VMOS power FET electrostatic fields which can break trolling low -power dc motors. Freedom allows the op amp to provide more volt- down the oxide layer, work is in progress from second breakdown, means less age to the VMOS power FET, insuring to solve this problem. One solution is to chance of damage from back emf or mo- it will deliver maximum output current. flatten the point of the V. This should mentary high current. Of course, the If desired, this project can be built as a ultimately make available high -voltage maximum ratings of the VMOS device battery charger or, with the filter capac- devices on a large scale. must be observed! itor increased in value, as an adjustable Automatic Battery Charger. The cir- high- current power supply. Some Simple Circuits. Now that cuit shown in Fig. 6 provides up to 12.5 you have a basic familiarity with VMOS A at 14 V. As the battery charges and "Flyback" DC -DC Converter. With a devices, let's look at how they can be its voltage rises, the circuit automati- +7 -to -18 -volt input, this circuit (Fig. 7) used in some simple circuits. VMOS de- cally reduces the charging current. can produce regulated -5 volts. The vices are now fairly easy to obtain. The When the battery is fully charged, the current output is limited, but should be VN66AF used in many of these circuits charger cuts off. Thus, a lead -acid stor- enough for an op amp or two. The cir- is available nationally through Radio age battery can be maintained fully cuit consists of a CMOS oscillator driv- Shack stores. Basically, the circuits run charged at all times. ing a VMOS . A full supply volt- the gamut from simple gadgets where age square wave appears across the 33- the VMOS power FET simply replaces ohm forming the VMOS load, a conventional power , to a and the negative, or "flyback" transi- high performance r -f switch where only tion, is rectified, filtered, and zener reg- VMOS will work properly. In all of the ulated to -5 volts. Since the oscillator circuits, simplicity is apparent, particu- FOR MORE INFORMATION operates at a high frequency, the filter larly in the drivers for the VMOS de- ON can have a small value, yet do vices. This is one of the benefits of high - YMOS TECHNOLOGY a good job. The circuit can be assembled impedance inputs. in little space and work with battery - Audio Alarm. A circuit that can be The following publications are available powered op -amp projects. used as a burglar alarm, keyboard beep- from Siliconix, Inc., 2201 Laurelwood Rd., VMOS R -F Switch. The circuit of er, timer alert, or audio tone generator is Santa Clara, CA 95054: Fig. 8 can switch r -f signals in 50 nano- shown in Fig. 4. The CMOS gate is seconds, far faster than any (20 to wired as an oscillator, whose pitch is Docu- 50 ms typical). Other advantages in- variable by changing the 200 -k1ì resis- ment clude 60 -dB isolation with a 10 -MHz, tor, or number the 0.001 -AF capacitor. The 20 -volt peak -to -peak input, and 1 -dB in- VMOS power FET is wired as a simple sertion loss. These are impressive AN79-1 A 500 KHz fea- switch that drives the speaker directly. Switching Inverter for 12 V Systems tures for such a simple circuit. Note a zener connected across the AN79-3 Dynamic Input Characteristics of Basically, the VMOS power FETs are gate and source of the VMOS device. a VMOS Power Switch wired as a "T" switch. When the V -con- This component is internal to the AN79 -4 Driving VMOS Power FETs trol input is 0 volt, the two 2N6660 VMOS, and protects the input against AN79.5 Using the VN64GA High Current, FETs are biased on, because 10 volts overvoltage. A zener is built into most High Power VMOS Power FET appear between their source and gate. VMOS devices to make them less sensi- AN79 -6 Using VMOS to In- The left 2N6660 turns off both power terface from IC Logic to High tive to static, so fewer handling precau- VMOS devices by pulling their gates to Power Loads tions are necessary. volts. At the same time, AN79-7 Applications of the VN10KM -10 the The circuit can be built on a small VMOS Power FET sources of both VMOS devices are piece of perf board and cemented to the AN80-1 A Key to the Advance of Switch- pulled low by the right 2N6660. Thus, rear of the speaker. If desired, the sup- ing Power Supplies both VMOS devices are turned off, and ply voltage can be raised to 12 V for AN80-2: Meet the VMOS FET Model the junction between them is grounded higher output power. If you want to AN80-3 Ultralinear Broadband to reduce leakage through the switch. tinker with the design, you can the AN8O-4 Enjoy VHF Power Amplifier De- Making the V- control input - 10 volts two unused gates in the CD -4011 for sign turns off the two 2N6660s and the another lower- frequency oscillator. Con- AN80.5 An Alternative Power Amplifier VMOS devices are allowed to turn on Design nect the output of this oscillator to the because of the 470 -ohm pull -up resistor. "strobe" input of the first oscillator. The input r -f signal passes through the This produces a two -tone "boop- beep" VMOS devices to the output. This sim- sound similar to some police sirens. ple circuit can be used for many applica- Lamp Dimmer. The problem in dim- tions where high r -f frequencies are ming conventional incandescent lamps is Basically, the . battery charger is a used. A good example would be in a low - that more power can be dissipated in the full -wave consisting of a pair of power transceiver, or to switch several controller than in the lamp! The circuit 1N249 silicon , filter capacitor receivers to one antenna. Build it on a of Fig. 5 solves this problem using a C2, and the VMOS series -pass element. piece of "ground plane" perf board or VMOS and pulse -width modulation. The VMOS device is driven by a simple board covered on one side with copper Control R2 changes the duty cycle, or op -amp error amplifier, that compares a sheet. This will insure maximum attenu- "on time," of the CMOS oscillator. As a portion of the output voltage, via the 5- ation when the switch is "off." result, the on time for the VMOS switch k1ì , with a constant -volt- From this and the other circuits varies along with lamp intensity. Since age reference source. The reference con- shown above, it is easily seen that the VMOS power FET isn't on contin- sists of a CR390, a 390 -µA current VMOS power FETs are not exotic, diffi- uously, it dissipates less power, reducing source, and the 1N4100 . cult -to -use devices. On the contrary, heat -sink requirements. This circuit can The power for the op amp and reference they not only offer performance advan- be constructed on perf board, and is derived from a separate source, which tages over bipolar devices in many appli- adapted to many different applications. produces a slightly higher operating cations, they are often considerably eas- For example, it should work well con- voltage than the main dc source. This ier to use.

64 POPULAR ELECTRONICS

www.americanradiohistory.com