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.
No comments:
Post a Comment