer feedback pro-

vides high IP3 and low noise figure at reasonable current consumption. In both configurations,

gain, input and output impedance can be adjusted by modification of external components.

The temperature compensated bias voltage (FMBIAS) for the base of the NPN transistor is

derived from an integrated voltage reference. The bias current of the FM amplifier is defined by

an external resistor.