Current feedback power amplifier circuit

Traditional power amplifier circuits usually use voltage feedback, which can achieve very low distortion, good frequency characteristics and large damping coefficients, but it is not easy to obtain a high conversion rate and the TIM distortion is also large. In order to further improve the sound quality, in recent years, people have introduced current feedback power amplifier circuits. The main feature of this circuit is that it uses a complementary push-pull circuit from the input stage to the output pole. It has a relatively simple structure, can easily obtain a conversion rate of hundreds of volts per microsecond, and has a very good driving ability for capacitive loads. Its typical application circuit is shown in the figure below. This circuit was an outstanding entry in a Japanese amplifier production competition.

The working principle of the circuit is: after the input signal voltage is converted into current by the buffer (BG1, BG2) with a gain of 1, it is converted into voltage by the IV converter (BG3, BG4), and then further amplified and output by BG5 and BG6 to drive the load. . After the feedback resistor Rf converts the output voltage into current, it is added to the emitter resistor Rs of the IV converter to form current negative feedback. Therefore, the current flowing through Rs is the difference between the current of the IV converter and the feedback current. Circuit gain Kf=1+Rf/Rs

Since the input buffer is not included in the feedback loop, special attention should be paid to the performance of this stage. Here, a Darlington tube combination is used between the input buffer and the IV converter, and the well-received NEC pair tube 2SA992/2SC1845 is selected, which can obtain higher input impedance and lower output impedance, and the comprehensive performance ratio Other combination methods are superior and have simple structure. The output stage uses Hitachi's high-power MOS-FET 2SK1056/2SJ160 (its symmetry is better than other FETs), and is connected to a common source push-pull amplification method, eliminating the need for an output buffer and further simplifying the circuit.

In current feedback amplifiers, the value of the feedback resistor Rf cannot be too large and should generally be <1KΩ. This circuit only takes 220Ω. The value of Rs is related to the distortion, frequency bandwidth and circuit gain. Here, the gain of 22Ω is about 20dB, and the feedback amount is about 30dB, which can obtain better overall performance. The circuit has excellent response to square waves, the conversion rate is as high as 412V/uS, the output power is 25W, the distortion at this time is <0.5%, and the damping coefficient DF=17. The resolution of this circuit is very high, the sound is clear and transparent, and it can work stably even when connected to a 1uF pure capacitive load, which reflects the characteristics of a current feedback amplifier.