About the working principle and debugging points

In the production test of the dynamic Class A power amplifier circuit, the linearity of the dynamic bias of most circuits is not good and the offset cannot be accurately controlled, but there is no choice but to create a shortcut to innovate but succeed. This circuit is quite different from the traditional OCL circuit. The following is a brief introduction to its working principle and debugging points. 1. Circuit principle and characteristics The MOS field effect transistors Q1, Q2 and Q4, Q5, Q9, Q10, Q11 and related components constitute a common source, common base, and common base input stage. Q1 and Q2 convert the input voltage into signal current. The settings of Q4 and Q5 are to reduce the power consumption of Q1 and Q2. Q9, Q10, Q11 are common base configuration, no current gain, can be regarded as part of the input stage, its role is to change the direction of the signal current, the current is converted into a voltage signal direct drive output tube through R17, R18. This circuit is simple and easy, and the bipolar transistors in all channels are driven by the current source, which will not produce the odd harmonics that worry the fans. There are two main differences between this circuit and the traditional circuit: One is that the input stage adopts super-conventional large quiescent current, each tube reaches 30mA. The benefit is that the driving force of the output tube is strong, and the dynamic is large, the distortion is small, and the linearity is good, which crosses the curved part of the characteristic curve of the field effect tube. The timbre response is quite different from that when a smaller current is used. The author tried to reduce the current of Q1 and Q2 to 8mA, and the original round and thick tone became a little dull. The second is the dynamic bias circuit. The traditional OCL circuit clamps the constant voltage tube Q8 between the two base (gate) poles of the output tube, and its potential floats up and down with the output voltage, but the potential of Q8 in this circuit is fixed, which is helpful to improve the dynamic bias Set the accuracy. When static, the constant voltage circuit composed of Q8 and so on provides stable bias to Q6 and Q7, and adjusts W2 to change the static current value. The dynamic bias process is: when the output is negative half cycle, the output signal is divided by R22 and R23, and the base of Q8 is shunted through D8, W2 and R21. , And then increase the output tube current. The half-wave rectifier circuit composed of D8 and C12 stores the negative potential on C12 when the output is negative half cycle, and provides the same offset as the negative half cycle for the positive half cycle output, and the time constant of C12 charging is less than its discharge Time constant, the voltage on it is relatively stable for a period of time. In this way, the circuit will automatically adjust the working current of the final tube according to the level of the output voltage to make it work in the Class A state. Reasonable selection of the ratio of R22 and R23 according to the load impedance value can obtain accurate control amount and excellent dynamic bias linearity. About the working principle and debugging points of dynamic class A power amplifier circuit Second, debugging points 1. Static debugging. After the installation is confirmed, the output tube is not installed, the output terminal is shorted to ground, and the power is turned on. Adjust W1 and W2 to make the voltage at both ends of R17 and R18 close to the threshold voltage of the output tube. Turn off the power and install the power tube to remove the short circuit wire. , Disconnect R22, adjust W2 to make the quiescent current of the output tube around 250mA, adjust W1 to make the output terminal zero potential, and readjust the heat engine in half an hour. 2. Debug of dynamic bias circuit. Connect R22, the output end is connected with a high power (wire wound) resistor that is consistent with the load impedance. Note that the resistance power P≥V square c1 / R, short circuit C6, making the circuit a DC amplifier, the upper output tube Q14 drain Connect an ammeter with a voltmeter at the output, and slowly adjust W1 to make the potential of the output gradually approach from zero to negative full scale. The current value of Q14 should slowly decrease as the potential of the output decreases, but never cut off. When it is negative full scale, the current of Q14 is more suitable at 50mA, otherwise the value of R23 should be changed. Call back W1 to reset the potential at the output to zero, remove the C6 short-circuit line and the dummy load at the output, and connect to the speaker to listen. Three, matters needing attention 1. The + 45V power supply must be stabilized, otherwise the quiescent current will fluctuate with the fluctuation of the power supply voltage, and the positive and negative 30V power supply must be full of energy; 2.Q4, Q5, Q9, Q10, Q11 must add a heat sink, the author will install it On the same heat sink, thermal coupling is used to make the zero potential of the output terminal very stable; 3. Q1 and Q2, Q14 and Q15 must be paired to be on the machine, and the other symmetrically used tubes can be paired best, but the requirements are not strict. 4. The junction tube cannot be used for Q1 and Q2, because the junction tube's transconductance is too low, it will seriously reduce the gain of the whole machine. If you need a junction tube, you need to increase the resistance of R17 and R18 and add a drive to the output tube. 5. If Q3 is thermally unstable, it will cause static current drift. If this phenomenon occurs, it should be considered for temperature compensation; 6. When welding MOS tube, attention should be paid to anti-static breakdown. 7. When Q1 and Q2 are not easy to purchase, K214 can be used instead, and Q4 and Q5 can be canceled to make the circuit more concise.