INTEGRATED TUNED AMPLIFIERS BASIC INFORMATION AND TUTORIALS

The tuned amplifiers studied so far have included a tuned load incorporated into a discrete component circuit. There are many integrated circuits on the market, and most of them include other circuit functions besides that of the IF amplifier.

The Motorola MC3372 is such a device and consists of an oscillator, mixer and IF amplifier fabricated on the same chip. All these integrated circuits, however, require external tuning circuits as coils cannot

be etched on to substrates.

Manufacturers frequently indicate physical dimensions for the tuning coils but occasionally they have to be calculated. Two integrated circuits will be considered in this text, namely the CA3028A manufactured by Harris and the MC1350 manufactured by Motorola. Both these chips can be used for IF and RF applications and have on board automatic gain facilities.

The CA3028A may be used as a cascade amplifier with the advantages of low input resistance, large bandwidth and low noise; alternatively it may be operated in the differential mode. It is designed to be used in communications and industrial equipment operating at frequencies between d.c. and 120 MHz and may be configured as a mixer or oscillator.

Some applications are shown in the data sheets, together with the electrical specifications and performance curves which are frequently used in design. The MC1350 is generally used as an IF amplifier and has applications in both radio and television.

It has a wide bandwidth, ranging from zero up to about 60 MHz, but this is reduced to the required value by the tuned circuits. A variety of different integrated RF amplifiers are available from several manufacturers, and a few have been mentioned above. Typical parameters for such integrated circuits are:

(i) voltage gain 20 dB, bandwidth 140 MHz, maximum input signal voltage 100 mV;

(ii) voltage gain 26 dB, bandwidth 100 MHz, maximum input signal voltage 50 mV;

(iii) voltage gain 80 dB, maximum input voltage 15 μV, input impedance 15 Ω in parallel with 100 pF.

The range is comprehensive but selection and design are normally determined by these parameters. Some calculations follow which help illustrate the requirements for certain applications.