Application analysis of field-effect transistors in audio digitization 3

2. MOSFET output efficiency is lower than * * *

It is well known that the loss of MOS transistor output stage is greater than that of bipolar transistor. Usually, in the same circuit, in order to achieve the same output power as bipolar transistors, the method used is to increase the power supply voltage by ± 5V to compensate for the losses of MOS transistors. However, actual production has proven that it is far more than that simple.

The parameters of commonly used field-effect transistor in audio systems are shown in the following table:

The main characteristic data of several representative MOS transistor can be obtained from the attached table. Let's take Hitachi's old LDMOS tube K135/J50 as an example to illustrate. The gate source turn-on voltage threshold of K135/J50 is between 0.15 and 1.45V. It was measured that VO.25V was used when Io=10mA, while V increased to 0.6-0.85V at the typical value of I~=100mA. It can be seen that the voltage control characteristics of field-effect transistors determine that the gate source loss voltage increases with the increase of drain current I (in contrast, the V of bipolar transistors is almost always 0.7V). The internal loss of MOS transistor mainly depends on the size of the drain source conduction resistance R. The parameter RDs1 is not directly given in the K135/J50, but the R of K135/J50 can be calculated using the formula RDs (oN)=UDs)/ID based on the drain source conduction voltage VDs (sat): 12V and ID7A. This is equivalent to connecting a 1.7Q resistor in series with the load, and for a standard 8Q load, the power loss is close to 20%. If we consider the sudden drop in impedance of the speaker at low frequencies and the negative temperature voltage current characteristic of the field-effect transistor (i.e., the current decreases when the temperature rises, that is, R increases at this time), then the actual internal loss of the MOS transistor will be greater. In contrast, the situation of bipolar transistors is quite different. For example, Toshiba's A1265/C3182 has a voltage of 2V when Ic=7A. If the output is a two-stage emitter follower, then adding the loss of the last preceding stage (<1V), the total V. (sat)<3V. It is self-evident which is better or worse compared to the VDs (sat) of K135/J50=12V.