Current negative feedback power amplifier production
Author: Li Changqing
The production process of the current negative feedback power amplifier, Fig. 1 is the post-level schematic, the input stage uses the JFET field effect tube, due to the R14, the source voltage of T1, T2 pulls "ground" potential, so TL, T2 The drain source voltage drop does not exceed its unilateral power supply voltage, allowing 2SK246 / 2SJL03 tubes to be used in the circuit of the upper 100 watt.
Since the current negative feedback does not need a differential circuit, the common mode suppression ratio is a difference, which requires the circuit to be highly symmetrically symmetrical, and must be paired at all levels before the installation.
T4, T5 can be tested directly with a digital multimeter. T1, T2 β value test is carried out by means of Figure 2. Since the noise coefficient of the field effect tube is not I. Increased increase.
The big work current can increase the driving force, so the working current of TL, 12 is set at 2.5mA. Tucalyptu-produced triode multi-color separation, 2SK246 / 2SJL03 "BL" this gear IM>7mA. The current meter in the figure is used by the maca macadium, the voltage meter of the machine. Adjusting the W the current table in Figure 2 is 2.5mA, at which time the reading on the digital table is the V-shaped value at which the tube is in IO = 2.5 mA, and it is paired with this value. Since the W-both voltage is known, the current flowing is 2.5 mA, then the W value can be calculated, and the value of R3 and R4 can be determined.
For 2SK214 / 2SJ77, 2SKL058 / 2SJL62 selected a "soil" approach: one of the 300W electric furnace. The strap is hung on a small piece of wood, and the four nodes in Figure 3 use the crocodile clip, and the replacement pipe is fast and safe. Power-on, first harsen, a crocodile clip, double-knife double throw switch is in the "2" position, the field effect is installed on a sufficiently large radiator, then turn on the 6V voltage regulator power supply and polar power supply, hold a crocodile one hand The sandwich is slid on the electric furnace, slowly slowing the reading of the digital table voltage gear (2SKL058 / 2SJL62 is 450mv2sk214 / 2sj77 is 100mV). After a certain amount of reading, the reading will not be closed in the A crocodile clip.
If the heat sink is large enough, the reading of the digital table will be stabilized in 20 seconds.
The current flowing through the tube is 450 mA. The result of simulating the actual working state should be trusted. Then, the switch throws to "L", the value displayed on the digital table is the V under the working current of the 450mA, records, and waits until all the pipes are finished after the analysis is classified. The actual installation indicates that the paired pipe operating current is very small.
It is important to point out that in the current negative feedback amplifier, the open loop gain is not too high. To reduce the open loop gain, the method of applicable is to increase the emitter resistance of T4, T5, which does not affect the frequency characteristics, and does not increase the phase shift, and a large number of emitter resistors will increase stability. However, it cannot be added too much. Otherwise, W1, R9 is too large, T4, T5 collecting extreme pressure is small, unfavorable to dynamic range, reference, W1, R9 take 500Ω 1 620Ω. One more, the feedback resistance of the current negative feedback amplifier cannot be too large.
Otherwise there is no sufficient feedback current. However, the resistance is small, and the power consumption is increased, so R15 uses a 1.2kΩ3W US DALE resistor.
T6 ~ T17 and 13 are all in a radiator. In the welding field effect, in order to prevent electrostatic breakdown, the field effect tube should be short-circuited, and the short circuit is removed after welding.
Before everything is ready, you can start debugging. First adjust the WL to 500Ω, W2 is transferred to the maximum, and the voltage flowing through the voltage of 0.27Ω resistor can be measured. Then slowly adjust the small W2, and T8 ~ T17 will gradually heat. When the current rises to 250 mA, some is hot when it reaches 350mA.
The finger is pinched, because the heat sink is too small. Finally, it was transferred to 120 mA. The fine adjustment W1 causes the output in the output to be zero. After it is completely stable, the WL, W2 will be replaced with a corresponding fixed resistance.
After a period of time, the aging is agglomerated, it is connected to a previously produced AA class, with a common volume. Monitor the output midpoint potential with a digital table. The results show that the DC drift is within 30mY, and the other is within 40 mV. This drift is safe. There is no need to add DC servo.
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