# 40 Practical Analog Circuit Tips That 90% of People Will Ignore

2023-03-31Archive

1. When designing an electrical interface, reflection attenuation typically gets worse at high frequencies. This is because reflection of a lossy transmission line is frequency related. In this case, it is extremely important to keep PCB track as short as possible. as much as possible.

2. A zener diode is a diode that stabilizes operating voltage of a circuit. Due to special characteristics of internal structure, it is suitable for working state of reverse breakdown. As long as current is limited, this breakdown is impossible. -destructive.

3. The PN junction has a good mathematical model: switch model → a diode is born → another PN junction is born, a triode is born.

4. In high-frequency circuits, it is necessary to take into account influence of capacitance of PN junction (forward bias - diffusion capacitance, reverse bias - barrier capacitance).

5. In cases of high density, crosstalk can occur because send and receive signals are close to each other. When connecting, 3W principle must be observed, that is, distance between center lines of adjacent PCB tracks must be greater than 3 times width of PCB line. There should be plenty of ground pins at junction between plug-in board and connector to provide a good RF loop.

6, Bipolar lamps are current control devices that control more collector current through a small base current; MOS lamps are voltage control devices that control source and drain voltage through gate voltage. On-resistance between .

7. The triode works by moving carriers. Take npn emitter follower as an example, when base is supplied with or without voltage, pn junction formed by base region and emitter region should prevent many carriers (The main region is holes, emission region is electrons) , and p-n junction will induce an electrostatic field (that is, a built-in electric field) from emission region to base region.

8. Schottky diodes (Schottky, SBD) are suitable for high frequency switching circuits, with low forward and reverse voltage drop (0.2V), but low reverse breakdown voltage and low leakage current.

9.Most of jitter characteristics depend on characteristics of output chip. However, if PCB layout is not correct, power supply filter is insufficient, and frequency reference is too aggressive, this will also increase jitter component. Signal line termination directly affects jitter. In particular, chip contains a frequency multiplication function, and its phase noise is relatively large.

10. Pole type selection refers to whether BJT uses PNP or NPN tubing, which should be considered when determining shape of power supply. The body of some triodes is connected to an electrode, which is often collector for silicon lamps. This factor should be taken into accountb when grounding of a certain pole is required.

11. There is a big difference between a FET and a bipolar transistor in workflow: charge carrier in a bipolar transistor is a hole or a small amount of "minority" that is knocked out, while charge in an FET is a relatively large number of free electrons, "many sons".

12. The emitter is forward biased and collector is reverse biased. This is a prerequisite for BJT to operate in an extended working state. Three connection methods: common base, common emitter (most because current, voltage and power can be amplified), common collector. Methods to distinguish three configurations: common emitter, base input, collector output, common collector, base input, emitter output, common base, emitter input, collector output.

13. The main parameters of triode are: current amplification factor β, reverse current between electrodes, (maximum allowable collector current, maximum allowable collector power dissipation and reverse breakdown voltage = 3 important limiting parameters that determine operating state of BJT safe zone).

14. Since Rgs of J-FET is very high, you must first pay attention to non-static operation when using it, otherwise gate breakdown will easily occur; In addition, you must carefully consider each limit parameter when designing a circuit, and it must not exceed range. When using J-FET as a variable resistor, make sure device is properly biased and cannot get into DC region.

15. Emitter bias circuit: used to eliminate temperature influence on static operating point (double power supply is better).

16. Comparison of three BJT amplifier circuits: common emitter amplifier circuit, both current and voltage can be amplified. Common Collector Amplifier Circuit: Amplifies current only, follows voltage, input R is large and output R is small, used as input stage and output stage. Common-base amplifier circuit: amplifies voltage only, follows current, and has good high-frequency performance.

17. Decoupling Capacitor: The output signal capacitor is grounded to filter out high frequency signal noise. Bypass Capacitor: The input signal capacitor is grounded to filter out high frequency signal noise. AC signals are treated as short circuits for these two capacitors.

18.Besides correct parameter selection and correct calculation when using MOSFETs, most important point to pay attention to is problem of antistatic operation.Antistatic procedures must be strictly observed during circuit debugging, welding, and installation.

19. The main direction is IC from emitter to collector and bias current is Ib from emitter to base. Compared to main circuit, circuit supplying current nand base is so-called bias circuit.

20. Three aluminum FET electrodes: gate g, source s, drain d. Corresponding to base b, emitter e and collector c of triode, respectively. , where a P-type substrate is usually connected to gate r.

21. An advanced FET must rely on a gate-to-source voltage, Vgs, to operate (turn-on voltage, Vt), while a depleted type FET does not require a gate-to-source voltage. By positive Vds, there will be a large drain current direction Source (when negative voltage Vgs is applied, cutoff may occur, and voltage at this time becomes cutoff voltage Vp ***important feature***: it can work with positive and negative gate). source voltage)

22. N-channel MOSFETs require positive Vds (equivalent to Vcc added to collector of triode) and positive Vt (equivalent to Vbe of base and emitter of triode), while P-channel MOSFETs require negative Vds and negative Vt.

23. VMOSFET has advantages of high input impedance, low drive current, fast switching speed, good high frequency performance, negative current temperature coefficient, no thermal vicious cycle, and excellent thermal stability.

24. When op amps are used, negative feedback currents are usually used.

25. Differential amplifier circuit: differential mode signal: difference between two input signals. Common mode signal: sum of two input signals divided by 2. Thus: using definition of differential mode and common mode to represent two input signals, one can obtain an important mathematical model: any input signal = common mode signal ± differential mode signal/2.

26. The differential amplifier circuit only amplifies differential mode signals and rejects common mode signals. By using this function, influence of changes in external factors such as temperature on operation of circuit can be well suppressed. Specific performance indicators: common mode rejection ratio Kcmr.

27. When diode switches from forward to reverse bias, a large reverse recovery current will flow from cathode to anode, and reverse current will first rise to a peak value and then fall to zero.

28. Under ideal conditions, if current and voltage waves of two transistors of a push-pull circuit are perfectly symmetrical, there will be no even harmonic components in output current, and push-pull circuit consists of known even harmonic components. In fact, since characteristics of two lamps are always different, circuit cannot be completelysymmetrical, so output current will also have even harmonic components. To reduce non-linear distortion, try to choose paired lamps as much as possible.

29. In order to obtain a large output power, voltage and current supplied to power transistor are very large, and transistor operates in a large signal state. Therefore, safe operation of transistor becomes an important issue for power amplifier, and generally does not exceed limit parameters (Icm, BVceo, Pcm) of lamp as a limit.

30. Amplifier circuit noise: 1. Keep power supply away from amplification circuit 2. Shield input stage.

31. Four negative feedback amplifier circuit configurations: negative feedback with serial voltage (stable output voltage), negative feedback with parallel voltage, negative feedback with series current (stable output current), negative feedback with parallel current.

32. Voltage and current feedback evaluation method: output short circuit method, set RL=0, if there is no feedback signal, it is voltage feedback, otherwise it is current feedback.

33. Serial and parallel feedback evaluation method: method of summing feedback signal and input signal, if it is in form of voltage, it is series feedback, and if it is in form of current, it is parallel feedback.

34. For NPN circuits, for common emitter configurations, this can be roughly understood as taking VE as a "fixed" reference point, driving VBE (VBE=VB-VE) by driving VB, thereby driving IB and further driving IC (flows into C pole from a higher potential location, column C can also be considered as an ascending funnel for water inflow).

35. For digital circuits, VCC is supply voltage of circuit, VDD is operating voltage of microcircuit (usually Vcc>Vdd), VSS is ground point, in FETs (or COMS devices), VDD is drain, VSS is source, VDD and VSS refers to component pins, not supply voltage.

36. The oscilloscope probe has a ground wire and a signal wire. The ground wire is connected to shell of oscilloscope's input connector. It usually takes form of a clip. The signal wire usually has a hook probe. If you connect it, you are connecting ground of oscilloscope. Connect wire to ground of your device and connect signal wire terminal to your signal terminal. Please note that if signal to be measured is not isolated from mains, it cannot be measured directly.

37. There are two cases of insufficient control ability: one is that input impedance of device is too small, and output waveform will be deformed, such as TTL level cannot drive relay; other is that input impedance of device is large enough, but powerdevices can not be reached, such as low-power power amplifier driving a powerful speaker, speaker can sound, but volume is very small, in fact, output voltage is not large enough.

38.Filtration circuit: using reactive element energy storage function, it can play a very good filtering role. Inductance (series connection, high power) and capacitance (parallel connection, low power) can play a smoothing role.

39. Switching regulated power supply and linear power supply: The linear power supply has low efficiency and strong heat dissipation, but output is very stable. The switching power supply has a high efficiency and average heat dissipation, but output ripple is large and needs to be smoothed out.

40. The types of failures caused by internal factors in electronic circuits include: failures caused by changing characteristics of electronic components such as transistors, capacitors and resistors, failures caused by poor contact of corresponding lines in electronic circuits. Types of electronic circuit failures caused by external factors include: technicians do not follow instructions when using electronic circuits; technicians have incorrect and unscientific maintenance procedures.