Broadband RF preamplifier for Vlf Lf and AM BCB

In many situations, a broadband (as opposed to tuned) RF amplifier is needed. Typical applications include boosting the output of RF signal generators (which tend to be normally quite low level), antenna preamphfication, loop antenna amplifier, and in the front ends of receivers. A number of different, circuits published, including some by me, but one failing that I've noted on most of them is that they often lack response at the low end of the frequency range. Many designs offer —3 dB frequency response limits of 3 to 30 MHz, or 1 to 30 MHz, but rarely are t he VLF, LF, or even the entire AM broadcast, band (540 to 1700 kHz) covered.

The original need for this amplifier was that 1 needed an amplifier to boost AM BCB DX signals. Many otherwise fine communications or entertainment-grade "general coverage" receivers operate from 100 kHz to 30 MHz or so, and that range initially sounds really good to the VLF through AM BCB DXer, But when examined closer, it turns out that the receiver lacks sensitivity on the bands below either 2 or 3 MHz, so it fails somewhat in the lower end of the speci mm. Although most listening on the AM BCB is to powerful local stations (where receivers with no RF amplifier and a loop-stick antenna will work nicely), those who are interested in DXing are not well served. In addition to the receiver, I wanted to boost my signal generator 50-fl output to make it easier to develop some AM and VLF projects that I am working on and to provide a preamplifier for a square-loop antenna that tunes the AM BCB,

Several requirements were developed for the RF amplifier. First, it had to retain the 50-fl input and output impedances that are standard in RF systems. Second, it had to have a high dynamic range and third-order intercept point in order to cope with the bone crunching levels on the AM BCB, One of the problems of the AM BCB is that those sought after DX stations ten<1 to be buried under multikilowatt local stations on adjacent channels. That's why high dynamic range, high-intercept point loop antennas tend to be required in these applications. I also wanted the amplifier to cover at least two octaves (4:1 frequency ratio), but it achieved a decade (J0:1) response (250 to 2500 kHz).

Furthermore, the amplifier circuit had to be easily modifiable to cover other frequency ranges up to 30 MHz. This last requirement, would make the amplifier useful to a large number of readers as well as extend its usefulness to me.

Consider a number of issues when designing ari RF amplifier for the front end of a receiver. The dynamic range and intercept point requirements were mentioned previously. Another issue is the amotmt of distortion products (related to third-order intercept point) that are generated in the amplifier. It does no good to have a high capability on the preamplifier only to overload the receiver with a lot of extraneous RF energy it can't handle—energy that was generated by the preamplifier, not from the stations being received. These considerations point to the use of a push-pull RF amplifier design.

Push-pull RF amplifiers

The basic concept of a push-pull amplifier is demonstrated in Fig, 7-9. This type of circuit consists of two identical amplifiers that each process half the input sine wave signal. In the circuit shown, this job is accomplished by using a center-tapped transformer at the input to split the signal and another at the output to recombine the signals from the two transistors. The transformer splits the signal because its center tap is grounded; thus, it serves as the common for the signals applied to the two transistors. Because of normal transformer action, the signal polarity at end "A" will be opposite that at end "B" when the center tap ("CT") is grounded. Thus, the two amplifiers are driven 180° out of phase with each other; one will be turning on while the other is turning off and vice versa.

100khz 30mhz Preamp

7-9 Push-pull broadband amplifier block diagram.

The push-pull ampliOer circuit is balanced, and as a result it has a very interest-iiig property: even-order harmonies are canceled In the output, so the amplifier output signal will be cleaner than for a single-ended amplifier using the same active amplifier devices.

There are two general categories of push-pull RF amplifiers: tuned amplifiers and wideband amplifiers, The tuned amplifier will have the inductance of the input and output transformers resonated to some specific frequency, in some circuits, the nontapped winding might be tuned, but in others, a configuration such as Fig, 7-10 might be used. In this circuit, both halves of the tapped side of the transformer are individually tuned to the desired resonant frequency. Where variable tuning is desired, a split-stator capacitor might be used to supply both capacitances.

The broadband category of circuit is shown in Fig. 7-11 A. In this type of circuit, a special transformer is usually needed. The transformer must be a broadband RF transformer, which means that it must be wound on a suitable core so that the windings are bifilar or trifilar. The particular transformer in Fig, 7-11A has three windings, of which one is much smaller than the others. These must be trifitar wound for part of the way and bifilar wound the rest of the way. This means that all three windings are kept parallel until no more turns are required of the coupling link. Then, the remaining two windings are kept parallel until they are completed. Figure 7-1 IB shows an example for the case where the core of the transformer is a ferrite or powdered-iron toroid.

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