U4254BM

TELEFUNKEN Semiconductors

Rev. A1, 08-Jul-97

3 (12)

Pin Description

FMIN

FMIN is the input of the FM amplifier. It is the base of a

bipolar transistor. A resistor or a coil is connected

between FMIN and VREF2. If a coil is used, noise perfor-

mance is excellent.

ESD

1

FMIN

12390

Figure 3.

GND1

To avoid crosstalk between AM and FM signals, the

circuit has two separate ground pins. GND1 is the ground

for the FM part.

FMGAIN

The DC current of the FM amplifier transistor is adjusted

by an external resistor which is connected between

FMGAIN and GND1. In order to influence the AC gain

of the amplifier, a resistor is connected in series to an

capacitor between FMGAIN and GND1. The capacitor

has to be a short at frequencies of 100 MHz.

ESD

3

FMGAIN

12391

Figure 4.

AGC

DC current flows into the AGC pin at high FM antenna

input signals. This current has to be amplified via the cur-

rent gain of an external PNP transistor that feeds a

PIN-diode. This diode dampens the antenna input signal

and protects the amplifier input against overload. The

maximum current which flows in the AGC pin is approxi-

mately 1 mA. In low end applications, the AGC function

is not necessary and therefore the external components

can be omitted.

ESD

4

AGC

12392

Figure 5.

AGCADJ

The threshold of the AGC can be adjusted by variing the

DC current at pin AGCADJ. If pin AGCADJ is connected

directly to GND1, the threshold is set to 96 dB

µV at the

FM amplifier output. If a resistor is connected between

AGCADJ and GND1, the threshold is shifted to higher

values with increasing resistances. If AGCADJ is open,

the threshold is set to 106 dB

µV.

ESD

13

65 k

W

12397

AGCADJ

Figure 6.

FMOUT

The FM amplifiers output is an open collector of a bipolar

RF-transistor. It should be connected to VS via a coil.

ESD

15

12398

FMOUT

Figure 7.