Circuit design of triode based circuit design

- Aug 28, 2018-

IO ports are often used in embedded circuits to control the switching function of certain circuits. At this time, the triode can be used as a switching device.When used as a switching device, the switching triode, such as 9014 and 9015, should be used. At this time, the triode is in a saturated state.An example is given to illustrate the characteristics of this kind of circuit. 

As the simulation circuit diagram is not very complete, this circuit is the crystal oscillator closing function circuit, in which VO is connected with the MCU crystal oscillator input terminal such as (XIN). 

If the base poles of Q1 and Q3 are both low, Q2 leads and VO is 0, resulting in the shutdown of the processor.We analyze R3 and R4(actual circuit 470K) to saturate Q2 and Q3.Q3 is Q1 collector load. When adjusting the resistance value of R5, Q1 can be controlled in saturated state or amplified state.In order to make Q2 base electrode conduction, Q1 must provide sufficient current to satisfy the condition. 

When R5=100K, the simulation diagram is as follows:

Circuit design of triode based circuit design

When R5=470K, the simulation figure is as follows:

Circuit design of triode based circuit design

Based on the above analysis, it can be concluded that only when the current is large enough can the Q2 be switched off and the crystal oscillator is closed. 

Power device

In the design of embedded circuit, power amplification circuit is rarely used. Yesterday, I got a feeling after reading through the contents of the transistor in the university's moduli textbook. Although the moduli thought they were good at learning at that time, they found out after re-reading that they were just memorizing but did not really understand. 

The static working point not only determines the distortion, but also affects dynamic parameters such as voltage amplification factor and input resistance.However, in the actual circuit, the static operating point is made stable due to the change of the ambient temperature, which makes the dynamic parameters unstable, and even worse, the circuit can not work normally.Among all environmental factors, temperature has the greatest influence on dynamic parameters. 

When the temperature increases, the transistor amplification factor becomes larger and ICE becomes significantly larger.Take the common emitter circuit as an example, the Q point will move to the saturated region when the temperature increases.As the temperature decreases, Q points will move towards the cut-off area. 

The following figure is a typical static operating point circuit

Circuit design of triode based circuit design

Figure AB has the same equivalent dc circuit.In order to stabilize the Q operating point, it is usually necessary to satisfy I1>>IBQ

VBQ = Rb1 * VCC/rb2’s much-publicised + Rb1

With this design the VBQ will remain essentially the same no matter how the ambient temperature changes. 

When the temperature rises, ICE becomes larger and VEQ becomes larger. VBE= vbq-veq, so VBE becomes smaller.As VBE becomes smaller, IBE will become smaller, and ICE will become smaller. 

The use of RE makes the Q point more stable by introducing the dc negative feedback. Generally speaking, the stronger the feedback is, the more stable the Q point is. 

Other stable Q operating point circuits

Circuit design of triode based circuit design

The above is the temperature compensation circuit using the directional and forward characteristics of the diode. 

For figure A, since IRB=ID+IBE, ICE and ID become larger when the temperature rises (directional current increases with the temperature rise), which will cause IBE to decrease and ICE to decrease.