Three-stage differential amplifier circuit

This circuit shown uses Hitachi's power field effect complementary pair tube 2SK134/2SJ49 as the power
output tube. This is a pair of MOS field effect tubes that are recognized as the best. They have been widely used since their development and are
one of the most commonly used output tubes in field effect amplifiers.
    This circuit uses a three-stage differential amplifier circuit, which is one more stage than the two-stage differential amplifier, but its working principle is not much different.
VTi, VT2 is the differential input stage, VT3 is its constant current source, VDi, v]t is the voltage reference of the constant current source. The collector current of the two tubes is adjusted to 120VA respectively. Correspondingly, its voltage drop
on R7 and R is 1.8V, this voltage drop serves as the bias voltage of the second stage differential amplifier. The collector current of the second differential amplifier is respectively V, and the voltage drop of this current on Ris and R16 is 1.7V. This voltage drop is used as the base bias voltage of the third differential amplifier, so that the third differential amplifier VT6 The collector current of , Vr7 is about 15mA respectively. The voltage drop of this current on RPi is about 0,sv, which is used to bias the final output tube, so that its quiescent current is about 1~200mAo.     In order to improve the performance of the circuit, in addition to biasing the operating points of all levels at the lowest level of nonlinear distortion In addition to the status quo, the following measures have also been taken.     The usual circuit uses two-stage differential amplifiers instead of three-stage differential amplifiers. The purpose of adding one-stage differential amplifiers is to increase the open-loop gain, and to deepen the negative feedback to further reduce the nonlinear distortion of the circuit and the device.     In order to improve the possible causes of deepening the negative feedback To reduce the transient intermodulation distortion, a two-stage low-pass filter K, c and R5, C are used at the input end of the first differential amplifier to limit the cut-off frequency (-3dB) of the high-frequency end of the circuit to about 65kHz, that is, to limit the input signal The maximum rise rate makes it lower than the maximum rise rate of the circuit. At the same time, current negative feedback (code, Rio) is introduced at the emitters of VTi and Vtz , which expands the effective input voltage range of the input stage.     The input capacitance of the field effect transistor is large, and the input capacitance difference between the complementary transistors is also very large. In order to make the circuit more symmetrical, capacitors c8 and C14 are added between the gate and source of VTio and VT12. In order to improve this capacitance Due to the transient characteristics of the linear load, the quiescent current of the third differential amplifier is particularly large to     reduce distortion. In addition to carefully designing the ground wire layout, this machine also uses "floating ground" technology. The so-called "floating ground" is to use a low-resistance resistor to connect between the input signal ground, the load ground and the power supply ground, so that the two are suspended against the signal ground . As long as the floating resistor R3 is larger than the load impedance (generally 8 to 1∞), it can prevent the parasitic signals from the output high current loop from entering the input loop at the input end.     C4, Cr5, island, and R19 are phase compensation components.     The quiescent current of this machine can be 100~200mA, and the midpoint voltage should be within the range of ±30mV.