ROHM BA1442A Datasheet

Audio ICs

AM radio / FM IF stereo system IC

BA1442A

The BA1442A is an AM radio and FM IF stereo system IC developed for radio cassette players. The FM circuit is com­prised of a differential IF amplifier, a double-balance type quadrature detector, and a PLL-type stereo MPX circuit. The
AM circuit is comprised of a local oscillator, a double-balanced mixer, an IF amplifier, a detector, and an AGC circuit.

Applications
Radio cassette players

Features

1) The “S” curve characteristic of the FM detector can
be used for upper heterodyne (“N” curve) operation.

2) Audio high cut and low cut can be used to improve the
AM fidelity.

3) A reference voltage supply is provided to ensure
good frequency stability in the shortwave bands.

4) Good FM stability.

5) Built-in damping resistor (3.3kΩ) for the quadrature
coil.

6) The FM MPX uses a Laser Locked Loop, making
adjustment of the VCO and addition of external
components unnecessary .

7) Built-in forced monaural function for the MPX (VCO
stops, LED off).

8) VCO of the MPX is switched off during AM opera­tion.

9) The stereo indicator pin has a constant drive cur­rent, and a current limiting resistor is not required.

10) Few external components required.

Absolute maximum ratings (Ta = 25C)

Recommended operating conditions (Ta = 25C)

337

Audio ICs BA1442A

Block diagram

Pin descriptions

338

Audio ICs BA1442A

339

Audio ICs BA1442A

340

Audio ICs BA1442A

Electrical characteristics (unless otherwise noted, Ta = 25C, VCC = 5V, FM IF MPX: fin = 10.7MHz,

400Hz modulation, 22.5kHz dev (30%), 19kHz 7.5kHz dev (10%)
AM: fin = 1000kHz, fm = 400Hz 30%)

Measurement circuit

341

Audio ICs BA1442A

Technical reference materials (unless otherwise noted, Ta = 25C and VCC = 5V)

Circuit operation

(1) The FM IF amplifier is a three-stage differential am­plifier. The gain of this stage is approximately 60dB. The
bias for the first-stage amplifier is taken from the bypass
filter (pin 2). This improves the FM stability of the first­stage amplifier by reducing noise. The input impedance
is set by a resistor on the chip (330Ω), and can be con­nected to a ceramic filter.
(2) The FM detector circuit is comprised of a quadra­ture detector. The IF amplified signal is input to the detec­tor via a limiter amplifier with a gain of 13dB.
(3) The AM mixer is a double-balance type, and has
been designed for low local oscillator leakage. The mixer
bias current is added from Vreg via antenna coil. If you
lower the mixer conversion gain for mid- to high-strength
electric fields, AGC will operate, and for high-strength
electric fields, the input shunt AGC gives good strong-in­put characteristics.

342

(4) The AM local oscillator circuit consists of a differen­tial positive feedback loop. T o improve the frequency sta­bility , particularly in the SW band, the circuit is operated
with bias referenced to Vreg. T o improve the startup char­acteristics, a 100Ω resistor is inserted in series with the
tank circuit.
(5) The AM IF amplifier consists of two-stage differen­tial amplifier. Both amplifiers use capacitors to block DC,
so bypass capacitors are not necessary. The first stage
amplifier has AGC. The gain of this stage is 47dB, and
the input impedance is 3kΩ.
(6) AM detector circuit
A built-in envelope detector that uses a minute current
serves as the AM detector. External components are not
required.