U4254BM-M Datasheet, PDF (3/14 Pages) ATMEL Corporation – Low-noise AM/FM Antenna Impedance Matching IC

Pin Description
FMIN
GND1
FMGAIN
U4254BM-M
FMIN, a bipolar transitor’s base is the input of the FM amplifier. A resistor or a coil is
connected between FMIN and VREF2. If a coil is used, the noise performance is
excellent.
Figure 3. Internal Circuit at Pin FMIN
1
FMIN
ESD
To avoid cross-talk between AM and FM signals, the circuit has two separate ground
pins. GND1 is the ground for the FM part.
The DC current of the FM amplifier transistor is adjusted by an external resistor which is
connected between FMGAIN and GND1. To influence the AC gain of the amplifier, a
resistor is connected in series to a capacitor between FMGAIN and GND1. The capaci-
tor has to shorten frequencies of 100 MHz.
Figure 4. Internal Circuit at pin FMGAIN
AGC
FMGAIN
ESD
3
DC current flows into the AGC pin at high FM antenna input signals. This current has to
be amplified via the current gain of an external PNP transistor that feeds a PIN diode.
This diode dampens the antenna’s input signal and protects the amplifier input against
overload. The maximum current which flows in the AGC pin is approximately 1 mA. In
low-end applications, the AGC function is not necessary and the external components
can therefore be omitted.
3
4772B–AUDR–11/03