The parameter optimization and distortion suppression of triode in the amplifier circuit are crucial to improving the circuit performance.
In various amplifier circuits such as audio and radio frequency, triode is a core device, and its performance directly affects the quality of signal amplification. Improper parameter optimization can easily lead to static operating point offset and cause nonlinear distortion; and when the signal amplitude exceeds the linear region of triode, saturation or cutoff distortion will occur. Distortion not only reduces the sound quality and affects the communication quality, but also may cause system misjudgment. Therefore, it is of key significance to improve the fidelity and stability of the amplifier circuit by reasonably selecting circuit parameters and adopting effective suppression methods.
The setting of the static operating point is the basis for suppressing distortion. By accurately calculating the base bias resistor, the static operating point is stabilized by using a voltage divider bias circuit so that it is located in the center of the linear region of the triode output characteristic curve. The bias circuit analysis is simplified by using the Thevenin theorem, and the resistance value that satisfies ICQ=βIBQ is calculated in combination with the triode input-output characteristic equation. For small signal amplifier circuits, the operating point current is generally 1-2mA, and the voltage is about half of the power supply voltage. At the same time, considering the influence of temperature on triode parameters, temperature compensation elements such as thermistors are introduced to dynamically adjust the bias voltage to ensure that the operating point remains stable when the temperature changes and reduce the distortion caused by temperature drift.
Different circuit topologies have different effects on distortion suppression. The common emitter circuit has a higher voltage gain, but there is a larger nonlinear distortion, which is suitable for occasions where distortion requirements are not high; although the common collector circuit has no voltage gain, it has the characteristics of high input impedance and low output impedance, which can effectively reduce the mutual influence between the signal source and the load, and is often used in impedance matching and power amplifier pre-stage; the common base circuit has good high-frequency characteristics and small distortion, which is suitable for high-frequency amplification. In multi-stage amplifier circuits, a common emitter-common base combination circuit can be used to give full play to the high-frequency characteristics of the common base circuit and the voltage amplification capability of the common emitter circuit, while suppressing nonlinear distortion and improving overall performance.
Introducing negative feedback is an effective means to suppress distortion. Voltage series negative feedback can stabilize the output voltage, reduce the output impedance, and improve the load capacity; current parallel negative feedback can stabilize the output current and increase the input impedance. Reasonably design the feedback coefficient and determine the feedback depth according to the required gain and distortion. For audio amplifier circuits, a feedback depth of 10-20dB is usually used, which can effectively suppress distortion without excessively reducing gain. At the same time, pay attention to the self-oscillation problem that may be caused by negative feedback. By adding a phase compensation capacitor in the feedback network, the self-oscillation condition is destroyed to ensure stable operation of the circuit.
It is crucial to select triodes and peripheral components with matching parameters. The difference in the β value of the paired triode should be controlled within 5% to reduce the distortion caused by inconsistent parameters. For power amplifier circuits, select triodes with high characteristic frequency fT to ensure sufficient gain-bandwidth product within the operating frequency range. Optimize the values of coupling capacitors and bypass capacitors to make them present low impedance at the operating frequency, reducing the attenuation and distortion of the signal during transmission. At the same time, reduce the parasitic capacitance and inductance in the circuit, shorten the transmission path of high-frequency signals, and reduce high-frequency distortion.
Establishing a distortion detection and compensation mechanism can improve the amplification performance in real time. The harmonic components in the output signal are detected by a harmonic analyzer to calculate the total harmonic distortion (THD). When it is detected that the distortion exceeds the threshold, the bias circuit parameters or feedback coefficients are adaptively adjusted. For example, a digital potentiometer is used to dynamically adjust the bias resistor, or a microcontroller is used to adjust the digital programmable gain amplifier (PGA) in the feedback network. In addition, the pre-distortion technology is used to pre-introduce characteristics opposite to the nonlinear distortion of the triode in the input signal, so that the distortion of the final output signal is offset, further improving the amplification quality.
Special optimization measures are required in high-frequency and power amplifier circuits. For high-frequency amplification, a low-noise, high-cutoff frequency triode is selected, and a microstrip line or coaxial cable is used to transmit the signal to reduce signal radiation and interference. In the power amplifier circuit, Class A, B or Class B working states are used to reduce static power consumption, but attention should be paid to the crossover distortion problem, which is eliminated by setting an appropriate bias voltage. At the same time, a heat sink is equipped for the triode to ensure stable temperature during high-power operation to avoid distortion caused by changes in the β value due to temperature increase. In addition, a push-pull circuit structure is adopted, and two triodes are used to work alternately to effectively suppress even-order harmonic distortion and improve power amplifier efficiency and sound quality.
