01 7805 Application Circuit
1. Introduction
7805 is a series three-terminal regulator, the three ports are the voltage input terminal (IN), the ground wire (GND), and the voltage regulator output (OUT). When the voltage between IN-GND exceeds 7V, the voltage between the output terminal and GND will stabilize at 5V. The following is the most common application circuit of 7805.
▲ 图1.1.1 7805典型稳压工作电路
If a bias voltage V off V_{off} is added between the GND terminal of the 7805 and the circuit groundINoff, then the output of the circuit will also become V off + 5 V_{off} + 5INoff+5 . If the bias voltage is regarded as an input change signal, there is also a signal component in the same direction as the input signal in the output of 7805.
▲ 图1.1.2 7805输出电压调整
Second, follow the amplifier
Using the constant difference between the GND and OUT of the 7805 to follow the characteristics, it can form a follower amplifier. The input signal has the GND input of 7805, and its output signal includes the input signal. Since the 7805 has a strong current output capability, this follower amplifier has a power amplification function.
▲ 图1.2.1 基于7805 构成的功率放大器
Below are the input and output signal waveforms of the 7805 follower amplifier as observed by an oscilloscope. Due to the function of the 7805 internal circuit, the frequency bandwidth of this amplifier circuit is relatively low, only a few hundred hertz.
▲ 图1.2.2 7805功率放大器输入（蓝色）输出信号（青色）
3. Capacitive three-point oscillator
Since the 7805 has a large current amplifying ability from GND to OUT, it can be used to form a Colpits capacitor three-point oscillator. The figure below shows the oscillator circuit diagram. This circuit is composed of L1, C3, C2 resonant tank, using the current amplification capability of 7805 to continuously supplement inductance in C3, so as to maintain the LC loop oscillation.
▲ 图1.3.1 基于7805 的 Colpits 电容三点式振荡器
The following is a sinusoidal oscillation waveform displayed at the output of the measurement 7805, the oscillation frequency is 18.99kHz. Calculate the resonant frequency of the L1, C3, C2 loops as follows, it can be seen that this frequency is very close to the frequency of the measured oscillator.
▲ 图1.3.2 基于7805 的 Colpits振荡器输出波形
The signal output by this oscillator is also very strange, that is, it has a double-cycle characteristic, that is, its waveform is different in two adjacent oscillation cycles, which is a typical feature of chaotic circuits.
4. Summary
Of course, the above two circuits have no special use value. Through these two special usages, we can have a deeper understanding of the 7805 integrated chip.
● Links to related charts:
- Figure 1.1.1 7805 typical voltage regulator working circuit
- Figure 1.1.2 7805 output voltage adjustment
- Figure 1.2.1 Power amplifier based on 7805
- Figure 1.2.2 7805 power amplifier input (blue) output signal (cyan)
- Figure 1.3.1 Colpits Capacitor Three-Point Oscillator Based on 7805
- Figure 1.3.2 Colpits oscillator output waveform based on 7805