Introduction: LM7805 is originally a series regulated power supply, but it can actually be regarded as an emitter follower, with a large current gain between its output and input. Using this feature, a Colpits oscillator can be formed based on it using an LC circuit.

关键词: LM7805 , Colpits , Capacitor

01 7805 Oscillator

7805 is a three-terminal voltage regulator integrated circuit. In electronic products, common three-terminal voltage regulator integrated circuits include 78×× series with positive voltage output and 79×× series with negative voltage output. As the name implies, a three-terminal IC refers to an integrated circuit used for voltage regulation. There are only three pin outputs, namely the input terminal, the ground terminal and the output terminal. It looks like an ordinary triode, the standard package of TO-220, and the TO-92 package of 9013.

The LM7805 is originally a series voltage regulator integrated chip, which maintains a stable 5V voltage between its output and GND. Actually the output forms a follower with GND. Using this one, it is possible to add LC to its output to form a sinusoidal oscillation circuit. The following experiment will test this idea.

First, the circuit principle

▲ Figure 1.1.1 Schematic diagram of the experimental circuit

▲ 图1.1.1 实验电路原理图

Second, the experimental circuit

Build the above test circuit on a breadboard.

▲ Figure 1.2.1 Experimental circuit

▲ 图1.2.1 实验电路

3. Experimental results

After the input voltage exceeds 8V, the output begins to oscillate. The following is the measured oscillation signal after the input is 11V.

▲ Figure 1.3.1 Output waveform of Vout

▲ 图1.3.1 Vout的输出波形

After the circuit oscillated, the LM7805 started to heat up badly. Later, when the input voltage was too high, the 7805 burned out.

4. Capacitor oscillator

The 7805 based oscillator described above is basically the same as the Colpits oscillator.

▲ Figure 1.4.1 Colpits capacitor oscillator

▲ 图1.4.1 Colpits 电容振荡器

▲ Figure 1.4.2 The waveform after the Colpits oscillator is started

▲ 图1.4.2 Colpits振荡器启动后的波形

※ Summary ※

The LM7805 is originally a series regulated power supply, but it can actually be regarded as an emitter follower with a large current gain between its output and input. Taking advantage of this feature, an LC circuit can be used to form a capacitive Colpits oscillator based on it.

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Figure 1.1.1 Schematic diagram of the experimental circuit
Figure 1.2.1 Experimental circuit
Figure 1.3.1 Output waveform of Vout
Figure 1.4.1 Colpits Capacitor Oscillator
Figure 1.4.2 Waveforms after Colpits Oscillator Starts

Alternative use of the LM7805: forming a sinusoidal oscillator