(5) Multisim-based radio transmission system: buffer design

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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

Please see the previous article (3) Multisim-based radio transmission system: design of high-frequency power amplifier.

Design of low frequency power amplifier

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

Buffer design

purpose

The buffer isolation stage isolates the oscillation stage from the power amplifier stage to reduce the impact of the power amplifier stage on the oscillation stage. Because the output signal of the power amplifier stage is large, changes in the working state will affect the frequency stability of the oscillator (or waveform distortion or output voltage reduction). small).

Multisim circuit diagram and analysis

In order to reduce the mutual influence between stages, a buffer isolation stage is usually inserted in the middle. The buffer isolation stage often uses an emitter follower circuit. The buffer in the picture is composed of LM1875T , which has the characteristics of high input impedance, low output impedance, and voltage amplification close to 1 .

Simulation results