模拟设计的100条圣经(一)

张衍波

希望有能力的牛人把它翻译后贴出来
1/ Capacitors and resistors have parasitic inductance, about 0.4nH for surface mount and 4nH for a leaded component.
2/ If you don"t want a high bandwidth transistor to oscillate place lossy components in at least 2 of the 3 leads. Ferrite beads work well.
3/ When taking DC measurements in a circuit and they don"t make sense, suspect that something is oscillating.
4/ Opamps will often oscillate when driving capacitive loads.
5/ The base-emitter voltage Vbe of a small signal transistor is about 0.65v and drops about 2mV/deg C. Vbe goes down with increasing temp.
6/ Multiply 0.13nV by the square root of the ohmic value of a resistor to find the noise in a 1Hz bandwidth. Then multiply by the square root of the BW in Hz gives the total noise voltage.
7/ Johnson noise current goes down with a increase in resistance.
8/ The impedance looking into the emitter of a transistor at room temp is 26Ohm/Ie in mA
9/ All amplifiers are differential in that they are referenced to ground somewhere.
10/ Typical metal film resistor has a temp coef of about 100 ppm/deg C
11/ The input noise voltage of a quiet op amp is 1nv/sqrt(Hz) but there are plenty available with 20nV/sqrt(Hz). Op amps with bipolar front-ends have lower voltage noise and higher current noise than those with FET front-ends
12/ Using an LC circuit as a power supply filter can actually multiply the power supply noise at the filter"s resonant frequency. Use inductor with low Q to overcome this.
13/ Use comparators for comparing and op amps for amplifying and don"t even think of mixing the two.
14/ Ceramic caps with any other dielectric other than NPO should only be used for bypass applications.
15/ An N-channel enhancement-mode FET needs +ve voltage on the gate-source to conduct form drain-source.
16/ Small signal JFETS work very well as low-leakage diodes by connecting drain & source together in log current-to-voltage converters and low leakage input protection. Small signal bipolars with b-c tied together will also make nice low-leakage diodes.
17/ With low pass filter use Bessel for least amount of overshoot in the time domain, and Cauer (or elliptic) for fastest rolloff in the freq domain.
18/ dB is always 10 times the log of the ratio of 2 powers.
19/ At low frequencies, the current in the collector of a transistor is in phase with the applied current at the base. At high frequencies the current at the collector lags by 90deg. You must appreciate this simple fact to understand high frequency oscillators.
20/ The most common glass-epoxy PCB material (FR4) has a dielectric constant of about 4.3 To make a trace with a characteristic impedance of 100 Ohm, use a trace thickness of about 0.4 times the thickness of the board with a ground plane on the opposite side. For a 50Ohm trace make it 2 times the thickness.
21/ If you need a programmable dynamic current source, find out about operational transconductance amps. Most of the problem is figuring out when you need a programmable dynamic current source.
22/ A CMOS output with an emitter follower can drive a 5V relay nicely as the relays normally have a must-make spec of 3.5V. This saves power and require no flyback components.
23/ Typical thermocouple potential is 30uV/degC. Route signals differentially, along the same path, avoid temp gradients. DPDT latching relays won"t heat up when multiplexing these signals.
24/ You SHOULD be bothered by a design that looks messy, cluttered or indirect. This uncomfortable feeling is one of the few indications that there"s a better way.
25/ Avoid drawing any current from the wiper of a potentiometer. The resistance of the wiper contact will cause problems (local heating, noise offsets etc.)
26/ Most digital phase detectors have a deadband where the analog output does not change over the small range where the 2 inputs are coincident. This often-ignored fact has helped to create some very noisy PLL"s (Use a high val bleeding resistor to always ensure current flow in the deadband)
27/ The phase noise of a phase-locked VCO will be at least 6dB worse than the phase noise of the divided reference for each octave between the comparison frequency and the VCO output frequency. Avoid low-comparison frequencies.
28/ You can almost always determine the leads of a bipolar transistor with an ohm meter. b-e and b-c junctions will measure like a diode with the b-c junction reading slightly lower than the b-e junction when forward biased.
29/ For low distortion, the drains (or collectors) of a differential amp"s front-end should be bootstrapped to the source (or emitter) so that the voltages on the part are not modulated by the input signal.
30/ If your design uses a $3 op amp, and you will be making a thousand of them, you have just spend $3000. Are you smart enough to figure out how to use a $.30 op amp instead?
31/ The Q of an LC tank circuit is dominated by the losses in the inductor in terms of series R. Q=omega.L/R
32/ Leakage current doubles for every 10degC increase in temp.
33/ When inputs to most JFET op amps exceed the common-mode range for the part, the output may reverse polarity. This artifact will haunt the designers of these parts for the rest of their lives, as it should!
34/ Understand the difference between "make-before-break" and "break-before-make" when you specify switches.
35/ 3 Terminal voltage regulators in the TO-220 packages are wonderful parts. They are cheap, rugged, thermally protected and very versatile. Use them virtually any place where you need a protected power transistor. They also make nice AM power-modulators.
36/ Use step recovery diode where you need fast edges under 100pS (hot-carrier is even faster)
37/ The old 723 regulator is still one of the lowest noise regulators around! (2.5uVrms 100Hz-10k)
38/ You can make a very simple oscillator with one diac, cap and a resistor.
39/ NPN transistors are normally superior to their PNP counterpart in performance.
40/ Typical spec in some databooks should read "Seen it once". Always work with the worst spec of the part when doing a design.
41/ Don"t just copy circuits from application notes without understanding completely how it operates, and the reason for the choice of values.
42/ Dealing with crystals, make sure you understand the difference between series and parallel resonant. In a circuit, crystal frequency can generally be slightly lowered by placing a inductor in series and increased by a capacitor in series.
43/ Power MOSFETS on-resistance will have a -ve temp coef and not +ve at low current levels. This is important to remember when paralleling devices.
44/ Lowest noise figure of a RF transistor is not normally where the input is perfectly matched.
45/ Many un-stable RF devices can be made stable by loading the input or the output by a simple resistor, either in series or parallel.
46/ You trade gain for bandwidth.
47/ Push-pull power invertors using bipolars are risky and can saturate the core because of hysteresis stepping (use power fets)
48/ The Al value of a core will increase up to 50% or more under current transients.
49/ Be aware of leakage inductance when switching. V=L(dI/dt)
50/ The harder you turn-on a power transistor, the longer it will take to turn off.( the part where you burn the joules in the device)