Field effect tube amplifier circuit with beautiful sound

1. Working Principle

The circuit principle is shown in the figure below, which is a typical OCL power amplifier. ICl-A is the input stage; BG1 and BG2 are the excitation stage; BG3 and BG4 are the complementary output stage composed of MOSFET; IC1-B is the DC zero servo circuit.

IC1-A plays the role of voltage amplification. Due to the limitation of power supply voltage, the signal output amplitude of this stage does not exceed ±15V. If the latter stage adopts emitter follower output, the output power is too small. This circuit should not only give full play to the characteristics of simple design and excellent performance of the operational amplifier, but also meet the appropriate output power. Therefore, this circuit is powered by two sets of power supplies (±15V, ±36V), the regulated ±15V is supplied to the operational amplifier, and the ±36V is supplied to the latter stage. BG1 and BG2 are common emitter circuits, R8 and R9 are negative feedback resistors, and R1, R5, D1, D2, R18, and R19 provide stable DC bias for BG1 and BG2 to ensure that they are in the on state. In addition to the voltage amplification function, this stage is more important.

It realizes the signal coupling of different power supply circuits. Field effect tube is a voltage amplifier device. Although there is no special requirement for the driving current, the influence of the input capacitance of the field effect tube on the high-frequency characteristics must be considered. Therefore, the internal resistance of the driving signal should be small enough. The driving signals of BG3 and BG4 are taken from the loads R4 and R3 of BG2 and BG1 respectively. The collector load resistance of BG1 and BG2 is divided into two sections. This is because the gate-source driving voltage of the field effect tube shall not be greater than 20 volts. BG3 and BG4 are the drain output stage composed of MOSFET pairs. R11 and R12 are source negative feedback resistors. The role of ICl-B is to stabilize the current voltage at the midpoint of the final stage to realize the DC conversion of the power amplifier. R13 and R7 are loop negative feedback resistors. R20 and R21 are set to prevent high-frequency self-excitation of MOS tubes; the role of C2 and R16 is to prevent circuit self-excitation.

2. Installation and debugging

IC1, BGI~BG4, you can choose the devices shown in the schematic diagram, or you can choose devices with similar parameters. BG1 and BG2 require β value greater than 100, Vceo greater than 100V, Icm greater than 1A, PCM greater than 10W; BG3 and BG4 parameters are VDSS=140V; ID=8A; PD=100W. Each complementary tube should be paired as much as possible. Resistors R2, R3, R4, R10, and R16 should use IW metal film, R11 and R1.2 should use 3W resistors, and other resistors can use 1/4W five-ring resistors. R20 and R21 should be installed as close to the MOS tube gate as possible. Use high-quality capacitors.

After the static test is correct, turn on the power supply and check whether the midpoint voltage is zero volts; measure the gate-source voltage of BG3 and BG4, which is generally around 1V. If you want to change to pure Class A amplification, you can increase the resistance of R18 and R19. After everything is normal, you can connect the speaker and sound source for a test.