Keywords: differential transistor amplifier production
This article describes a power amplifier in the input stage and the differential voltage amplifier stage circuit of two asymmetrical structure, good linearity of amplification, wide frequency response, power fluctuations and drift effects on inhibition of strong, sweet quality, full flavor, worth a try .
A circuit schematic of FIG. 1 is a brief analysis of the main power amplifier amplifying circuit, VT2, VT3 constituting the input stage differential circuit, VT1, LED1, R4, R9 and C2 composing the constant current source circuit of the input stage differential circuit. When LED1 is normal, its positive negative voltage is constant between 1.8V to 2V, and noise is smaller than the regulator diode, which is often used in the power amplifier circuit. Its positive and negative terminals of a voltage difference of about 1.9V role in the emitter circuit VT1 so VT1 shot - in the constant current collectors (1.9V ~ 0.6V) /680Ω≈1.9mA. In VT2, VT3 case of the differential input circuit parameters completely symmetrical, flowing VT2, VT3 shot - 1.9mA current collector, i.e., a half of 0.95mA. RP2 change VT2, VT3 emitter feedback resistor, so VT2, the symmetrical variation of the quiescent operating point VT3, eventually changing the output DC potential of the mid-point. The voltage drop in R7 and R8 is 2.2 kΩ × 0.95 mA≈2.1 V, and the transmitting junction bias voltage of the voltage amplification stage VT7 and VT8 differential circuit is used. The current flowing through VT7 and VT8 - the current (2.1 V ~ 0.6V) /R13≈4.5 mA. VT4, VT5 constitutes VT7, VT8 differential voltage amplifier mirror current source load. VT6 connected in a common base state, as a load resistance of VT7. VT9, R12 and RP3 driver stage configuration, the output stage bias circuit, and functions as a feedback control action on the final power tube temperature. RP3 VT9 set adjustment may be changed - between the emitted voltage, thereby changing the driver stage and the output stage quiescent bias current. On the other hand, the power level VT9 tube VT12, VT13 mounted on the same heat sink, play of VT12, VT13 temperature feedback control action, prevent VT12, VT13 temperature is too high causes the output current is too large burn. Temperature feedback control principle is that, when the VT12, VT13 output current increases, the temperature increase exceeding the set VT9 - increases the emission current collector - emitter voltage drop, thereby decreasing the pushing and output stages of the quiescent output current of the power tube VT12, VT13 current and temperature control in a safe range. VT10, VT11 driver stage constituting an emitter resistors R19, R20 and the DC voltage drop on a power output stage VT12, VT13 of the bias voltage, can be adjusted to change the RP3 VT12, VT13 of the static output current. R26, C9 and R27 constitute a feedback circuit from AC voltage of the machine is 52 times magnification (Av + = 1 + R26 / R2 7 = 52). Feedback tap taken at the midpoint of symmetry of the drive stage, the maximum impact of the loudspeaker to avoid the small signal input stage, which output stage compared to tap approach generally symmetrical with the mid-point feedback, quality improvement is obvious, the sound field strengthen control, the transient better. Figure 2 is a power supply and protection circuit of a power amplifier. This only introduces the protection circuit here. The protection circuit having a power output stage and the start delay the midpoint of the DC overvoltage protection function. Just machines, point A right channel 12V supply voltage via a protective circuit R31, R33 to charge C17, VT16 base voltage at this time is low, in the off state, and resulting in D7, VT17 off relay K1 does not pull at the start instant to avoid the impact of the surge current speaker. Over time, C17 is charged to a certain extent saturated conduction VT16, VT17 also result in saturated conduction, the relay K1 pull, delay the completion of the boot process. When the left and right channel power output stage symmetry (A point A in Figure 1), the VT14 or VT15 will cause the VT14 or VT15 to conduct the VT16 to discharge, the relay is released, and the relay is released. It is not damaged by the protecting the speaker. C15, C16 into contact with the positive and negative non-polarized capacitor, may be positive and negative charging, while avoiding erroneous operation of the protection circuit transient excessive voltage.
Second, the component selection and production debugging production before the component must be carefully selected. RP2, RP3 multi-turn precision potentiometer, elements R5, R6 and VT2, VT3, etc. used in pairs, each error should be kept to a minimum, the only way to reduce the problem of debugging, production increased chance of success. Production testing can be divided into blocks, a first soldering the first stage differential circuit (R2 ~ R9, VT1 ~ VT3), adjusted to the intermediate position RP2, input is grounded and a 100kΩ resistor VT3 base is grounded, measuring R7, R8 on the DC voltage drop should be about 2.1 V. Welding a second stage differential circuit (VT4 ~ VT9) i.e. R15, R16 left circuit measures R10, the voltage on the R11, R13 and R14 are each 1.45V, while adjusting RP3, VT9 collector emitter voltage may be within a certain range in changes, so that the front two normal circuit operation. Next, the welding promotion stage, withdraw the VT3 base 100 k pair ground resistance, and the feedback branch is discharged, and two important debugging are performed. RP3 adjusted so VT9 collector-emitter voltage is about 2.5 V, the driver stage VT10, VT11 output current determination about 6.35mA, R19, R20 of the voltage drop across each 0.64V. RP2 digital multimeter regulate the DC millivolt measurement block driver stage symmetry point (i.e. R19, R20 junction) voltage, the voltage controlled within ± 5mV. Connected to the power output stage, the trimming RP3 VT12, VT13 quiescent current is set to 80mA, R23, the voltage drop across R24 each 17.6mV. Measuring the power output of the DC voltage level symmetric point (i.e. R23, R24 connection), if VT12, VT13 not the symmetry point may not be zero when the static voltage, the voltage at point RP2 adjusted likewise controlled within ± 5mV . At this point, the whole production essential for the success. The next step is adding audio audition debugging, able to do so you can use the oscilloscope waveform observation machine and bandwidth.
Third, the whole overall performance and technical indicators quiet background, sound field open, high, very good low-frequency response, sound sweet, rhyme feeling full. Measured performance indicators are as follows:
Passband: 10Hz ~ 230 kHz (-3dB)
Conversion rate: 20V / 1us
Standard Output Power: 45W × 2 (8Ω)
Maximum undistorted output power: 72W × 2 (8Ω)
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