May 1984

4

Philips Semiconductors

Product specification

AM receiver circuit

TDA1072A

FUNCTIONAL DESCRIPTION

Gain-controlled r.f. stage and mixer

The differential amplifier in the r.f. stage employs an a.g.c. negative feedback network to provide a wide dynamic range.

Very good cross-modulation behaviour is achieved by a.g.c. delays at the various signal stages. Large signals are

handled with low distortion and the S/N ratio of small signals is improved. Low noise working is achieved in the differential

amplifier by using transistors with low base resistance.

A double balanced mixer provides the i.f. output signal to pin 1.

Oscillator

The differential amplifier oscillator is temperature compensated and is suitable for simple coil connection. The oscillator

is voltage-controlled and has little distortion or spurious radiation. It is specially suitable for electronic tuning using

Gain-controlled i.f. amplifier

This amplifier comprises two cascaded, variable-gain differential amplifier stages coupled by a band-pass filter. Both

stages are gain-controlled by the a.g.c. negative feedback network.

Detector

The full-wave, balanced envelope detector has very low distortion over a wide dynamic range. Residual i.f. carrier is

blocked from the signal path by an internal low-pass filter.

A.F. preamplifier

This stage preamplifies the audio frequency output signal. The amplifier output has an emitter follower with a series

resistor which, together with an external capacitor, yields the required low-pass for a.f. filtering.

A.G.C. amplifier

The a.g.c. amplifier provides a control voltage which is proportional to the carrier amplitude. Second-order filtering of the

a.g.c. voltage achieves signals with very little distortion, even at low audio frequencies. This method of filtering also gives

fast a.g.c. settling time which is advantageous for electronic search tuning. The a.g.c. settling time can be further reduced

by using capacitors of smaller value in the external filter (C16 and C17). The a.g.c. voltage is fed to the r.f. and i.f. stages

via suitable a.g.c. delays. The capacitor at pin 7 can be omitted for low-cost applications.

Field strength indicator output

A buffered voltage source provides a high-level field strength output signal which has good linearity for logarithmic input

Standby switch

This switch is primarily intended for AM/FM band switching. During standby mode the oscillator, mixer and a.f.

preamplifier are switched off.

Short-circuit protection

All pins have short-circuit protection to ground.