When working with complex circuits, it is useful to use various technical tricks that allow you to achieve your goal with little effort. One of them is the creation of transistor switches. What are they? Why should they be created? Why are they also called "electronic keys"? What are the features of this process and what should I pay attention to?
What are transistor switches made of
They are made using field-effect or bipolar transistors. The former are further divided into MIS and keys that have a control p–n junction. Among bipolar ones, non-saturated ones are distinguished. A 12 Volt transistor key will be able to satisfy the basic needs of a radio amateur.
Static mode of operation
It analyzes the private and public state of the key. The first input contains a low voltage level, which indicates a logic zero signal. In this mode, both transitions are in the opposite direction (a cutoff is obtained). And only thermal can affect the collector current. In the open state, at the input of the key there is a high voltage level corresponding to the logical one signal. It is possible to work in two modessimultaneously. Such performance may be in the saturation region or the linear region of the output characteristic. We will dwell on them in more detail.
Key Saturation
In such cases, the transistor junctions are forward biased. Therefore, if the base current changes, then the collector value will not change. In silicon transistors, approximately 0.8 V is needed to obtain a bias, while for germanium transistors, the voltage fluctuates within 0.2-0.4 V. How is key saturation achieved in general? This increases the base current. But everything has its limits, as does increasing saturation. So, when a certain current value is reached, it stops increasing. And why carry out key saturation? There is a special coefficient that displays the state of affairs. With its increase, the load capacity that transistor switches have increases, destabilizing factors begin to influence with less force, but performance deteriorates. Therefore, the value of the saturation coefficient is chosen from compromise considerations, focusing on the task that will need to be performed.
Disadvantages of an unsaturated key
And what happens if the optimal value has not been reached? Then there will be such disadvantages:
- The voltage of the public key will drop and lose to about 0.5 V.
- Noise immunity will deteriorate. This is due to the increased input resistance that is observed in the keys when they are in the open state. Therefore, interference such as power surges will also lead tochanging the parameters of transistors.
- Saturated key has significant temperature stability.
As you can see, this process is still better to carry out in order to ultimately get a more perfect device.
Performance
This parameter depends on the maximum allowable frequency when the signals can be switched. This, in turn, depends on the duration of the transient, which is determined by the inertia of the transistor, as well as the influence of parasitic parameters. To characterize the speed of a logic element, the average time that occurs when a signal is delayed when it is transmitted to a transistor switch is often indicated. The diagram that displays it usually shows just such an average response range.
Interaction with other keys
Connection elements are used for this. So, if the first key at the output has a high voltage level, then the second one opens at the input and works in the specified mode. And vice versa. Such a communication circuit significantly affects the transient processes that occur during switching and the speed of the keys. This is how a transistor switch works. The most common are circuits in which the interaction takes place only between two transistors. But this does not mean at all that this cannot be done by a device in which three, four or even more elements will be used. But in practice, it is difficult to find an application for this,therefore, the operation of a transistor switch of this type is not used.
What to choose
What is better to work with? Let's imagine that we have a simple transistor switch, the supply voltage of which is 0.5 V. Then, using an oscilloscope, it will be possible to capture all changes. If the collector current is set to 0.5mA, then the voltage will drop by 40mV (the base will be about 0.8V). By the standards of the task, we can say that this is a rather significant deviation, which imposes a restriction on the use in a number of circuits, for example, in analog signal switches. Therefore, they use special field-effect transistors, where there is a control p–n junction. Their advantages over their bipolar cousins are:
- Small amount of residual voltage on the key in the state of wiring.
- High resistance and, as a result, a small current that flows through a closed element.
- Low power consumption, so no significant control voltage needed.
- It is possible to switch low-level electrical signals that are units of microvolts.
The transistorized relay key is the ideal application for the field. Of course, this message is posted here solely so that readers have an idea of their application. A little knowledge and ingenuity - and the possibilities of implementations in which there are transistor switches, a great many will be invented.
Work example
Let's take a closer look,how a simple transistor switch works. The switched signal is transmitted from one input and removed from another output. To lock the key, a voltage is applied to the gate of the transistor, which exceeds the values of the source and drain by a value greater than 2-3 V. But in this case, care should be taken not to go beyond the permissible range. When the key is closed, its resistance is relatively large - more than 10 ohms. This value is obtained due to the fact that the reverse bias current of the p-n junction additionally affects. In the same state, the capacitance between the switched signal circuit and the control electrode fluctuates in the range of 3-30 pF. Now let's open the transistor switch. The circuit and practice will show that then the voltage of the control electrode will approach zero, and is highly dependent on the load resistance and the switched voltage characteristic. This is due to the whole system of interactions of the gate, drain and source of the transistor. This creates some problems for interrupter mode operation.
As a solution to this problem, various circuits have been developed that stabilize the voltage that flows between the channel and the gate. Moreover, due to the physical properties, even a diode can be used in this capacity. To do this, it should be included in the forward direction of the blocking voltage. If the necessary situation is created, the diode will close, and the p-n junction will open. So that when the switched voltage changes, it remains open, and the resistance of its channel does not change, between the source and the input of the key, you canturn on the high-resistance resistor. And the presence of a capacitor will significantly speed up the process of recharging the tanks.
Transistor key calculation
For understanding, I give an example of calculation, you can substitute your data:
1) Collector-emitter - 45 V. Total power dissipation - 500 mw. Collector-emitter - 0.2 V. Limiting frequency of operation - 100 MHz. Base-emitter - 0.9 V. Collector current - 100 mA. Statistical current transfer ratio – 200.
2) 60mA resistor: 5-1, 35-0, 2=3, 45.
3) Collector resistance rating: 3.45\0.06=57.5 ohm.
4) For convenience, we take the value of 62 Ohm: 3, 45\62=0, 0556 mA.
5) We consider the base current: 56\200=0.28 mA (0.00028 A).
6) How much will be on the base resistor: 5 - 0, 9=4, 1V.
7) Determine the resistance of the base resistor: 4, 1 / 0, 00028 \u003d 14, 642, 9 Ohm.
Conclusion
And finally, about the name "electronic keys". The fact is that the state changes under the influence of current. And what does he represent? That's right, the totality of electronic charges. This is where the second name comes from. That's all. As you can see, the principle of operation and the arrangement of transistor switches are not something complicated, so understanding this is a feasible task. It should be noted that even the author of this article needed to use some reference literature to refresh his own memory. Therefore, if you have questions about terminology, I suggest recalling the availability of technical dictionaries and searching for a newinformation about transistor switches is there.
