An inexpensive Integrated Circuit has been available for some time which contains the complete gain, detection and age functions required in 455 kHz AM i-f strips, Intended for use with ceramic filters and an outboard front-end converter, the LM372 is presently designed into the AM section of several commercial AM/FM high fidelity receivers, where it replaces the hand-assembled "i-f can" type strip formerly used.
The LM372 is. in reality, a much broader-band device than its 455 kHz applications would suggest. It has useful sensitivity throughout the AM broadcast baud, the 160 and SO meters bands, and even-into the 40 meter band, when used to amplify pretuned AM signals directly from an antenna. Sensitivity, of course, is not as great as when used with the additional gain of a superheterodyne conversion stage, but is adequate for urban area broadcast reception, or local 160 and 80 meter net reception.
As may be seen in Fig. 1, there is considerable difference between the LM372 and a conventional i-f strip. The IC is basically a very high gain dc amplifier, with all its gain in one "lump," as opposed to the transformer coupled individual gain stages usually userd. To keep the dc biasing constant, a negative feedback loop is used, with capacitor C3 making the loop effective for dc only; thus, ac (rf) gain is not reduced by the feedback system.
In conventional strips, age is performed by shifting the dc biasing of the individual gain stages, effectively reducing their gain for strong input signals. Unfortunately, such bias shifting detunes the conventional interstage transformers. In the IC, however, the gain stages operate at maximum efficiency at all times; they are preceded by a separate age attenuator stage, which can cut incoming signals by as much as 80 dB (1 0,000 to one).
Conventional diode-type AM detectors require dc biasing through the last i-f "can" in the strip, and are inherently nonlinear, causing distortion. The LM372 uses a more sophisticated detector, adapted from the op-amp type peak-following circuits used in analog computers. In this "active detector," the diode is inside an amplifier's feedback loop, which automatically biases the diode for linearity with small signals, actually gives voltage gain, rather than the loss of conventional detectors, and is directly, rather than transformer coupled, to the output of the gain stages. In Fig. 1, C5 is the detector smoothing capacitor, with the R4, C4 combination forming the age loop.
The schematic of Fig. 2 illustrates in detail how the blocks of Fig. 1 operate. With no age, Q2 is simply an emitter follower, with unity gain. Its output is coupled to the three gain stages, Q6. Q7 and Q8, whose output is directly coupled to the active detector, formed by Q11, Q1 2 and Q14. Dc feedback for the gain stages is performed through R8, C3, Q5, R6 and R7.
As input signals become stronger, the dc output level of the detector, at pin 6, rises
Fig. 1. LM3 72 block diagram.
R4 and C4 couple this voltage, with the audio component filtered out, to Q4 and Q3. Stronger signals begin to turn on Q3; since Q3 and Q2 form a differential pair, this tends to turn off Q2, giving an age action that is both series (turning off Q2) and shunt (turning on the low-impedance emitter of
The string of diodes, Dl—D7, and the various emitter followers attached to the string, act as separate power supply regulators for each stage of the LM372, which also eliminates the need for separate rf decoupling of each stage's supply line.
As can be seen, there is no internal tuning in the LM372; it will amplify, detect, and age all signals applied to its input. While in a superheterodyne receiver, it would be fed by an already bandpassed 455 kHz signal, the circuit of Fig. 3 allows it to roughly select any signal in the 550-1650 kHz band. The Tune Radio Frequency circuit has of course, poor selectivity, since it uses only a single tuned circuit which also acts as an antenna. Its selectivity is sufficient, however, to peak the desired station, which will then operate the age so that overall gain is minimum required for that station. Thus, other stations will be far down on the single LC circuit's selectivity curve. More sophisticated designs might use multisection tuning ahead of the strip.
The prototype was constructed using very inexpensive imported "transistor radio" components. In fact, virtually any junked transistor radio will supply nearly all of the required external components for broadcast band use. A ferrite "loopstick" antenna, LI, resonates with a small, polyethylene dielectric tuning capactior. If operation is desired above the broadcast band, a smaller capacitor may be used, or the broadcast type may be connected in series with another capacitor, and then shunted across the coil, to lower the net capacitance.
The LM372 performs its gain function just as it would in an i-f application, but in this case directiy drives a class A power amplifier. Since the dc voltage at pin 6 is relatively constant (varying from 2.1 to 2.4V
pin numbers appear in circles
Fig. 2. LM372 schematic.
pin numbers appear in circles
Fig. 2. LM372 schematic.
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