(4) Multisim-based radio transmission system: design of low-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

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

Design of low frequency power amplifier

purpose

The audio signal is amplified and then input to the modulation circuit.

Multisim circuit diagram and analysis

The low-frequency power amplifier circuit is shown in the figure.
The input frequency is $f_0=1.65kHz$Modulated signal and its $V_{MAX}=2V$。

Simulation results and analysis

The simulation result is shown in the figure, the output wave VMAX = 13 V V_{MAX}=13V\, after passing through the low-frequency power amplifier$V_{MAX}=13V$, And the transient analysis of the low-frequency power amplifier shows that the circuit has obtained enough output power.

Buffer design

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