(3) Multisim-based radio transmission system: design of high-frequency power amplifier

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1. This series is divided into five chapters , including (1) oscillator design, (2) amplitude modulator design, (3) high frequency power amplifier design, (4) low frequency power amplifier design and (5) buffering The design of the device and the software used are Multisim14.
2. The next series is a superheterodyne receiving system based on Multisim, so stay tuned.
3. Free to share the original files of the Multisim circuit design of the entire radio transmission system, just leave a comment.
4. Please indicate the original author for reprinting, thank you.

System Requirements

1. Carrier signal frequency $535-1605kHz$
2. IF signal frequency $465kHz$
3. Modulation signal frequency $500Hz-10kHz$

Fundamental

The radio transmission system uses free space as the transmission channel, and loads the signals to be transmitted into high-frequency oscillations and transforms them into electromagnetic waves and sends them to a remote receiving point.
The overall framework of the radio transmission system is shown in the figure.
In order to improve the frequency stability, an improved capacitor three-terminal oscillator- Schiller oscillator is used , and a buffer is added behind it to weaken the influence of the latter stage on the main oscillator. The amplitude modulator is the core of the transmitter, and an analog multiplier is used to modulate the carrier signal and the input signal. Although AM modulation has low power utilization and poor anti-interference ability, its receiving equipment is simple and is still widely used in radio transmission systems. The high-frequency power amplifier amplifies the power of the modulated signal for signal transmission.

Oscillator design

Please see the previous article (1) Multisim-based radio transmission system: oscillator design.

Design of Amplitude Modulator

Please see the previous article (2) Multisim-based radio transmission system: the design of amplitude modulator.

Design of high frequency power amplifier

Principle of high frequency power amplifier

The schematic diagram of the high-frequency power amplifier is shown in the figure. Its function is to amplify the high-frequency signal so that the final stage of the transmitter can obtain enough transmission power to meet the requirements of the transmission power, and then radiate it to the space through the antenna to ensure that Receivers in the area can receive satisfactory signal levels.

Multisim circuit schematic diagram and analysis

The circuit simulated with Multisim14 software is shown in the figure.
1. In order to ensure that the triode is working in the amplified state, in $u_{BE}$和$u_{CE}$Place a multimeter in between to observe and adjust the working state of the transistor at any time;
2. When $R_1$When it is too large, the waveform will be distorted and the magnification will not be uniform. Don't be afraid, fine-tune the circuit until it meets the requirements.

Experimental results and analysis

The simulation result of the high frequency power amplifier is shown in the figure.
1. Input AM wave $V_{MAX}=500mV$, Output AM wave after high frequency power amplifier $V_{MAX}=1.5V$;
2. Analysis of the transient power amplifier circuit found sufficient to obtain a high-frequency output power.

Design of low frequency power amplifier

Please see the next article (4) Multisim-based radio transmission system: design of low-frequency power amplifier.

Buffer design

Please see the next article (5) Multisim-based radio transmission system: buffer design.