Today I want to share with you a very practical little common sense of analog circuits.
1. In the design of electrical interface, the reflection attenuation usually becomes worse at high frequencies. This is because the transmission line reflection with loss is related to frequency. In this case, it is extremely important to shorten the PCB trace as much as possible.
2. The Zener diode is a diode that stabilizes the working voltage of the circuit. Due to the special internal structure characteristics, it is suitable for the working state of reverse breakdown. As long as the current is limited, this breakdown is non-destructive.
3. The PN junction has a good mathematical model: switch model → the diode is born → another PN junction, the triode is born.
4. In high-frequency circuits, the influence of PN junction capacitance must be considered (forward bias is diffusion capacitance, reverse bias is barrier capacitance).
5. In high-density occasions, crosstalk is prone to occur because the sending and receiving signals are close together. When wiring, the 3W principle must be followed, that is, the centerline spacing of adjacent PCB traces should be greater than 3 times the PCB line width. There should be many grounding pins at the position where the plug-in card and the connector are connected to provide a good radio frequency loop.
6. The bipolar tube is a current control device, which controls a large collector current through a small base current; the MOS tube is a voltage control device, which controls the on-resistance between source and drain through the gate voltage.
7. The triode works by the movement of carriers. Take the npn emitter follower as an example. When the base is applied with or without voltage, the pn junction formed by the base region and the emitter region is to prevent many electrons (the base region is The diffusion movement of holes, electrons in the emitter region will induce an electrostatic field (that is, a built-in electric field) from the emitter region to the base region at the pn junction.
8. Schottky diodes (Schottky, SBD) are suitable for high-frequency switching circuits, with low forward voltage drop and reverse voltage drop (0.2V), but low reverse breakdown voltage and large leakage current.
9. Most of the jitter characteristics depend on the characteristics of the output chip. However, if the PCB layout is improper, the power filter is not sufficient, and the clock reference source is too aggressive, it will also increase the jitter component. Signal line matching has a direct impact on jitter. In particular, the chip contains a frequency multiplication function, and its phase noise is relatively large.
10. Pole type selection refers to whether the BJT uses a PNP or NPN tube, which should be considered when determining the power supply form. The shell of some triodes is connected to an electrode, which is often the collector for silicon tubes. This factor should be taken into account when base grounding is required.
11. There is a big difference between the field effect transistor and the BJT in the working process: the charge carrier in the BJT is a hole or a small amount of "minority" that is knocked out, while the charge in the FET is a relatively large number of orders of magnitude. Electronics, "many sons".
12. The forward bias of the emitter and the reverse bias of the collector are the prerequisites for the BJT to work in the enlarged working state. Three connection methods: common base, common emitter (most, because current, voltage, and power can be amplified), common collector. Distinguishing methods of three configurations: common emitter, input from the base, output from the collector; common collector, input from the base, output from the emitter; common base, input from the emitter, output from the collector.
13. The main parameters of the triode: current amplification factor β, reverse current between electrodes, (the maximum allowable current of the collector, the maximum allowable power dissipation of the collector, and the reverse breakdown voltage=3 important limit parameters determine that the BJT works in a safe area) .
14. Because the Rgs of J-FET is very high, you should first pay attention to non-static operation when using it, otherwise gate breakdown will easily occur; in addition, you should carefully consider each limit parameter when designing the circuit, and it should not exceed the range. When using the J-FET as a variable resistor, it should be ensured that the device has the correct bias and cannot enter the constant current region.
15. Emitter bias circuit: used to eliminate the influence of temperature on the static operating point (dual 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: only amplifies the current, follows the voltage, the input R is large, and the output R is small, used as an input stage and an output stage. Common base amplifier circuit: only amplifies the voltage, follows the current, and has good high-frequency characteristics.
17. Decoupling capacitor: The output signal capacitor is grounded to filter out high-frequency clutter of the signal. Bypass capacitor: The input signal capacitor is grounded to filter out high-frequency clutter of the signal. AC signals are treated as short circuits for these two capacitors.
18. In addition to the correct selection of parameters and correct calculations in the use of MOS-FETs, the most important thing to emphasize is still the problem of anti-static operation. In the process of circuit debugging, welding, and installation, it is necessary to strictly follow the anti-static procedures.
19. The mainstream is the IC from the emitter to the collector, and the bias current is Ib from the emitter to the base. Relative to the main circuit, the circuit that supplies current to the base is the so-called bias circuit.
20. Three aluminum electrodes of field effect tube: gate g, source s, drain d. Corresponding to the base b, emitter e, and collector c of the triode respectively. <The source needs to emit something, so corresponding to the emitter e, the English name of the gate is gate, which exists like a gate, and has almost the same function as the base> where the P-type substrate is generally connected to the gate g.
21. The enhanced FET must rely on the gate-source voltage Vgs to function (turn-on voltage Vt), while the depletion-type FET does not require a gate-source voltage. Under the action of positive Vds, a large drain current flows to the source ( If a negative Vgs is applied, pinch-off may occur, and the voltage at this time becomes the pinch-off voltage Vp***important characteristic***: it can work under positive and negative gate-source voltages)
22. N-channel MOS transistors require positive Vds (equivalent to the Vcc added to the collector of the triode) and positive Vt (equivalent to the Vbe of the base and emitter of the triode), while P-channel MOS transistors require negative Vds and negative Vt.
23. VMOSFET has the advantages of high input impedance, low drive current; fast switching speed, good high-frequency characteristics; negative current temperature coefficient, no thermal vicious cycle, and excellent thermal stability.
24. When the operational amplifier is applied, the negative feedback current is generally applied.
25. Differential amplifier circuit: differential mode signal: the difference between two input signals. Common mode signal: the sum of the two input signals divided by 2. Thus: using the definition of differential mode and common mode to represent two input signals can get 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 suppresses common mode signals. Utilizing this feature, the influence of changes in external factors such as temperature on circuit performance can be well suppressed. Specific performance indicators: common mode rejection ratio Kcmr.
27. When the diode is switched from forward bias to reverse bias, a large reverse recovery current will flow from the cathode to the anode, and the reverse current will first rise to the peak value and then drop to zero.
28. In an ideal situation, if the current and voltage waves of the two transistors of the push-pull circuit are completely symmetrical, there will be no even-order harmonic components in the output current, and the push-pull circuit is affected by known even-order harmonics. In fact, because the characteristics of the two tubes are always different, the circuit cannot be completely symmetrical, so the output current will also have even harmonic components. In order to reduce nonlinear distortion, try to select the paired tubes as much as possible.
29. In order to obtain large output power, the voltage and current applied to the power transistor are very large, and the transistor works in a state of large signal. In this way, the safe operation of the transistor becomes an important issue of the power amplifier, and generally does not exceed the limit parameters (Icm, BVceo, Pcm) of the tube as the limit.
30. Interference of the amplifier circuit: 1. Keep the power supply away from the amplifier circuit. 2. Shield the input stage.
31. Four configurations of negative feedback amplifying circuit: voltage series negative feedback (stable output voltage), voltage parallel negative feedback, current series negative feedback (stable output current), current parallel negative feedback.
32. Judgment method of voltage and current feedback: output short circuit method, set RL=0, if the feedback signal does not exist, it is voltage feedback, otherwise, it is current feedback.
33. Judgment method of series and parallel feedback: the summation method of the feedback signal and input signal, if it is in the form of voltage, it is series feedback, and if it is in the form of current, it is parallel feedback.
34. For NPN circuits, for common emitter configurations, it can be roughly understood as taking VE as a "fixed" reference point, controlling VBE (VBE=VB-VE) by controlling VB, thereby controlling IB, and further controlling IC (from The place with a higher potential flows into the C pole, and you can also regard the C pole as an upward funnel for water inflow).
35. For digital circuits, VCC is the power supply voltage of the circuit, VDD is the working voltage of the chip (usually Vcc>Vdd), and VSS is the ground point; in a field effect transistor (or COMS device), VDD is the drain, and VSS is Source, VDD and VSS refer to component pins, not supply voltages.
36. The oscilloscope probe has a ground wire and a signal wire. The ground wire is the one connected to the shell of the oscilloscope input terminal. It is usually in the shape of a clip. The signal wire generally has a probe hook. If you connect it, you connect the ground wire of the oscilloscope to the The ground of your equipment, connect the signal line terminal to your signal terminal. Note that if the signal to be measured is not isolated from the mains, it cannot be measured directly.
37. There are two cases of insufficient driving ability: one is that the input resistance of the device is too small, and the output waveform will be deformed, such as the TTL level cannot drive the relay; the other is that the input resistance of the device is large enough, but the power of the device cannot be reached, such as small A high-power amplifier drives a high-power speaker. The speaker can sound, but the volume is very low. In fact, the output voltage is not large enough.
38. Filtering circuit: Using the energy storage function of the reactance element, it can play a very good filtering role. Inductance (series connection, high power) and capacitance (parallel connection, low power) can both play the role of smoothing.
39. Switching regulated power supply and linear power supply: linear power supply has low efficiency and strong heat generation, but the output is very stable. Switching power supply has high efficiency and average heat generation, but the output ripple is large and needs to be smoothed.
The above is all the content shared today. I hope that after reading this article, it will be helpful to you.
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