Ni pato, tejede Circuit ọkọ (PCB) are made of electrical linear materials, i.e. their impedance should be constant. Nitorinaa kilode ti PCB ṣe ṣafihan aiṣedeede sinu ifihan kan? Idahun ni pe ipilẹ PCB jẹ “spatially non-linear” ibatan si ibiti ṣiṣan lọwọlọwọ.

Boya ampilifaya gba lọwọlọwọ lati orisun kan tabi omiiran da lori polarity lẹsẹkẹsẹ ti ifihan lori fifuye. 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; Awọn miiran excites awọn input resistor titi ti fori kapasito ni ami awọn. Figure 1 illustrates these two paths. The backflow current is what’s really causing the problem.

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Nigbati a ba gbe awọn kapasito fori ni awọn ipo oriṣiriṣi lori PCB, lọwọlọwọ ilẹ n ṣàn nipasẹ awọn ọna oriṣiriṣi si awọn kapasito fori, eyiti o jẹ itumọ ti “aiṣedeede aaye”. 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. When one polarity component is changed but the other polarity is not, distortion occurs and is manifested as the second harmonic distortion of the output signal. Nọmba 2 fihan ipa iyọkuro yii ni fọọmu abumọ.

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When only one polar component of the sine wave is disturbed, the resulting waveform is no longer a sine wave. Ṣiṣeduro ohun ampilifaya ti o dara pẹlu fifuye 100-and ati idapọ lọwọlọwọ fifuye nipasẹ alatako 1-into sinu foliteji ilẹ lori polarity kan ti ifihan nikan, awọn abajade ni eeya 3. Fourier transform shows that the distortion waveform is almost all the second harmonics at -68 DBC. At high frequencies, this level of coupling is easily generated on a PCB, which can destroy the excellent anti-distortion characteristics of an amplifier without resorting to much of the special nonlinear effects of a 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

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. Awọn eerun Quad-ampilifaya ni awọn ebute igbewọle ni ẹgbẹ kọọkan, nitorinaa ko si aye fun awọn iyika fori ti o dinku idamu si ikanni titẹ sii.

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Nọmba 5 fihan ọna ti o rọrun si ipilẹ amplifier mẹrin. Most devices connect directly to a quad amplifier pin. Ilẹ lọwọlọwọ ti ipese agbara kan le ṣe idiwọ foliteji ilẹ titẹ sii ati lọwọlọwọ ilẹ ti ipese agbara ikanni miiran, ti o fa iyọkuro. For example, the (+Vs) bypass capacitor on channel 1 of the quad amplifier can be placed directly adjacent to its input; Kapasito fori (-V) le ṣee gbe ni apa keji ti package. The (+Vs) ground current can disturb channel 1, while the (-vs) ground current may not.

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Lati yago fun iṣoro yii, jẹ ki ilẹ lọwọlọwọ ṣe idawọle titẹ sii, ṣugbọn jẹ ki PCB lọwọlọwọ ṣan ni aṣa laini aaye. 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. Nitorinaa, ṣatunṣe awọn kapasito fori meji lẹgbẹẹ ara wọn ki wọn pin aaye ilẹ kan. Nitori awọn paati pola meji ti lọwọlọwọ lọwọlọwọ wa lati aaye kanna (asomọ asomọ iṣelọpọ tabi ilẹ fifuye) ati awọn mejeeji ṣan pada si aaye kanna (asopọ ilẹ ti o wọpọ ti kapasito fori), rere/odi lọwọlọwọ n kọja nipasẹ ọna kanna. 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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To verify the above inference, two different PCB layouts were used: a simple layout (Figure 5) and a low-distortion layout (Figure 6). Iyọkuro ti iṣelọpọ nipasẹ FHP3450 quad-operational ampilifaya nipa lilo semikondokito itẹwọgba ni a fihan ni tabili 1. Iwọn igbohunsafẹfẹ aṣoju ti FHP3450 jẹ 210MHz, ite naa jẹ 1100V/wa, aiṣedeede titẹ sii jẹ 100nA, ati pe ṣiṣiṣẹ lọwọlọwọ fun ikanni jẹ 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. O han ni, ikanni ampilifaya ti a fun ni idamu kii ṣe igbewọle tirẹ nikan, ṣugbọn titẹ sii ti awọn ikanni miiran paapaa. The earth current flows through all the different channel inputs and produces different effects, but is influenced by each output, which is measurable.

Table 2 shows the harmonics measured on other undriven channels when only one channel is driven. The undriven channel displays a small signal (crosstalk) at the fundamental frequency, but also produces distortion directly introduced by the ground current in the absence of any significant fundamental signal. Ifilelẹ ipalọlọ kekere ni Nọmba 6 fihan pe irẹpọ keji ati lapapọ awọn abuda idapọpọ ibaramu (THD) ti ni ilọsiwaju pupọ nitori imukuro nitosi ti ipa lọwọlọwọ ilẹ.

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Akopọ nkan yii

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. Iwọn ifihan agbara igbohunsafẹfẹ giga n ṣàn pada si kapasito kekere fori. Awọn ṣiṣan igbohunsafẹfẹ kekere, gẹgẹ bi awọn ti awọn ifihan agbara ohun, le ṣan nipataki nipasẹ awọn kapasito fori nla. Paapaa lọwọlọwọ ipo igbohunsafẹfẹ kekere le “ṣe aibikita” agbara agbara fori ni kikun ati ṣiṣan taara pada si itọsọna agbara. Ohun elo kan pato yoo pinnu iru ipa ọna lọwọlọwọ jẹ pataki julọ. 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.

Ofin goolu fun apẹrẹ HB PCB ni lati tọju kapasito fori HF nitosi isunmọ agbara ti a ṣajọ bi o ti ṣee ṣe, ṣugbọn lafiwe ti Nọmba 5 ati Nọmba 6 fihan pe iyipada ofin yii lati mu awọn abuda abuku dara ko ṣe pupọ ti iyatọ. The improved distortion characteristics came at the expense of adding about 0.15 inches of high-frequency bypass capacitor wiring, but this had little impact on the AC response performance of the 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. Lower frequency signals such as audio have much stricter distortion requirements. 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.