Yn feite, printplaat (PCB) are made of electrical linear materials, i.e. their impedance should be constant. Dat wêrom yntroduseart in PCB nonlineariteit yn in sinjaal? It antwurd is dat de PCB-yndieling “romtlik net-lineêr” is relatyf oan wêr’t de hjoeddeistige streamt.

Oft de fersterker stroom ûntfangt fan ien of oare boarne, hinget ôf fan ‘e instantane polariteit fan it sinjaal op’ e lading. Current flows from the power supply, through the bypass capacitor, through the amplifier into the load. The current then travels from the load ground terminal (or shielding of the PCB output connector) back to the ground plane, through the bypass capacitor, and back to the source that originally supplied the current.

The concept of minimum path of current through impedance is incorrect. The amount of current in all different impedance paths is proportional to its conductivity. In a ground plane, there is often more than one low-impedance path through which a large proportion of ground current flows: one path is directly connected to the bypass capacitor; De oare eksiteart de ynputweerstand oant de bypass -kondensator wurdt berikt. Figure 1 illustrates these two paths. The backflow current is what’s really causing the problem.

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As de rûnwei -kondensatoren op ferskate posysjes op ‘e PCB wurde pleatst, streamt de grûnstream troch ferskate paden nei de respektivelike rûnwei -kondensatoren, wat de betsjutting is fan “romtlike netlineariteit”. If a significant portion of a polar component of the ground current flows through the ground of the input circuit, only that polar component of the signal is disturbed. If the other polarity of the ground current is not disturbed, the input signal voltage changes in a nonlinear manner. As ien polariteitskomponint wurdt feroare, mar de oare polariteit net is, komt ferfoarming foar en wurdt manifesteare as de twadde harmonyske ferfoarming fan it útgongssignaal. Figuer 2 toant dit ferfoarmingseffekt yn oerdreaune foarm.

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As mar ien poalkomponint fan ‘e sinusgolf wurdt fersteurd, is de resultearjende golffoarm net langer in sinusgolf. Simulearje in ideale fersterker mei in 100-ω lading en koppeljen fan de ladingstroom fia in 1-ω wjerstân yn ‘e grûnspanning op mar ien polariteit fan it sinjaal, resulteart yn figuer 3. Fourier transform shows that the distortion waveform is almost all the second harmonics at -68 DBC. By hege frekwinsjes wurdt dit nivo fan koppeling maklik genereare op in PCB, wat de treflike anty-ferfoarmingskenmerken fan in fersterker kin ferneatigje sûnder in protte fan ‘e spesjale net-lineêre effekten fan in PCB te brûken. When the output of a single operational amplifier is distorted due to the ground current path, the ground current flow can be adjusted by rearranging the bypass loop and maintaining distance from the input device, as shown in Figure 4.

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Multiamplifier chip

The problem of multi-amplifier chips (two, three, or four amplifiers) is compounded by the inability to keep the ground connection of the bypass capacitor far from the entire input. This is especially true for four amplifiers. Quad-fersterker-chips hawwe ynputterminalen oan elke kant, dus d’r is gjin romte foar bypass-sirkels dy’t steuring foar it ynfierkanaal beheine.

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Figuer 5 toant in ienfâldige oanpak foar in layout mei fjouwer fersterker. De measte apparaten ferbine direkt mei in quad -fersterker -pin. De grûnstream fan ien stroomfoarsjenning kin de ynputgrûnspanning en grûnstroom fan ‘e oare kanaalfoarsjenning fersteure, wat resulteart yn ferfoarming. Bygelyks, de (+Vs) bypass -kondensator op kanaal 1 fan ‘e quad -fersterker kin direkt neist syn ynfier wurde pleatst; De (-Vs) bypass-kondensator kin oan ‘e oare kant fan it pakket wurde pleatst. De (+Vs) grûnstream kin kanaal 1 fersteure, wylst de (-vs) grûnstream net kin.

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Om dit probleem te foarkommen, lit de grûnstream de ynfier fersteure, mar lit de PCB -stroom op romtlik lineêre manier streamje. To achieve this, the bypass capacitor can be arranged on the PCB in such a way that the (+Vs) and (– Vs) ground currents flow through the same path. If the input signal is equally disturbed by positive and negative currents, distortion will not occur. Rjochtsje dêrom de twa bypass -kondensatoren njonken inoar sadat se in grûnpunt diele. Om’t de twa poalkomponinten fan ‘e ierdstroom út itselde punt komme (de útfierferbining beskermet as de ladinggrûn) en beide werom streamje nei itselde punt (de mienskiplike grûnferbining fan’ e bypass -kondensator), streamt de positive/negative stroom troch itselde paad. If the input resistance of a channel is disturbed by (+Vs) current, (– Vs) current has the same effect on it. Because the resulting disturbance is the same regardless of the polarity, there is no distortion, but a small change in the gain of the channel will occur, as shown in Figure 6.

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Om de boppesteande ynferinsje te ferifiearjen, waarden twa ferskillende PCB-yndielingen brûkt: in ienfâldige yndieling (ôfbylding 5) en in yndieling mei lege ferfoarming (ôfbylding 6). De ferfoarming produsearre troch de FHP3450 quad-operasjonele fersterker mei help fan fairchild healgelieder wurdt werjûn yn tabel 1. De typyske bânbreedte fan ‘e FHP3450 is 210MHz, de helling is 1100V/us, de ynfierbiasstroom is 100nA, en de bestjoeringsstroom per kanaal is 3.6 mA. As can be seen from Table 1, the more distorted the channel, the better the improvement, so that the four channels are nearly equal in performance.

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Without an ideal quad amplifier on a PCB, measuring the effects of a single amplifier channel can be difficult. Fansels fersteurt in bepaald fersterkerkanaal net allinich de eigen ynput, mar ek de ynfier fan oare kanalen. The earth current flows through all the different channel inputs and produces different effects, but is influenced by each output, which is measurable.

Tabel 2 toant de harmonika’s mjitten op oare net -riven kanalen as mar ien kanaal wurdt oandreaun. It net -oandreaune kanaal toant in lyts sinjaal (oerspraak) op ‘e fûnemintele frekwinsje, mar produseart ek ferfoarming dy’t direkt wurdt yntrodusearre troch de grûnstroom by it ûntbrekken fan in wichtich fûnemintaal sinjaal. De yndieling mei lege ferfoarming yn figuer 6 lit sjen dat de twadde harmonyske en totale harmonyske ferfoarming (THD) skaaimerken sterk binne ferbettere fanwegen it hast eliminearjen fan it grûnstromeffekt.

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Dit gearfetting fan dit artikel

Simply put, on a PCB, the backflow current flows through different bypass capacitors (for different power supplies) and the power supply itself, which is proportional to its conductivity. De heechfrekwinsjele sinjaalstream streamt werom nei de lytse bypass-kondensator. Streamen mei lege frekwinsje, lykas dy fan audiosignalen, kinne foaral troch gruttere bypass-kondensators streamje. Sels in legere frekwinsjestream kin de folsleine bypass -kapasitânsje “negearje” en direkt werom streamje nei de stroomlieding. De spesifike applikaasje sil bepale hokker aktuele paad it meast kritysk is. Fortunately, it is easy to protect the entire ground current path by using a common ground point and a ground bypass capacitor on the output side.

De gouden regel foar HF PCB -yndieling is om de HF -bypass -kondensator sa ticht mooglik by de ferpakte power -pin te hâlden, mar in fergeliking fan figuer 5 en figuer 6 lit sjen dat it feroarjen fan dizze regel foar it ferbetterjen fan ferfoarmingskenmerken net folle fan in ferskil makket. De ferbettere skaaimerken fan ferfoarming kamen op rekken fan it tafoegjen fan sawat 0.15 inch heechfrekwinsje bypass-kondensatorbedrading, mar dit hie net folle ynfloed op ‘e AC-antwurdprestaasjes fan’ e FHP3450. PCB layout is important to maximize the performance of a high-quality amplifier, and the issues discussed here are not limited to hf amplifiers. Sinjalen mei legere frekwinsje lykas audio hawwe folle strangere easken foar ferfoarming. The ground current effect is smaller at low frequencies, but it may still be an important problem if the required distortion index is improved accordingly.