March 1989

4

Philips Semiconductors

Product specification

AM receiver circuit

TDA1072AT

FUNCTIONAL DESCRIPTION

Gain-controlled RF stage and mixer

The differential amplifier in the RF stage employs an AGC negative feedback network to provide a wide dynamic range.

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

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

differential amplifier by using transistors with a low base resistance.

A double balanced mixer provides the IF output 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 IF amplifier

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

stages are gain-controlled by the AGC negative feedback network.

Detector

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

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

AF preamplifier

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

resistor which, together with an external capacitor, provides the required low-pass filtering for AF signals.

AGC amplifier

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

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

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

reduced by using capacitors of smaller value in the external filter. The AGC voltage is fed to the RF and IF stages via

suitable AGC 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 primariIy intended for AM/FM band switching. During standby mode the oscillator, mixer and demodulator

are switched off.

Short-circuit protection

All pins have short-circuit protection to ground.