In the embedded circuit, IO port is often used to control the switching function of some circuits. At this time, the triode can be used as a switching device. When it is used as a switching device, it needs to use switch triode, such as 9014 and 9015, and the transistor is in saturation state. An example is given to illustrate the characteristics of this kind of circuit
In order to simulate the circuit diagram is not very complete, the circuit is a crystal oscillator off function circuit, where VO is connected to MCU crystal oscillator input terminal such as (Xin).
If Q1 and Q3 bases are low at the same time, Q2 turns on and VO is 0, which causes the crystal oscillator to stop oscillation and shut down the processor. We analyze R3 and R4 (actual circuit 470K) to make Q2 and Q3 in saturated state; Q3 is Q1 collector load, and Q1 can be controlled in saturated state or amplification state when R5 resistance is adjusted. In order to make Q2 base conduction, Q1 must provide enough current to meet the conditions, only Q1 is in the amplified state;
When rk = 1005, the simulation diagram is as follows:
When R5 = 470K, the simulation diagram is as follows:
Through the above analysis, it can be concluded that only when the current is large enough, Q2 can be turned on and the crystal oscillator can be turned off. Power devices, in the design of embedded circuits, rarely use power amplifier circuits. Yesterday, I read through the contents of transistor in college analog electronics textbook. Although I thought that I had learned well at that time, I found out after Rereading that I only learned by rote and did not really understand.
The static operating point not only determines the distortion, but also affects the dynamic parameters such as voltage magnification and input resistance. However, in the actual circuit, the static working point compensation is stable due to the change of the ambient temperature, which makes the dynamic parameters unstable, and more seriously, it may cause the circuit not to work normally; among all the environmental factors, the temperature has the greatest impact on the dynamic parameters.
When the temperature increases, the transistor magnification increases and ice increases significantly. Taking the common emitter circuit as an example, when the temperature rises, the q-point moves to the saturation region; when the temperature decreases, the q-point moves to the cut-off region.
The figure below shows a typical static operating point circuit
Figure AB has the same equivalent DC circuit. In order to stabilize the Q working point, it is usually necessary to satisfy I1 > > IBQ so that vbq = RB1 * VCC / Rb2 + RB1. By this design, vbq will remain basically unchanged no matter how the ambient temperature changes. When the temperature rises, ice becomes larger, and VEQ becomes larger. Because VBE = vbq – VEQ, VBE will be smaller; because VBE is smaller, IBE will also be smaller, so ice will be smaller. In general, the stronger the feedback is, the more stable the q-point is.
Other stable q-point circuits
The above is a temperature compensation circuit using diode directional and forward characteristics. For figure a, because IRB = ID + IBE, ice and ID increase when the temperature rises (directional current increases with temperature), which will make IBE decrease and ice decrease.