Power amplifier principle
Power amplifier, referred to as 'power amplifier', in many cases, the rated output power of the host can not be competent to drive the entire audio system. At this time, it is necessary to install a power amplifier between the host and the playback device to supplement the required power gap, and the power The amplifier plays a pivotal role in 'organizing and coordinating' in the entire sound system, to some extent dominates whether the whole system can provide good sound quality output. Definition and use of power amplifier The power amplifier, referred to as the power amplifier, is a voltage and current comprehensive amplification of the audio signal to obtain power amplification. The position of the power amplifier in the system diagram is in front of the speaker system, and its output is directly sent to the speaker system to drive the speaker system. Since the output sensitivity of the power amplifier is generally around 0db, the input signal to the power amplifier is generally taken from 0db output signal of mixer or peripheral equipment. For low-level output signals such as microphones, the power amplifier cannot be driven until the preamplifier or mixer voltage is amplified. The preamplifier, mixer or peripheral equipment output voltage signals, which can only output a very small current, not a power signal, so they cannot be used to drive the speaker system, and the audio signal must be further amplified by the power amplifier. Finally, the current and power are amplified so that it has sufficient power output to be sufficient to promote the work of the speaker system and radiate sound, that is, to promote the normal operation of the speaker. Power amplifier principle Power amplifier principle The current control function of the transistor or the voltage control function of the field effect tube is used to convert the power of the power supply into a current that changes according to the input signal. Because the sound is a wave of different amplitudes and different frequencies, that is, an AC signal current, the collector current of the transistor is always β times the base current, and β is the AC magnification of the transistor. Applying this, if a small signal is injected into the base, Then the current flowing through the collector will be equal to β times of the base current, and then this signal is isolated with a DC blocking capacitor to obtain a large signal whose current (or voltage) is β times the original. This phenomenon becomes the amplification of the triode effect. After continuous current amplification, power amplification is completed. 1. Balanced input and unbalanced input jacks 2. Balance-unbalance conversion stage, its role is to convert balanced input signal into unbalanced signal. 3. The line output isolation stage, whose function is to isolate the signal input to the power amplifier through active isolation and then output it to the outside. When a signal needs to drive multiple power amplifiers at the same time, the simple parallel mode will reduce the total synthesized output impedance. The result is that the actual output signal amplitude of the pre-stage equipment is reduced, which is the input signal amplitude actually obtained by a power amplifier. Reduced, if switched in this way, the load impedance of no signal is equivalent to the input impedance of a power amplifier. 4. The volume adjustment level actually takes the required amount from the total input signal through the potentiometer and adds it to the subsequent level, so that the output power is the required value. 5. The main task of the input stage and the secondary is to act as a buffer, and at the same time provide a certain amount of voltage amplification, and give a wave cancellation instruction if the power amplifier clips, so that the operator can appropriately reduce the volume. Sets often use the form of differential amplifier circuits. 6. The main voltage amplifier stage, this stage provides a large voltage amplification factor, the open-loop voltage amplification factor of the entire power amplifier is mainly provided by this stage. 7. The pre-push stage, because the main voltage amplifier stage can only provide a very small output signal current, so this stage is mainly to amplify the main voltage and provide the small signal current to initially amplify the signal current to several dozen times to one It is more than a hundred times, and not only does not amplify the signal voltage, but reduces it slightly. This stage uses an emitter follower circuit, which is a common collector circuit. 8. Pushing stage, further amplify the signal current that has been amplified by the pre-pushing stage. The magnification of the signal current is about several tens to more than one hundred times, in order to provide sufficient signal driving current to the power output stage. Like the driving stage, the signal voltage is not only not amplified, but slightly reduced. This stage also uses an emitter follower circuit. 9. The power output stage, this episode, will amplify the signal current again. Like the pre-push stage and the push stage, the signal voltage is not only not amplified, but slightly reduced. This stage also uses an emitter follower circuit. , This stage is the final output stage of this channel of the entire power amplifier, its output voltage depends on the driving signal voltage added to this stage, and the output current mainly depends on the ratio of the output signal voltage to the load impedance, which is mainly determined here Yu means that the output current cannot increase indefinitely with the infinite decrease of the load impedance. If the product of the current magnification beyond this stage and the drive signal current added to this stage, this stage will be unable to provide the maximum output signal current It is also limited by the output current of the DC working power supply provided for this level of work. In fact, the more important thing is the parameter limitation of the output power transistor, so when using the power amplifier, be sure not to overload the power amplifier, otherwise it may exceed the output power transistor Damage to the power amplifier.