TA7642 AM Radio IC

The TA7642 is a single chip AM Radio Integrated Circuit (IC) with AGC. It replaces the ZN414Z, and the MK414 radio IC. This IC has many equivalents such as CD7642, UTC7642, and LMF501.

Internal Organisation

From what I can see, the input stage consists of a long-tail pair with very high input impedance. Then what follows are transistor stages performing high gain RF amplification. The final transistor stage performs the detection.

Note that this chip does not perform regeneration, but instead consists of multiple high-gain RF amplifier stages. These amplifiers operate within the frequency range of the AM band. If you had an AM Crystal Radio that was producing a nice and clear signal, then this chip would certainly perform well when connected to it.

Pinout

This is a three pin IC in a TO-92 package and the pinout is very simple. When the flat side is facing you, the leftmost pin is ground. The centre pin is the signal input, and the rightmost pin is the signal output. The signal output pin 3 also doubles as the power input, and it usually connects to the positive side of the voltage rail through a resistor.

If you are new to this type of device, then it might appear to be confusing to see circuit diagrams where the signal output pin connects to the positive rail of the power supply!

Circuit: Power Resistor Rp

The resistance value of Rp is typically 1 kΩ when powering from a 1.5 V battery. Obviously, this value may require increasing if the power source is greater than that.

In my radio project, the circuit was powered by a 9 V battery, and therefore the most suitable value for Rp was found to be 150 kΩ.

Although the absolute maximum operating voltage is 6 V, I decided to keep the DC voltage at pin 3 as low as possible. I was able to make it work with only 0.7 V. With higher voltages, you run the risk of damaging the IC, and in addition, the IC will produce feedback noise.

Solar Powered

This chip is truly remarkable because it has a typical quiescent current rating of 0.2 mA, and yet provides a sensitivity of 600 µV. Many die-hard radio old timers will tell you that it is impossible to achieve such sensitivity with such little current; however, Toshiba has always been at the forefront of innovation. Some of their best integrated circuits are in high-end vintage walkmans. Due to the low power consumption, this type of radio also works well with solar power supply, hence this is something that I might try if I can find some broken solar panels.

Detector Output

The detector output voltage is typically 15 mV. I was able to connect my crystal earpiece between pin 3 and ground to listen to the output signal, and it was loud and clear!

AGC

This chip also has Automatic Gain Control (AGC), which can be set through the 100 kΩ resistor R1.

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