Feedforward power amplifier

The power circuit principle of this amplifier is shown in Figure 3-29. Only one channel circuit is shown here, the other channel circuit is exactly
the same.
    Enter the letter. After the signal is amplified by AP500A, it drives a pair of complementary VMOS tubes VTl and VT2 for Class A push-pull amplification. The
amplified signal is output from the source and added to the load through resistors Rl and R2. At the same time, VTl and VT2 also serve as the voltage driver stage of the final stage
. The signal voltages on Rl and R2 respectively drive a pair of high-power complementary transistors VT3 and VT4 for Class A and B push-pull amplification.
The output is added to the load through resistors R7, R8 and RO. . Since the resistance values ​​of Rl and R2 are large relative to the speaker impedance, the dynamic range of VTi and VT2 is limited
. Its output power is only about 4W, and the resistance values ​​of R7, R8, and RO are very small, so VT3. VT4
is the main output power port of this machine. When the power supply voltage is ±33V, the maximum undistorted power is 30W. RO It is the feedforward sampling resistor. When the signal current
on the emitter of VT3.VT4 passes through Ro, the signal voltage on Ro is added to the inverting input terminal of AP500A through the internal feedback loop . After comparison and amplification by AP500A, a signal with distortion is generated. Signals with the same signal size and opposite phase are added to the load to offset the original distorted signal . This is the currently popular "error feedforward correction circuit". In terms of its circuit principle, the output tube of this power amplifier and the current pouring power amplifier have no bias current when static, and are pure Class B amplifiers. However, the output arm of this machine has bias current when static, and are Class A and B amplifiers, which can more effectively overcome the output stage generated of crossover distortion. R7 and R8 are the emitter resistors of wr3 and VT4. They have a certain current negative feedback effect and can improve the nonlinear distortion of the suffix. At the same time, R7 and R8 are also the overcurrent sampling resistor ports of the output tube. When the emitter current of the output tube is greater than 3 . When ZA, the voltage on R7 and R8 exceeds o. 7V. VT5 and VT6 are turned on, causing VT3 and Vding4 to be cut off, thereby avoiding the expensive high-power output tube from being burned. In order to eliminate the impact of the switch, prevent fixed zero drift, and load short circuit causing overcurrent damage to the sound playback system , this machine is equipped with a speaker protection circuit. Its principle is very simple and has been introduced in detail in many books and periodicals, so I won’t go into details here.