模拟设计的100条经典经验！ 开关电源设计制作学习园地.mht

收集的模电的一些资料

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=F5=BE=AD=B5=E4=BE=AD=D1=E9=A3=A1<BR>1/=20
            Capacitors and resistors have parasitic inductance, about =
0.4nH for=20
            surface mount and 4nH for a leaded component. <BR>2/ If you =
don"t=20
            want a high bandwidth transistor to oscillate place lossy =
components=20
            in at least 2 of the 3 leads. Ferrite beads work well. =
<BR>3/ When=20
            taking DC measurements in a circuit and they don"t make =
sense,=20
            suspect that something is oscillating. <BR>4/ Opamps will =
often=20
            oscillate when driving capacitive loads. <BR>5/ The =
base-emitter=20
            voltage Vbe of a small signal transistor is about 0.65v and =
drops=20
            about 2mV/deg C. Vbe goes down with increasing temp. <BR>6/ =
Multiply=20
            0.13nV by the square root of the ohmic value of a resistor =
to find=20
            the noise in a 1Hz bandwidth. Then multiply by the square =
root of=20
            the BW in Hz gives the total noise voltage. <BR>7/ Johnson =
noise=20
            current goes down with a increase in resistance. <BR>8/ The=20
            impedance looking into the emitter of a transistor at room =
temp is=20
            26Ohm/Ie in mA <BR>9/ All amplifiers are differential in =
that they=20
            are referenced to ground somewhere. <BR>10/ Typical metal =
film=20
            resistor has a temp coef of about 100 ppm/deg C <BR>11/ The =
input=20
            noise voltage of a quiet op amp is 1nv/sqrt(Hz) but there =
are plenty=20
            available with 20nV/sqrt(Hz). Op amps with bipolar =
front-ends have=20
            lower voltage noise and higher current noise than those with =
FET=20
            front-ends <BR>12/ Using an LC circuit as a power supply =
filter can=20
            actually multiply the power supply noise at the filter"s =
resonant=20
            frequency. Use inductor with low Q to overcome this. <BR>13/ =
Use=20
            comparators for comparing and op amps for amplifying and =
don"t even=20
            think of mixing the two. <BR>14/ Ceramic caps with any other =

            dielectric other than NPO should only be used for bypass=20
            applications. <BR>15/ An N-channel enhancement-mode FET =
needs +ve=20
            voltage on the gate-source to conduct form drain-source. =
<BR>16/=20
            Small signal JFETS work very well as low-leakage diodes by=20
            connecting drain &amp; source together in log =
current-to-voltage=20
            converters and low leakage input protection. Small signal =
bipolars=20
            with b-c tied together will also make nice low-leakage =
diodes.=20
            <BR>17/ With low pass filter use Bessel for least amount of=20
            overshoot in the time domain, and Cauer (or elliptic) for =
fastest=20
            rolloff in the freq domain. <BR>18/ dB is always 10 times =
the log of=20
            the ratio of 2 powers. <BR>19/ At low frequencies, the =
current in=20
            the collector of a transistor is in phase with the applied =
current=20
            at the base. At high frequencies the current at the =
collector lags=20
            by 90deg. You must appreciate this simple fact to understand =
high=20
            frequency oscillators. <BR>20/ The most common glass-epoxy =
PCB=20
            material (FR4) has a dielectric constant of about 4.3 To =
make a=20
            trace with a characteristic impedance of 100 Ohm, use a =
trace=20
            thickness of about 0.4 times the thickness of the board with =
a=20
            ground plane on the opposite side. For a 50Ohm trace make it =
2 times=20
            the thickness. <BR>21/ If you need a programmable dynamic =
current=20
            source, find out about operational transconductance amps. =
Most of=20
            the problem is figuring out when you need a programmable =
dynamic=20
            current source. <BR>22/ A CMOS output with an emitter =
follower can=20
            drive a 5V relay nicely as the relays normally have a =
must-make spec=20
            of 3.5V. This saves power and require no flyback components. =
<BR>23/=20
            Typical thermocouple potential is 30uV/degC. Route signals=20
            differentially, along the same path, avoid temp gradients. =
DPDT=20
            latching relays won"t heat up when multiplexing these =
signals.=20
            <BR>24/ You SHOULD be bothered by a design that looks messy, =

            cluttered or indirect. This uncomfortable feeling is one of =
the few=20
            indications that there"s a better way. <BR>25/ Avoid drawing =
any=20
            current from the wiper of a potentiometer. The resistance of =
the=20
            wiper contact will cause problems (local heating, noise =
offsets=20
            etc.) <BR>26/ Most digital phase detectors have a deadband =
where the=20
            analog output does not change over the small range where the =
2=20
            inputs are coincident. This often-ignored fact has helped to =
create=20
            some very noisy PLL"s (Use a high val bleeding resistor to =
always=20
            ensure current flow in the deadband) <BR>27/ The phase noise =
of a=20
            phase-locked VCO will be at least 6dB worse than the phase =
noise of=20
            the divided reference for each octave between the comparison =

            frequency and the VCO output frequency. Avoid low-comparison =

            frequencies. <BR>28/ You can almost always determine the =
leads of a=20
            bipolar transistor with an ohm meter. b-e and b-c junctions =
will=20
            measure like a diode with the b-c junction reading slightly =
lower=20
            than the b-e junction when forward biased. <BR>29/ For low=20
            distortion, the drains (or collectors) of a differential =
amp"s=20
            front-end should be bootstrapped to the source (or emitter) =
so that=20
            the voltages on the part are not modulated by the input =
signal.=20
            <BR>30/ If your design uses a $3 op amp, and you will be =
making a=20
            thousand of them, you have just spend $3000. Are you smart =
enough to=20
            figure out how to use a $.30 op amp instead?<BR>31/ The Q of =
an LC=20
            tank circuit is dominated by the losses in the inductor in =
terms of=20
            seriesR.Q=3Domega.L/R <BR>32/ Leakage current doubles for =
every 10degC=20
            increase in temp. <BR>33/ When inputs to most JFET op amps =
exceed=20
            the common-mode range for the part, the output may reverse =
polarity.=20
            This artifact will haunt the designers of these parts for =
the rest=20
            of their lives, as it should! <BR>34/ Understand the =
difference=20
            between "make-before-break" and "break-before-make" when you =
specify=20
            switches. <BR>35/ 3 Terminal voltage regulators in the =
TO-220=20
            packages are wonderful parts. They are cheap, rugged, =
thermally=20
            protected and very versatile. Use them virtually any place =
where you=20
            need a protected power transistor. They also make nice AM=20
            power-modulators. <BR>36/ Use step recovery diode where you =
need=20
            fast edges under 100pS (hot-carrier is even faster) <BR>37/ =
The old=20
            723 regulator is still one of the lowest noise regulators =
around!=20
            (2.5uVrms 100Hz-10k) <BR>38/ You can make a very simple =
oscillator=20
            with one diac, cap and a resistor. <BR>39/ NPN transistors =
are=20
            normally superior to their PNP counterpart in performance. =
<BR>40/=20
            Typical spec in some databooks should read "Seen it once". =
Always=20
            work with the worst spec of the part when doing a design. =
<BR>41/=20
            Don"t just copy circuits from application notes without=20
            understanding completely how it operates, and the reason for =
the=20
            choice of values. <BR>42/ Dealing with crystals, make sure =
you=20
            understand the difference between series and parallel =
resonant. In a=20
            circuit, crystal frequency can generally be slightly lowered =
by=20
            placing a inductor in series and increased by a capacitor in =
series.=20
            <BR>43/ Power MOSFETS on-resistance will have a -ve temp =
coef and=20
            not +ve at low current levels. This is important to remember =
when=20
            paralleling devices. <BR>44/ Lowest noise figure of a RF =
transistor=20
            is not normally where the input is perfectly matched. =
<BR>45/ Many=20