Nyatane, papan sirkuit cetak (PCB) are made of electrical linear materials, i.e. their impedance should be constant. Dadi, kenapa PCB ngenalake nonlinearity dadi sinyal? Wangsulane yaiku tata letak PCB “non-linear spatially” relatif ing endi aliran saiki.

Apa ampli nampa arus saka siji sumber utawa sumber liyane gumantung ing polaritas sinyal cepet ing beban kasebut. 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; Sing liyane nyenengake resistor input nganti kapasitor bypass. Gambar 1 nggambarake rong jalur kasebut. The backflow current is what’s really causing the problem.

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Nalika kapasitor bypass dilebokake ing posisi sing beda ing PCB, arus saiki mili liwat jalur sing beda menyang kapasitor bypass, sing artine “nonlinearity spasial”. 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. Yen polaritas arus lemah liyane ora diganggu, voltase sinyal input bakal diganti kanthi cara ora linier. Nalika salah sawijining komponen polaritas diganti nanging polaritas liyane ora, distorsi banjur ditampilake minangka distorsi harmoni nomer loro saka sinyal output. Gambar 2 nuduhake efek distorsi iki ing bentuk sing berlebihan.

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Yen mung siji komponen kutub saka gelombang sinus sing diganggu, gelombang sing diasilake ora ana maneh gelombang sinus. Simulasi ampli sing becik kanthi beban 100-ω lan gandeng beban saiki liwat resistor 1-into menyang voltase lemah mung ing siji kutub sinyal, asil ing gambar 3. Fourier transform shows that the distortion waveform is almost all the second harmonics at -68 DBC. Kanthi frekuensi dhuwur, level kopling iki gampang digawe ing PCB, sing bisa ngrusak karakteristik anti-distorsi ampli tanpa nggunakake efek nonlinear khusus PCB. 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

Masalah chip multi-amplifier (loro, telu, utawa papat amplifier) ​​ditambah karo ora bisa njaga koneksi lemah kapasitor bypass adoh saka kabeh input. Iki pancen cocog kanggo papat ampli. Kripik kotak-amplifier duwe terminal input ing saben sisih, mula ora ana ruang sirkuit bypass sing nyuda gangguan ing saluran input.

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Gambar 5 nuduhake pendekatan sederhana kanggo tata letak papat-amplifier. Umume piranti nyambung langsung menyang pin ampli kotak. Arus lemah saka siji pasokan listrik bisa ngganggu voltase lemah input lan arus lemah pasokan listrik saluran liyane, nyebabake distorsi. Contone, kapasitor bypass (+ Vs) ing saluran 1 saka ampli kotak bisa langsung dipasang ing jejer; Kapasitor bypass (-Vs) bisa dilebokake ing sisih liya paket. Arus lemah (+ Vs) bisa ngganggu saluran 1, dene arus lemah (-vs) bisa uga ora.

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Kanggo ngindhari masalah iki, wenehi arus sing saiki ngganggu input, nanging supaya arus PCB saiki mili kanthi gaya linier. Kanggo nggayuh iki, kapasitor bypass bisa diatur ing PCB kanthi arus (+ Vs) lan (- Vs) arus lemah mili liwat jalur sing padha. Yen sinyal input merata karo aliran positif lan negatif, distorsi ora bakal kedadeyan. Mula, selarasake loro kapasitor bypass ing sandhinge supaya bisa nuduhake titik dhasar. Amarga rong komponen kutub ing bumi saiki padha saka titik sing padha (konektor output output utawa landasan beban) lan kalorone mili bali menyang titik sing padha (sambungan dhasar umume kapasitor bypass), arus positif / negatif mili dalan sing padha. Yen resistensi input saka saluran bakal diganggu dening (+ Vs) saiki, (- Vs) saiki duwe efek sing padha. 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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Kanggo verifikasi inferensi ing ndhuwur, digunakake rong layout PCB sing beda: tata letak sing gampang (Gambar 5) lan tata distorsi rendah (Gambar 6). Distorsi sing diproduksi dening amplifier quad-operasional FHP3450 nggunakake semikonduktor fairchild ditampilake ing tabel 1. Bandwith khas FHP3450 yaiku 210MHz, kemiringan 1100V / us, arus bias input yaiku 100nA, lan arus operasi saben saluran yaiku 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. Temenan, saluran ampli sing diwenehake ora mung ngganggu input dhewe, nanging uga input saluran liyane. 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 nuduhake harmoni sing diukur ing saluran liyane sing ora disurung nalika mung siji saluran sing didorong. Saluran undriven nampilake sinyal cilik (crosstalk) kanthi frekuensi dhasar, nanging uga ngasilake distorsi sing langsung dikenalake karo arus lemah yen ora ana sinyal dhasar sing penting. Tata letak distorsi rendah ing Gambar 6 nuduhake yen karakteristik distorsi harmoni kaping loro lan distorsi harmoni (THD) luwih apik amarga efek samping saiki wis cedhak.

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Ringkesan artikel iki

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. Saiki sinyal frekuensi dhuwur mili bali menyang kapasitor bypass cilik. Aliran frekuensi rendah, kayata sinyal audio, bisa uga utamane mili liwat kapasitor bypass sing luwih gedhe. Malah saiki frekuensi sing luwih murah bisa “nglirwakake” kapasitansi bypass lengkap lan mili langsung bali menyang plumbungan listrik. Aplikasi tartamtu bakal nemtokake dalan saiki sing paling kritis. 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.

Aturan emas kanggo tata letak PCB HF yaiku supaya kapasitor bypass HF bisa cedhak pin daya rangkep, nanging mbandhingake Gambar 5 lan Gambar 6 nuduhake manawa ngowahi aturan iki kanggo nambah karakteristik distorsi ora ana bedane. Karakteristik distorsi sing luwih apik ngasilake biaya nambah udakara 0.15 inci kabel kapasitor bypass frekuensi tinggi, nanging iki ora ana pengaruh sing gedhe tumrap kinerja respons AC FHP3450. Tata letak PCB penting kanggo nggedhekake kinerja ampli berkualitas tinggi, lan masalah sing dibahas ing kene ora diwatesi mung kanggo ampli hf. Sinyal frekuensi ngisor kayata audio duwe syarat distorsi sing luwih ketat. Efek arus lemah luwih cilik kanthi frekuensi kurang, nanging bisa uga dadi masalah yen indeks distorsi sing dibutuhake ditambah apik.