Low Noise Economy Preampfor M

Low gain is almost a meaningless parameter Ln an rf preamplifier specification unless some notation is made about the noise characteristics of the circuit. A gain of 20-30 dB is not uncommon in a VHF preamplifier with a single semiconductor of modern vintage. But if the active device tends to generate and amplify noise, that healthy signal could be reduced to such an extent that copy is actually better with the preamp completely out of the receiver circuit. A signai-to-noise-ratio measurement made before installing the preamp should, under ideal conditions, read the same with the preamp in the circuit provided the specified gain is actually the effective gain — which, unfortunately, is rarely the case. In practice, the effective gain of a typical two meter preamp is considerably less than half the rated gain!

Bandwidth is another parameter worthy of a builder's consideration. A broad amplification curve might be a real blessing for the frequencv-hopper who operates at the low end of the band one night and at the other end the next, providing he's not trying to operate from an area noted for spectrum congestion. The fact is, the broader the bandwidth of the preamp, the more likely the receiver will be to be "swamped out" by Strong signals on adjacent frequencies. Obviously, then, for such applications as FM repeaters, and for metropolitan operation in general (where plenty of strong signals abound), selectivity would take precedence over broadband capability.

The two meter preamp described here represents what 1 feel to be a sound compromise with respect to bandwidth and gain. It is relatively broadbanded, but not so much as to allow tuning of the entire two meter band without some retuning. It is capable of resisting strong signals (above a millivolt) at adjacent 30 kHz points without desensitizing the receiver - a feature that qualifies it well for use with repeaters. The effective gain of this preamplifier is 14 dB; and the noise contribution is as low as the state of the art will allow.

The preamp owes its extremely low noise figure to the single HEP 802, a low-cost VHF junction-type field-effect transistor produced by Motorola for the hobbyist and experimenter market.1 FE-Ts in general have lower noise figures than ordinary bipolar transistors, and the junction FET has a significantly lower noise figure than the metal-oxide (MOSFET) type.

The circuit presented here — designed, by the way, by Motorola engineers — uses the FET in a common-gate configuration. While common-gate amplifiers don't exhibit the sometimes astronomical gain that can be achieved with common-source or common-drain circuits, other factors make it an ideal choice for VHF applications. Chief among those "other factors" is the devilish Millet-effect, the FET's traditional l'most serious threat" to VHF applications. The Miller effect is the name applied to a FET's characteristic high-frequency feedback capacitan ce problem; as the frequency goes up, the gate-to-drain capacitance increases and actually appears a cross the amplifier, making the input impedance in effect a capacitive reactance.

Fortunately, while the Miiler effect is horrendous with common-source amplifiers, and just plain terrible for common-drain configurations, it is a negligible consideration with the common-gate amplifier hookup. Clever engineers have developed methods for skirting the Miller effect, such as cas-coding (with an "o" not an "a") the common-gate FET with a common-source type2, but these add complexity - and complexity means expense. Considering the various tradeoffs, the most practical approach appears to be to settle for a little less overall gain than what is technologically possible in return for improved selectivity and a noise figure that is virtually incomparable.

Construction

The schematic diagram is shown in Fig. 1. As shown, the circuit represents a conservative use of components - a feature that is attractive from both the standpoint of economy and miniaturization. If you've worked with FETs at VHF before, you'll have no trouble with this excellent-performing circuit.

Just remember to keep the leads short; make good solder connections; don't overheat the FET leads (use sockets); and use effective shielding.

It's always a good idea to use printed circuit boards for projects like this, of course, but I didn't. The very good per-

Fig. 1. This single-FET preamp offers an ideal compromise between selectivity and gain; not easily swamped by adjacent frequency signals, it will provide better than 14 decibels of signal improvement for fixed-frequency two-meter operation. Coil values are given in the text.

formance of my own unit serves as proof that a small piece of Vector board and point-to-point wiring with solid conductors can be as effective (and as small) as a printed circuit - and it is a darn sight less trouble.

The preamp contains three coils wound on forms, plus one loop overwind. I used ceramic coil forms with 1/4" brass slugs. Inductor LI consists of 5-1/2 turns of 26 gauge solid wire (tinned copper) wrapped on the slug-tuned form. This coil should be tapped 1-1/4 turns in from the ground end. Coil L2, also on a slug-iuned form, is 9-1/2 turns of 34-gauge wire. L3 is 5 turns of 26 gauge wire, and L4 is 1-1 /4 turns of the same type of wire wrapped around the lower end of L3.

To avoid power-supply problems, I used a simple 9 V transistor radio battery to operate my preamp. But for some applications, such as those involving use with a repeater, a battery would be impractical in spite of the preamp's low power consumption. The preamplifier's input voltage range, however, is broad enough to allow connection to a 12V source (rectified and filtered repeater filament supply, for example, or auto battery for mobile installations) with no circuit modifications.

Performance Checkout if you plan to use the preamp with an AM receiver, you can adjust it for optimum performance in this fashion: first connect the antenna directly to the receiver and tune to a spot where there are no signals (but close to the frequency of maximum interest for you). If your S-meter has an adjustment

"HAMTRONICS" IS

0 0

Post a comment