Monolithic Microwave Integrated Circuits
Monolithic microwave integrated circuits (MMICs) and similar devices are used in a wide variety of receivers. These devices may be very wideband or relatively narrowband. Very wideband amplifiers have a bandpass (frequency response) of several hundred megahertz or more, typically ranging from sub-VLF to the low end of the microwave spectrum. An example might be a range of 100 kHz to 1000 MHz (i.e., 1 GHz), although somewhat narrower ranges are more common. These circuits have a variety of practical uses: receiver preamplifiers, signal generator output amplifiers, buffer amplifiers in RF instrument circuits, cable television line amplifiers, and many others in communications and instrumentation.
One reason why very wideband amplifiers are rarer than narrowband amplifier circuits is that they were difficult to design and build until the advent of monolithic microwave integrated circuit devices. Several factors contribute to the difficulty of designing and building very wideband amplifiers. For example, too many stray capacitances and inductances are in a typical circuit layout, and these form resonances and filters that distort the frequency response characteristic. Also, circuit resistances combine with the capacitances to effectively form low-pass filters that roll off the frequency response at higher frequencies, sometimes drastically. If the RC phase shift of the circuit resistances and capacitances is 180º at a frequency where the amplifier gain is ≥1 (and in very wideband circuits that is likely) and the amplifier is an inverting type (producing an inherent 180º phase shift), then the total end-to-end phase shift is 360º—the criteria for self-oscillation.
If you have ever tried to build a very wideband amplifier, it likely was a very frustrating experience. Until now. New, low-cost devices, called silicon MMICs, make it possible to design and build amplifiers that cover the spectrum from near DC to about 2000 MHz, using seven or fewer components. These devices offer 13–30 dB of gain and produce output power levels up to 40 mW (+16 dBm). Noise figures range from 3.5 to 7 dB. In this chapter, we use the MAR-X series of MMICs by Mini-Circuits Laboratories as representative.
The only connections are RF input, RF output, and two ground connections. The use of dual ground connections distributes the grounding, reducing overall inductance and thereby improving the ground connection. Direct current power is applied to the output terminal through an external network. But more on that shortly.
Although an IC, the device looks very much like a small UHF/microwave transistor. The body is made of plastic and the leads are wide metal strips (rather than wire) to reduce the stray inductance that narrower wire leads would exhibit. These devices are small enough that handling can be difficult; I found that hand forceps (tweezers) were necessary to position the device on a prototype printed circuit board. A magnifying glass or jeweler's eye loupe is not out of order for those with poor close-in eyesight. A color dot and a beveled tip on one lead are the keys that identify pin 1 (the RF input connection). When viewed from above, pin numbering (1, 2, 3, 4) proceeds counter-clockwise from the keyed pin.
INTERNAL CIRCUITRY
The MAR-X series of devices inherently matches 50 Ω input and output impedances without external impedance transformation circuitry, making it an excellent choice for general RF applications. These devices are silicon bipolar monolithic ICs in a two-transistor Darlington amplifier configuration. Because of the Darlington connection, the MAR-X devices act like transistors with very high gain. Because the transistors are biased internal to the MAR-X package, the overall gains typically are 13–33 dB, depending on the device selected and operating frequency. No external bias or emitter bias resistors are needed, although a collector load resistor to V+ is used.
The MAR-X series of devices inherently matches 50 Ω input and output impedances without external impedance transformation circuitry, making it an excellent choice for general RF applications. These devices are silicon bipolar monolithic ICs in a two-transistor Darlington amplifier configuration. Because of the Darlington connection, the MAR-X devices act like transistors with very high gain. Because the transistors are biased internal to the MAR-X package, the overall gains typically are 13–33 dB, depending on the device selected and operating frequency. No external bias or emitter bias resistors are needed, although a collector load resistor to V+ is used.
Neyker Stewart Zambrano
CRF
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