Power amplifier experimental device

Abstract

The invention discloses a power amplifier experiment device. The device comprises a power amplifier experiment circuit, the upper end of a loudspeaker is connected with a right contact of a change-over switch, and the upper end of a load resistor is connected with a left contact of the change-over switch. The center end of the change-over switch is connected with an output coupling capacitor of the power amplifier. And the bootstrap capacitor branch is connected in series with the switch. The circuit is provided with a damping circuit for eliminating high-frequency self-oscillation. Experiment circuit diagrams, instrument block diagrams of a signal source, an oscilloscope and the like and a circuit connection mode are printed on the front face of an instrument panel. The defects that in the prior art, similar products are large in experimental data error and prone to high-frequency self-oscillation to cause experimental failure, noise of fixed tones in a laboratory is noisy, and a bootstrap capacitor can not be made to improve the experimental content of the maximum output power are overcome. The method has the characteristics of low error, no noisy, multiple functions, no self-excitation and high wiring success rate. The device is used for improving experiment teaching of analog circuit courses in colleges and universities.

Description

Technical Field

[0001] The present invention relates to an analog circuit experiment device, and particularly to a power amplifier experiment device. Background Art

[0002] "Analog Circuit Experiment" is an experimental course required for students majoring in science and engineering in various domestic universities. In the analog circuit experiment projects, there is a power amplifier experiment project. The experimental equipment that can complete these experimental projects is usually an analog circuit experiment box or an electrical and electronic experiment bench. A power amplifier experimental circuit is installed on such an experiment box or experiment bench. Together with a low-voltage DC regulated power supply and several thin patch cords with small plugs at both ends for connecting the circuit, a power amplifier experimental device is formed. By connecting a signal source, an oscilloscope, a DC ammeter, an AC voltmeter, and a digital multimeter to this power amplifier experimental device, experiments such as the debugging of the static operating point of the power amplifier, the measurement of the maximum undistorted output power, the measurement of the frequency response, and the measurement of the efficiency can be carried out. The inventors of the present application found five defects in the products of the prior art during experimental teaching: First, the continuous noise of a fixed tone emitted by several speakers of each experimental group during the experiment is unbearable; Second, when measuring the frequency response of the power amplifier, since the voice coil of the speaker is a coil wound with enameled wire, which is equivalent to a combination of a resistor and an inductor, the impedance is different for different frequencies. Especially when it reaches tens of thousands of hertz, the impedance increases significantly, resulting in an incorrect measurement value (higher voltage) of the frequency response of the high-frequency signal; Third, the bootstrap capacitor in the bootstrap circuit is fixed and cannot be disconnected, resulting in the inability to experimentally verify the beneficial effect of the bootstrap capacitor on increasing the maximum undistorted output power, which is not conducive to students' perception of the important role of the bootstrap capacitor; Fourth, during the experiment, due to the relatively long output and input leads (patch cords with plugs), the long output lead is equivalent to a transmitting antenna, and the long input lead is equivalent to a receiving antenna. This forms a positive feedback electrical signal when the amplified output signal electromagnetic wave enters the input lead through space electromagnetic induction, resulting in high-frequency self-excited parasitic oscillation. When observing the audio amplification signal waveform with an oscilloscope, a very messy and unstable high-frequency parasitic oscillation signal waveform is superimposed, making it impossible to accurately read the data or even read the data; Fifth, the front side of the epoxy resin circuit board of the similar products in the prior art does not have instrument graphics such as a signal source, an oscilloscope, and a voltmeter and their connection methods, which is not convenient for students to correctly connect the wires during the experiment. The above five defects often lead to large errors in the experimental data of the power amplifier, and even the failure of the experiment due to self-excitation oscillation. In addition, there are defects such as the fixed-tone noise in the laboratory being very noisy and the inability to conduct the experiment on the beneficial effect of the bootstrap capacitor on increasing the maximum output power.

[0003] The object of the present invention is to provide a power amplifier experimental device with low error, noiseless, multifunctional, non-self-exciting, and high one-time wiring success rate, which can significantly improve the experimental teaching effect and quality. During the experimental teaching process, the power amplifier experimental device has no continuous noise, the load impedance does not change with frequency, significantly reducing the measurement error, adding an experiment to verify the bootstrap capacitor can increase the maximum output power, completely avoiding high-frequency self-excitation oscillation, and the circuit and instrument connections are clear at a glance. Summary of the Invention

[0004] To achieve the above object, the negative pole of the DC power supply 1 of the present invention is connected to the ground, and the output end of the signal source 2 is connected to the base of the preamplification triode T1 through the input capacitor C1. The preamplification triode T1 and each peripheral electronic component connected thereto form a preamplification circuit. The lower bias resistor R B1 is connected in parallel between the base of the preamplification triode T1 and the ground, and the upper bias resistor R B2 is connected in series with the potentiometer R W1 and then connected in parallel between the base of the preamplification triode T1 and the emitter of the lower power amplification triode T3. The emitter resistor R E1 is connected in parallel with the emitter capacitor C E1 and then connected between the emitter of the preamplification triode T1 and the ground. The collector of the preamplification triode T1 is connected to the negative pole of the diode D and the base of the lower power amplification triode T3. The positive pole of the diode D is connected to the trimming resistor R W2 , and the upper end of the trimming resistor R W2 is connected to the base and the lower end of the collector resistor R C1 of the upper power amplification triode T2. The upper end of the collector resistor R C1 is connected to the left end of the resistor R. The collector of the upper power amplification triode T2 is connected to the right end of the resistor R and is connected to the output end of the DC power supply 1 through the ammeter 3. The emitter of the upper power amplification triode T2 is connected to the emitter of the lower power amplification triode T3 and the left end of the output coupling capacitor C O . The collector of the lower power amplification triode T3 is grounded. Each electronic component and the circuit diagram are installed on the front of the epoxy resin circuit board 10 and the diagram corresponds to the physical object of the component one by one. The circuit connections are installed on the back of the epoxy resin circuit board 10. The upper end of the speaker 6 is connected to the right contact of the changeover switch 7, and the lower end of the speaker 6 is grounded. The upper end of the load resistor R L is connected to the left contact of the changeover switch 7, and the lower end of the load resistor R L is grounded. The center end of the changeover switch 7 is connected to the right end of the output coupling capacitor C O . The bootstrap capacitor C2 is connected in series with the switch 8 and then connected in parallel between the emitter of the upper power amplification triode T2 and the left end of the resistor R. The bootstrap capacitor C2, the switch 8, and the collector resistor R C1A bootstrap circuit that can be turned on and off is formed. The upper power amplifier triode T2 and the lower power amplifier triode T3, together with the various peripheral electronic components connected thereto, form a power amplifier circuit. Each electronic component is installed on the front side of the epoxy resin circuit board 10, and the circuit connections are installed on the back side of the epoxy resin circuit board 10. In order to eliminate the high-frequency self-excited oscillation that may occur during the experiment, a damping capacitor C3 is also connected in parallel between the base of the lower power amplifier triode T3 and the ground.

[0005] In order to improve the one-time success rate of students' wiring during the experiment class, the instrument block diagrams of the signal source 2, the ammeter 3, the AC voltmeter 4, the oscilloscope 5, and the DC voltmeter 9 are also printed on the front side of the epoxy resin circuit board 10 and are provided with the connections to the circuit.

[0006] Since the upper end of the speaker 6 of the present invention is connected to the right contact of the changeover switch 7, and the upper end of the load resistor R L is connected to the left contact of the changeover switch 7, and the center terminal of the changeover switch 7 is connected to the right end of the output coupling capacitor C O This enables the changeover switch 7 to be conveniently switched to the right contact for a moment during the experiment, so as to briefly turn on the speaker 6 to listen to the sound for a short time, thereby proving that the circuit is connected correctly and perceiving the sound effect of the power amplifier; then the changeover switch 7 is switched to the left contact to connect the load resistor R L The sound of the speaker disappears, and various experimental data can be measured calmly without disturbing others. Moreover, when measuring the frequency response, since the non-inductive load resistor R L is used instead of the inductive speaker 6, the experimental error is significantly reduced.

[0007] Since a switch 8 is connected in series on the capacitor C2 of the bootstrap circuit, the maximum output power of the power amplifier can be measured and compared in two cases when the switch 8 is disconnected and connected respectively, thereby verifying the important role of the bootstrap capacitor C2.

[0008] A damping capacitor C3 is connected in parallel between the base of the triode T1 and the ground, so that the high-frequency self-excited oscillation signals that may occur during the experiment are short-circuited to the ground and absorbed, and are completely suppressed, avoiding the defects that the oscilloscope waveform is superimposed with high-frequency oscillation signals and the experiment cannot be carried out or the experimental error is too large.

[0009] The present invention prints the instrument panel diagram including the instrument block diagrams such as the signal source and the oscilloscope on the front side of the epoxy resin circuit board 10, which makes the wiring principle of the experimental circuit and each instrument very intuitive, facilitates the students' wiring and understanding and memory, and improves the one-time wiring success rate during the experiment class. That is, the present invention has the characteristics of low error, non-disturbing, multi-functional, non-self-excited, and high one-time wiring success rate.

[0010] The cost increased by the present invention on the existing technology products is only a few yuan, but the improvement of the experimental teaching effect brought is very significant. Description of the Drawings

[0011] Figure 1 is the connection diagram of the circuit and peripheral instruments of the present invention.

[0012] Figure 2 is the instrument panel diagram of the present invention.

[0013] Figure 3 is the measured frequency response curve diagram of the present invention. Detailed Embodiment

[0014] Refer to Figure 1 . The negative pole of the DC power supply 1 is connected to the ground of the power amplifier (abbreviated as ground in the present invention). The output end of the signal source 2 is connected to the base of the preamplification triode T1 through the input capacitor C1. Any one of the existing technologies can be used for the signal source (also called the function signal generator). The lower bias resistor R B1 is connected in parallel between the base of the preamplification triode T1 and the ground. The upper bias resistor R B2 is connected in series with the potentiometer R W1 and then connected in parallel between the base of the preamplification triode T1 and the emitter of the lower power amplifier triode T3. The emitter resistor R E1 is connected in parallel with the emitter capacitor C E1 and then connected between the emitter of the preamplification triode T1 and the ground, which is used to generate DC negative feedback to stabilize the static operating point and has no negative feedback on the AC signal. The collector of the preamplification triode T1 is connected to the negative pole of the diode D and the base of the lower power amplifier triode T3. The positive pole of the diode D is connected to the trimming resistor R W2 . The upper end of the trimming resistor R W2 is connected to the base and the lower end of the collector resistor R C1 of the upper power amplifier triode T2. The series circuit of the diode D and the trimming resistor R W2 plays a role in adjusting and stabilizing the static current of the upper power amplifier triode T2 and the lower power amplifier triode T3, and is used to eliminate crossover distortion and reduce static power consumption. The upper end of the collector resistor R C1 is connected to the left end of the resistor R. The resistor R plays a role in decoupling and delaying the discharge speed of the bootstrap capacitor C2, and making the bootstrap capacitor C2 produce a bootstrap effect. The collector of the upper power amplifier triode T2 is connected to the right end of the resistor R and is connected to the output end (the +5V terminal) of the DC power supply 1 through the ammeter 3. Of course, the ammeter 3 is not required for each experimental content (only used when debugging the static current of the final-stage power amplifier and measuring the efficiency, and other experimental contents can be replaced by patch cords). In the experiment, the ammeter also acts as a wire. The ammeter 3 can be replaced by the 200 mA DC current range of a digital multimeter. The emitter of the upper power amplifier triode T2 is connected to the emitter of the lower power amplifier triode T3 and the output coupling capacitor C OThe left ends (positive extreme ends) of the three are connected together. The collector of the lower power amplifier triode T3 is grounded. The upper end of the speaker 6 is connected to the right contact of the changeover switch 7, and the lower end of the speaker 6 is grounded. The upper end of the load resistor R L is connected to the left contact of the changeover switch 7, and the lower end of the load resistor R L is grounded. Both the speaker 6 and the load resistor R L have a resistance value of 8 ohms, so that they can be substituted for each other when measuring experimental data. The center terminal of the changeover switch 7 is connected to the right end (negative extreme end) of the output coupling capacitor C O , and the right end of the output coupling capacitor C O is the output terminal of the power amplifier. The output terminal of the power amplifier is also connected to the input probe of the oscilloscope 5 and the input terminal of the AC voltmeter 4 (range 2 volts) for measuring the amplified waveforms and output voltage values under various conditions during the experiment. Both the oscilloscope 5 and the AC voltmeter 4 use products in the prior art. The DC voltmeter 9 is used to measure the static DC voltages of each pole of each triode to conduct a comprehensive debugging of the static operating point. The grounds of the signal source 2, the AC voltmeter 4, the oscilloscope 5, and the DC voltmeter 9 are all connected to the ground of the power amplifier (the negative pole of the DC power supply 1). The DC voltmeter 9 can also be replaced by the 20-volt DC voltage range of a digital multimeter.

[0015] The bootstrap capacitor C2 is connected in series with the switch 8 and then connected in parallel between the emitter of the upper power amplifier triode T2 and the left end of the resistor R to be used as a circuit for bootstrapping to increase the maximum output power (precisely speaking, it should be called the maximum non-clipping distortion output power). In this way, the maximum output powers of the power amplifier in two cases where the switch 8 is disconnected and connected can be measured respectively for comparison. When the switch 8 is connected, the maximum output power is high, and when the switch 8 is disconnected, the maximum output power decreases significantly (see the experimental data later), thus verifying the important role of the bootstrap capacitor C2 and deepening the students' understanding of the importance of the bootstrap capacitor. This adds an experimental function and has the feature of being multi-functional compared with the prior art.

[0016] In order to eliminate the possible high-frequency self-excited oscillation during the experiment, a damping capacitor C3 is also connected in parallel between the base of the lower power amplifier triode T3 and the ground. The principle is that when the high-frequency oscillation signal that may be generated during the experiment (proved by the inventors of the present invention through years of experimental teaching: high-frequency self-excited oscillation is extremely likely to occur when the input and output leads are too long or they are close to each other, and it almost occurs every time in class) is short-circuited to the ground by the damping capacitor C3 for AC and is filtered out, completely avoiding the occurrence of self-excited oscillation. And because the capacitance of the damping capacitor C3 is very small (only 0.005 microfarads), it is almost open-circuited and lossless to the audio experimental signals from 20 to 20,000 hertz and will not affect the experimental results.

[0017] The present invention only adds a few electronic components to the products in the prior art (such as the changeover switch 7, the load resistor R L, the switch 8, and the anti-vibration capacitor C3), the cost only increases by a few yuan, and the production is also very easy. However, it brings advantages such as significantly reducing experimental errors, increasing the experimental success rate, eliminating the occurrence of the phenomenon of long-term continuous noise disturbing people during the experiment, and adding experimental content to verify the important role of the bootstrap capacitor C2 in increasing the maximum output power (that is, realizing the multi-functional feature).

[0018] When conducting experiments with the present invention, the change-over switch 7 can be conveniently turned to the right contact point for a moment first, so as to briefly turn on the speaker 6 to listen to the sound for a short time, to prove that the circuit is connected correctly and to perceive the sound effect of the power amplifier; then turn the change-over switch 7 to the left contact point to connect the load resistor R L The sound of the speaker disappears, and various experimental data can be measured calmly without sound output disturbing people. And when measuring the frequency response, since the non-inductive load resistor R L is used instead of the inductive speaker 6, the experimental error is significantly reduced (see the actual measured data later).

[0019] The parameters of some components in this embodiment are as follows: Resistor R B1 、R B2 、R E1 、R C1 、R, R L have resistance values of 3.3, 2.4, 0.1, 0.68, 0.51, 0.008 kiloohms respectively. The potentiometer R W1 has a resistance value range of 0 - 10 kiloohms. The trimming resistor (R W2 ) has a resistance value range of 0 - 1 kiloohm. The diode D is a silicon rectifier diode 1N4007. The capacitances of the capacitors C1, C2, C3, C E1 、C O are 10, 100, 0.005, 100, 1000 microfarads respectively. The triodes T1 and T2 are NPN small-power triodes 2SC9011 and 2SC9013 respectively, and the triode T3 is a PNP small-power triode 2SC9012. The speaker 6 is a small moving coil speaker with an impedance of 8 ohms. The change-over switch 7 and the switch 8 can both be selected as small toggle double-position switches, but only one side of the switch 8 is used as a single-position switch.

[0020] The DC power supply 1 of the present invention can be made into a traditional series voltage regulator power supply with electronic components such as a 220V to 9V transformer in the prior art, a bridge rectifier composed of four 1N4007 diodes, a positive 5V three-terminal voltage regulator integrated circuit 7805, and a filter circuit composed of two 470 μF electrolytic capacitors, or any +5V DC power supply (or a +5V switching power supply) in the prior art. If the present invention is installed and used on an electrical and electronic experimental bench, the original +5V DC regulated power supply on the experimental bench can be used. The signal source 2, oscilloscope 5, AC voltmeter 4 (or millivoltmeter), DC voltmeter 9, and ammeter 3 used in the present invention are all products of the prior art. The DC voltmeter 9 and ammeter 3 can also be replaced by ordinary digital multimeters.

[0021] The present invention installs each component according to the layout with reference to Figure 1 (or Figure 2 ) on the front of the insulating epoxy resin circuit board 10 (which is also the substrate of the instrument panel), and then connects them from the back in the printed circuit board wiring mode. When doing experiments, small jacks are connected and fixed to the epoxy resin board at the places where students need to connect wires or instruments. Metal hooks can be soldered at multiple positions of the ground wire (such as at the left and right ends and the middle part of the ground wire) so that the black ground wire clips of the signal source, oscilloscope, and AC voltmeter can be clipped on them. The above several practices are similar to those of similar instruments in the prior art (for example: the electrical and electronic experimental bench produced by Hangzhou Tianhuang Teaching Instrument Company and the EL-DSDG-IIB type electrical and electronic experimental bench produced by a certain instrument company in Wuhan). The front of the epoxy resin circuit board 10 also needs to print the Figure 2 of the instrument panel. If the surface of the epoxy resin circuit board 10 of the instrument panel is dark, the lines on the instrument panel Figure 2 should be white, so that the circuit diagram on the instrument panel is clearer and more eye-catching; on the contrary, the Figure 2 of the instrument panel should be a white background with a black circuit diagram (black lines, black fonts, and black component diagrams - just like the Figure 2 color mode). It should also be noted that the graphic positions of each component on the instrument panel Figure 2 are the installation positions of the physical components, that is, the physical objects of each electronic component correspond one by one to the components on the diagram. Of course, the speaker 6 is large in volume and can be installed in other positions. Since the instrument panel Figure 2 includes the block diagrams of the signal source 2, AC voltmeter 4, DC voltmeter 9, oscilloscope 5 and the connection methods with the circuit, this can enable students to have a clear view of the circuit wiring, facilitating students' wiring and testing, and improving the one-time success rate of wiring in experimental classes.

[0022] To enable the public to better understand the present invention, the following is a comparison of the experimental data or phenomena of the products in the prior art with those of the present invention in five items through experiments actually conducted by the inventors of the present invention: The First Item: Comparison of Frequency Response Tests

[0023] Test conditions for this item: The power amplifier inputs a sine wave of 12.1 mV and keeps the voltage unchanged.

[0024] 1. Under the condition that the power output of the prior art product is connected to an 8-ohm speaker (equivalent to turning the switch 7 to the right contact point), the corresponding relationship between the actually measured experimental frequency and the output voltage (rms value) of the power amplifier is: 170 Hz at 255 mV, 200 Hz at 275 mV, 400 Hz at 336 mV, 500 Hz at 348 mV, 350 mV from 1000 to 4000 Hz, 385 mV at 5000 Hz, 403 mV at 7000 Hz, 411 mV at 8000 Hz, 437 mV at 10000 Hz, 420 mV at 15000 Hz, 403 mV at 20000 Hz, 350 mV at 25000 Hz, 348 mV at 30000 Hz, 298 mV at 35000 Hz, 271 mV at 40000 Hz, and 255 mV at 49600 Hz. Although the data between two adjacent experimental data above are not recorded, the oscilloscope waveform shows that the output voltage data between two adjacent experimental data (frequencies) do not exceed the output voltage range included between these two adjacent experimental data (that is, it will neither be lower than the minimum value of the output voltage of these two adjacent experimental data nor higher than the maximum value of the output voltage of these two adjacent experimental data). The oscilloscope waveform shows that the output voltage data waveform between the experimental data does not have a sudden increase or decrease, that is, the output voltage is gradually changing continuously. The frequency response curve drawn based on the above experimental data can be seen in the Figure 3 Frequency response curve 1 in the attached drawings of the specification.

[0025] Based on the output voltage at 1000 Hz as the reference for the above experimental data, when the output voltage drops by -3 dB (0.707 times the reference value), the corresponding frequency response is from 170 Hz to 49.6 kHz. The above data shows that due to the inductive reactance of the speaker voice coil increasing with the increase of frequency, resulting in a significant increase in the power output voltage from 8 to 20 kHz (seen from the Figure 3 frequency response curve 1 in the attached drawings, so a bulging package is generated), with the highest increase of 25%. This phenomenon is not based on the theory of the power amplifier itself and is completely wrong. It can be seen that the experimental results have a large error. That is, the bulging package in the frequency response curve 1 is not a characteristic of the power amplifier itself, so the experimental data and curve are obviously wrong.

[0026] 2. The product power output of the present invention is connected to an 8-ohm load resistor R L (with the switch 7 turned to the left contact), the relationship between the actually measured experimental frequency and the output voltage (rms) of the power amplifier is as follows: 170 Hz, 255 mV; 200 Hz, 275 mV; 400 Hz, 336 mV; 500 Hz, 348 mV; from 1000 Hz to 18000 Hz, 350 mV; 20000 Hz, 349 mV; 22000 Hz, 348 mV; 24000 Hz, 336 mV; 39000 Hz, 255 mV. Based on the output voltage at 1000 Hz for the above experimental data, the frequency response when the output voltage drops by -3 dB is from 170 Hz to 39 kHz. Although the data between any two adjacent experimental data above are not recorded, the oscilloscope waveform shows that the output voltage data waveform between adjacent experimental data does not have a sudden increase or decrease, that is, the output voltage changes gradually and continuously. The frequency response curve plotted based on the above experimental data can be seen in the Figure 3 frequency response curve 2 in the attached drawings of the specification. It can be seen that the frequency response curve is very flat and there is no such bulging phenomenon as described above where the voltage increases in some sections.

[0027] The comparison between the above prior art and the experimental data and experimental frequency response curve of the present invention shows that the present invention significantly improves the experimental accuracy and greatly reduces the experimental error. This is because the 8-ohm load resistor R L has no inductance and its impedance does not increase with the increase of frequency. Therefore, the frequency response curve does not have the defect that the output voltage in the high-frequency part of the frequency range increases significantly as described above when connected to a speaker. In the frequency response range, the frequency response curve is very flat, restoring the original true frequency response curve of the power amplifier. That is to say, the frequency response curve of the power amplifier measured by similar products in the prior art is incorrect: one error is that the data of the originally very flat frequency response curve of the power amplifier is wrongly output as a very uneven curve with a bulge in the 8 - 20 kHz frequency range where the signal amplitude is up to 25% higher than that at 1 kHz; the other error is that the frequency band width of the frequency response curve is wrongly extended from the actual value of 170 Hz to 39 kHz to 170 Hz to 49.6 kHz.

[0028] Second item: Comparison of the effect of preventing experimental noise 1. Under the condition that the power output of the prior art product is connected to an 8-ohm speaker (equivalent to turning the switch 7 to the right contact), the continuous and unchanging-pitch noise is unbearable and very noisy. Moreover, when doing this experiment in the experimental class, the noise emitted by more than a dozen experimental groups doing the experiment simultaneously is even more unbearable and noisy.

[0029] 2. The product power output of the present invention is connected to the load resistor R LUnder the condition that the switch 7 is turned to connect to the left contact point, the load resistor R L will not emit noisy sounds and the room will be quiet.

[0030] It can be seen that the present invention significantly improves the quality and effect of experimental teaching, but the increased cost is only a few yuan.

[0031] Item 3. The role of the bootstrap capacitor in increasing the maximum output power 1. In the products of the prior art, there is only a non-disconnectable mode in which the bootstrap capacitor C2 is fixedly welded in the circuit, and the experimental content for verifying the important role of the bootstrap capacitor in increasing the maximum output power cannot be carried out. Only the experimental content of measuring the maximum output power under the condition that the bootstrap capacitor C2 is always connected can be carried out. The measured maximum output power is 0.118 watts (the corresponding maximum undistorted voltage is 0.97 volts). It is necessary to explain the test rules: the maximum output power referred to in the present invention means the state when the sine wave on the oscilloscope connected to the output end of the power amplifier appears critical clipping distortion.

[0032] 2. When the switch 8 in the bootstrap circuit of the product of the present invention is turned to the on state (equivalent to the test state in the prior art), the measured maximum output power is 0.118 watts; when the switch 8 in the bootstrap circuit of the product of the present invention is turned to the off state, severe clipping distortion appears. Then, by reducing the amplitude of the sine wave signal input to the input end of the power amplifier and making the output signal waveform of the power amplifier return to the critical clipping distortion state again, the measured maximum output power is reduced to 0.06 watts (the corresponding maximum undistorted voltage is 0.69 volts). From the above experimental data, it can be seen that the connection of the bootstrap capacitor C2 can significantly increase the maximum output power of the power amplifier by nearly one time.

[0033] It can be seen that although the added switch 8 in the bootstrap circuit of the present invention costs only about 2 yuan, this enables students to obtain experimental data for comparing the maximum output power in two cases of the connection and disconnection of the bootstrap capacitor C2 through experiments, and to perceive and understand the importance of the bootstrap capacitor in the power amplifier through experiments, which can convince students both orally and mentally, but the products in the prior art cannot carry out this experimental content.

[0034] Item 4. Comparison of the non-self-excitation performance of the circuit 1. In the products of the prior art, when students connect the wires between the input terminal of the power amplifier and the signal source (the lengths of the patch cords vary) during wiring, and the wire between the selected oscilloscope and the output terminal of the power amplifier is too long, positive feedback will occur due to electromagnetic wave induction between the output and input wires. If the positive feedback is strong enough, high-frequency self-oscillation will occur. On the oscilloscope, the waveform will be chaotic and unstable noise instead of a sine wave, and the measured voltage will also be incorrect. That is, once self-oscillation occurs, this experiment cannot be carried out. (The inventor of this patent application often encounters this situation in every classroom teaching of this experimental project all year round. Once this situation occurs, a small-capacity capacitor of 5000 picofarads is connected in parallel between the base of the lower power amplifier triode T3 and the ground, and the self-oscillation immediately stops.)

[0035] 2. In the present invention, a damping capacitor C3 is connected in parallel between the base of the lower power amplifier triode T3 and the ground. The damping capacitor C3 presents a very low impedance to high-frequency self-excitation signals, thereby filtering out the high-frequency self-excitation signals and making the amplification factor of high-frequency signals very low, completely overcoming the drawback that self-oscillation occurs during the experiment and the experimental class cannot be completed. The cost increase of this improvement in the present invention is only a few cents, but the effect is remarkable.

[0036] Fifth item: Performance comparison of printing the instrument block diagram on the panel of the experimental instrument 1. In the products of the prior art, the instruments necessary for this experiment, such as signal sources and oscilloscopes, are not printed on the panel of the experimental instrument, which makes it easy for students to make mistakes when connecting the circuit.

[0037] 2. In the present invention, the instruments necessary for this experiment, such as signal sources and oscilloscopes, and the wires connecting them to the circuit are also printed on the panel of the experimental instrument, which facilitates students to correctly connect the instruments during the experiment and improves the success rate of the experiment for each wiring. This improvement in the present invention hardly increases the cost, but the teaching effect is improved.

[0038] As can be seen from the above content, the present invention ingeniously and significantly reduces the experimental error of the experimental data of the same type of power amplifier experimental instrument in the prior art, eliminates the instrument noise in the experimental class without being noisy, adds the experimental content of verifying the effect of the bootstrap capacitor on improving the maximum output power of the power amplifier, completely avoids the self-oscillation phenomenon of the experimental circuit, and improves the one-time success rate of students' wiring. These improvement effects all significantly improve the teaching quality of this experimental teaching project. In short, the present invention has the characteristics of low error, not noisy, multi-functional, non-self-exciting, and high wiring success rate, and significantly improves the teaching effect of this experimental teaching project.

[0039] The present invention can be used either as an independent experimental device or to improve the analog circuit experimental box or the electrical and electronic technology experimental bench in the prior art. It only needs to add the improved part of the present invention to these experimental boxes or experimental benches.

[0040] Although the present invention is directed to a power amplifier experimental circuit of a domestic most popular discrete component without output transformer (OTL), it can also be extended to other types of power amplifier experimental circuits.

Claims

1. A power amplifier experimental device, the negative electrode of the DC power supply (1) is connected to the ground, the input capacitor (C1) is connected to the base of the preamplifier transistor (T1), and the lower bias resistor (R B1 ) is connected in parallel between the base of the preamplifier transistor (T1) and ground, and the bias resistor (R B2 ) and the potentiometer (R W1 ) are connected in series and then in parallel to the base of the preamplifier transistor (T1) and the emitter of the lower power amplifier transistor (T3). The emitter resistor (R E1 ) and the emitter capacitance (C E1 ) are connected in parallel between the emitter of the preamplifier transistor (T1) and the ground, the collector of the preamplifier transistor (T1) is connected to the cathode of the diode (D) and the base of the lower power amplifier transistor (T3), and the anode of the diode (D) is connected to the trimmer resistor (R W2 ), fine-tuning resistor (R W2 ) is connected to the base of the power amplifier transistor (T2) and the collector resistor (R C1 ) at the lower end, the collector resistor (R C1 ) is connected to the left end of the resistor (R), the collector of the upper power amplifier transistor (T2) is connected to the right end of the resistor (R) and is connected to the output end of the DC power supply (1) through the ammeter (3), the emitter of the upper power amplifier transistor (T2) is connected to the emitter of the lower power amplifier transistor (T3) and the output coupling capacitor (C O ), the collector of the lower power amplifier transistor (T3) is grounded, various electronic components and circuit diagrams are mounted on the front of the epoxy resin circuit board (10), and the diagrams correspond to the actual components one by one, and the circuit wiring is mounted on the back of the epoxy resin circuit board (10), which is characterized by: The upper end of the speaker (6) is connected to the right contact of the switch (7), the lower end of the speaker (6) is grounded, and the load resistor (R L ) is connected to the left contact of the transfer switch (7), and the load resistor (R L ) is grounded, and the center terminal of the switch (7) is connected to the output coupling capacitor (C O ), a bootstrap capacitor (C2) is connected in series with a switch (8) and then connected in parallel to the emitter of the upper power amplifier transistor (T2) and the left end of the resistor (R), and a vibration elimination capacitor (C3) is connected in parallel between the base of the lower power amplifier transistor (T3) and the ground.

2. The power amplifier experimental device according to claim 1 is characterized in that The front side of the epoxy resin circuit board (10) is also printed with a block diagram of an instrument including a signal source (2), an ammeter (3), an AC voltmeter (4), an oscilloscope (5), and a DC voltmeter (9), and with wiring connecting the circuits.

Images