Field effect transistors are becoming more widely used today in the field of audio digitization. The principle, advantages and common sense of use have been discussed in some reference books and newspapers, and will not be repeated here. This article illustrates the rational application of FETs by two points that are easily overlooked by enthusiasts, especially beginners. At present, field effect transistors used in the audio field include junction transistors (JFETs) and insulated gate field effect transistors (MOsFETs), which are further divided into LDMOS, VMOS, and UHC, IGBT, etc., which have emerged in recent years, and are still evolving. With perfection.
1. JFETs lack complementary tubes in paired tolerances The maturity of today's bipolar transistor manufacturing processes has narrowed the pairing error of NPN and PNP complementary transistors to a level that is widely accepted by a wide range of professional manufacturers and audiophiles. In contrast, the selection of FETs is much more difficult, and the JFETs used as amplifier input stages are lacking the complementary pairs that meet the requirements (this is determined by current manufacturing levels). The table lists the main characteristic data comparisons of Toshiba's twin field effect transistor (DualFET) K389/Jl09. It can be seen from the attached table that the difference between K389 and J109 is V C C and NF, where the values ​​of C and C are 5 times larger than the difference between N groove and P groove. I once bought 8 pairs of K389/J109, but the results before the installation were quite disappointing: 1 The so-called twin tube is only the same performance of the two tubes in the same tube, while the 8 pairs of tubes purchased at the same time The difference between the N groove and the P groove is quite different; the Idss, gm and Vgs of 2K389 and J109 are different, and the actual waveform test is also asymmetrical. Finally, the author can only select two tubes with an error of 3.8% from K389 as the one-sided differential input stage. (In the past, it was not difficult to control the error of the same polarity tube when using bipolar tube. 1%, the matching error of the heteropolar tube will not be greater than 3%). Through the above data comparison and actual test, the following revelation can be obtained: when the JFET is used in the complementary input stage, the dispersion of V and I may cause a large offset of the static working point of the circuit, thereby changing the stability of the circuit. Poor; the inherent differences of gm, Cis, Cis affect the dynamic indicators such as the upper and lower waveform symmetry and transient response speed of the entire push-pull stage. In this regard, some well-known foreign manufacturers have long formed a consensus. For example, the products of Tianlong and Marantz often see the field effect differential input stage made by K389, but it is always difficult to see the shadow of J109. Maybe K389 /J109 was originally a "lalang match."
Compared with JFET, the withstand voltage, power consumption, and transconductance of MOS transistors are easily made higher. In addition, in addition to the input stage of the amplifier (such as the push stage, the output stage), the complementary pairing requirements can be relatively relaxed, and some pairing defects can be overcome by careful design of the circuit. Therefore, the application of the MOS tube at the final stage of the power amplifier is not a serious problem. The problem with MOS tubes for the output stage of the power amplifier is not the complementary pairing. The key is the low efficiency.
2, MOSFET output efficiency is lower than the *** low MOS tube output stage loss is more common than bipolar transistors. Usually in the same circuit, in order to obtain the same output power as the bipolar transistor, the method is to increase the power supply voltage by ±5V to compensate the loss of the MOS transistor. However, the actual production proves that it is much more than that simple.
The parameters of common FETs for audio are as follows: 
The main characteristic data of several representative MOS tubes can be found in the attached table. Here is an example of Hitachi’s old LDMOS tube K135/J50. The gate-to-source turn-on voltage threshold of K135/J50 is 0.15~1.45V. Measured when Io=10mA VO. 25V, and when I~=100mA typical value, V increases to 0, 6~0, 85V. It can be seen that the voltage control characteristics of the FET determine that the gate-source loss voltage rises as the drain current I increases (relative to this, the V of the bipolar transistor is almost constant 0.7V). The internal loss of the M OS tube mainly depends on the size of the drain-source on-resistance R. The RDs 1 is not directly given in the parameters of K1 35/J50, but can be calculated by the drain-source turn-on voltage VDs(sat): 12V and ID 7A, using the formula RDs(oN)=UDs)/ID. The R of K135/J50 is about 1.7 Q. This is equivalent to placing a 1.7 Q resistor in series with the load, which is nearly 20% lost for a standard 8 Q load. If the speaker is considered to have a sudden drop in impedance at low frequencies and a negative temperature-voltage characteristic of the FET (ie, the current drops when the temperature rises, that is, when R increases), the actual internal loss of the MOS transistor will be greater. In contrast, the case of bipolar transistors is quite different. For example, Toshiba's A1265/C3182, when Ic=7A, V, = 2V. If the output is a secondary emitter follower, then add the loss of the last stage (<1V), the total V. (sat) <3V. Compared with the V135s(sat)=12V of the K135/J50, it is natural that it is better and worse.
(2) Line current output current of NOS power car tube As mentioned above, only increasing the power supply voltage by ±5V under the same circuit condition does not enable the MOS power output stage to obtain the equivalent power of the bipolar tube. We often underestimate the dynamic loss of the MOS tube during actual operation. Set a 100W post stage. When the load is 8 ohms, the power supply voltage for the bipolar is V_Gc (8P. × RL) + 2 × [VcE(sat) + I ( ) × RE such as IcM = 5A, vc. 1.5V, RE = 0.22 Q, then VC 85.2V (±43V). For MOS tubes, V?=99.2V (±50V) can also be calculated. Of course, this is theoretical and the value at maximum power. In practice, the no-load voltage when using an unregulated power supply is obviously higher.
In fact, the AC secondary side supply of a 100W/8 Q bipolar transistor amplifier is approximately AC 33V × 2. When the AC 38V×2 is used as the MOS power amplifier according to the usual practice, P can only reach 70-80W, and the high R of the Hitachi MOS tube relies on the power amplifier NFB network to improve the overall internal resistance. Therefore, the actual hearing loss of the large output is lacking. (When the amplifier is designed to have no feedback, it is even worse). Then there are various misunderstandings about the MOS tube, such as the poor linearity of the MOS tube at high current.
In fact, compared with bipolar transistors, MOS tubes have excellent high-frequency characteristics and distortions with even-order harmonics. Because there is no secondary breakdown, Hitachi gives the application current of MOS tubes such as K135/J50. Recommended limits. The linear current of the VMOS tube can reach several tens of amperes, and the pulse current of the UHC-MOS tube is as high as 300A or more. Then why is the MOS sound tube misunderstood as a large current linear difference? The root cause is that there is not enough knowledge of the power loss caused by the internal resistance of the MOS tube and the corresponding countermeasures. Although the internal resistance of VMOS and UHC-MOS tubes is small, the V at high current is as high as 5V or more, so it should be taken seriously.
3, MOSFET output stage efficiency improvement
The loss of the MOS power output stage is always larger than that of the bipolar transistor, which is determined by its inherent characteristics. The improvement mentioned here should actually be how to reduce the power loss of the MOSFET output stage.
1 If the same output power as the bipolar transistor is to be obtained under the same circuit, the power supply voltage of the MOS output stage should be higher than ±10V when using the bipolar transistor. When the MOS tube loss is to be reduced, the voltage level and the current level can be separately supplied. At this time, the current level and the voltage level are Nl:k, and the bipolar tube output stage is ±5V high and ±10V.
2 use multi-tube parallel output stage. Multiple tubes are connected in parallel to reduce the equivalent on-resistance of the MOS power tube, rather than so-called to improve the linearity of the MOS tube. In addition to increasing the current driving force, the parallel connection of multiple MOS tubes can greatly reduce the power loss, and improve the equivalent internal resistance of 1/4 of the single tube when the K135/J50 4 groups are connected in parallel in the open loop. Below Q, the load power loss for 8 ohms is correspondingly reduced from 20% to 5%). In addition, the parallel parameter error of the MOS tube can be appropriately relaxed compared with the bipolar tube, that is, the parallel tube error is slightly larger and the hearing inductance is not deteriorated.
3 uses a common source output stage, that is, the collector output form of the bipolar tube. This output mode is more suitable for VMOS tubes. This is because the VMOS tube has a large current and a small internal resistance. When the circuit design is reasonable, good efficiency, low distortion, and low output impedance can be considered. The power supply voltage at this time may be ±3 to ±5 V higher than the bipolar circuit.
I used the op amp OPA604~DVMOS tube IRF540/9540~1 to make a common source output stage amplifier. The specification is v for ±40V, Po is 60W/8 Q, 100W/4 Q. The actual audition results show that the driving force is not weak, and the heat is not necessarily larger than the bipolar tube. As an audiophile, I certainly hope that in the near future, I will be able to fully meet the current bipolar tube pairing requirements and even surpass the bipolar tube's mutual ~+FET, high voltage UHC-MOS complementary power tube. This is not an illusion for today's ever-changing world, but one thing, I hope that the price of those "tonics" will not be too high.
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