Circuit module analysis exercise 6 (switch)

k1,k2 It's a two button tap switch , Press down to lock , Press again to restore .

In the circuit ,k1 Press after ,k1 Signal is grounded

k2 Press after ,tool0 Signal is grounded

Both resistors are current limiting ,c21 It is the function of filtering .

After the switch is disconnected and restored ,k1 and tool0 It's all high （vcc）

Switch circuit ：

One 、 Basic switching circuits ： Basic switch circuit is an important circuit with wide application , It mainly includes digital switch circuit 、 Analog switch circuit and mechanical switch circuit .

（1） Digital switch circuit ： Mainly by transistors or MOS Tube composition , This switching circuit is widely used in switching power supply 、 Motor drive 、LED Drive and relay drive applications , Is one of the most commonly used switching circuits .

（2） Analog switch circuit ： Because the anti-interference ability of analog signal is not as good as that of digital signal , The base current of a transistor, for example, will cause great interference to the analog signal , So... Is often used MOS Tubes, not transistors . This analog switch circuit is widely used in high frequency antenna switch 、 Sensor analog switch 、 Audio and video analog switch and other analog signal switching applications , This is also a very common switching circuit .

（3） Mechanical switch circuit ： Such as single pole double pole switch 、 Relay switch and so on all belong to the category of mechanical switch . The obvious disadvantage of mechanical switch is that the switching frequency is very low , The switching device is large in size , And the life span is short ; The advantage of mechanical switch is that the switching loss is very small , The isolation is very good , And it can realize the function of keeping power off . The principle of mechanical switch circuit is very simple , No more details here .

Two 、 Transistor digital switch circuit ：

2.1、 Basic composition ： It is a kind of counting connection - Disconnect the collector of the transistor - A circuit in which the current between the emitters is used as a switch , At this time, the transistor works in the cut-off region and the saturation region . Transistor digital switch circuit mainly includes emitter ground switch circuit and emitter follower switch circuit , The emitter grounding switch circuit can be changed into open set output by removing the collector resistance (OC) circuit ; Similarly, the emitter follower type switching circuit can be changed into a push-pull output circuit .

When it is necessary to output a large load current , Due to collector current ( Load current ) It comes from amplifying the base current , Therefore, it must be possible to provide more than from the input 1/Hfe The base current , This is powerless for the base drive circuit under the condition of large load current at the output end . Therefore, Darlington tube can be used to solve this problem , Darlington tube is a kind of composite tube , Its DC amplification factor Hfe A very large . As shown in the figure below , When using Darlington tube connection , The emitter current of the front tube becomes the base current of the back tube , So the total DC amplification factor Hfe Is the DC amplification factor of each transistor Hfe Product of (Hfe1 * Hfe2). This requires only a small base current , A larger load current can be obtained , To drive high-power load equipment . But here's the thing , The starting voltage of the Darlington tube is generally 1.2~1.4V, For two Vbe.

2.2、 Performance parameters of transistor switching circuit ：

(1)、 Switching frequency ： Adopt emitter follower switch circuit , It can obviously increase the switching frequency , Realize high-speed switching .

(2)、 Isolation ： It indicates how many input signals leak to the output when the switch is cut off . When the input signal frequency is low , There will be few leaks ; But when the input signal frequency increases , The input signal will pass through the collector of the transistor - The inter electrode capacitance between emitters leaks to the output , This is because the isolation parameter becomes very important .

(3)、 On resistance ： Indicates the collector of the transistor when the switch is on - Equivalent resistance between emitters . The on resistance is too high , The signal and power losses are also very large , Therefore, the smaller the on resistance, the better .

2.3、 Application of transistor switching circuit ：

(1)、 Logic level conversion circuit .

(2)、 Relay drive circuit .

(3)、LED Display driver circuit .

(4)、 Optocoupler transmission circuit .

3、 ... and 、MOS Tube digital switch circuit ：

3.1、 Basic composition ： It is a kind of counting connection - To break off MOS The leakage pole of the tube - A circuit in which the current between the sources is used as a switch , At this time MOS The tube works in the cut-off zone and the constant current zone .MOS Tube digital switch circuit mainly includes source ground switch circuit and source follower switch circuit , The source grounding switch circuit can be changed into open drain output by removing the drain resistance (OD) circuit ; Similarly, the source follower type switching circuit can be changed into a push-pull output circuit .

Link to the original text ：https://blog.csdn.net/weixin_40877615/article/details/94900293