When working with complex schemes, 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 features of this process are there and what should I pay attention to?
What are transistor keys made of?
They are performed using field or
bipolar transistors. The former are further divided into TIRs and keys, which have a control p – n junction. Among bipolar, non-saturated are distinguished. A transistor switch of 12 volts will be able to satisfy the basic requirements of the amateur radio.
Static mode
It analyzes the closed and open state of the key. In the first input there is 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 collector current can be affected only by thermal. In the open state, at the input of the key there is a high voltage level corresponding to the signal of a logical unit. It is possible to work in two modes at the same time. Such operation 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, the value on the collector will not change. In silicon transistors, approximately 0.8 V is required to obtain a bias, while for germanium, the voltage fluctuates within 0.2-0.4 V. But how is key saturation achieved? To do this, the base current increases. But everything has its limits, as well as an increase in saturation. So, when a certain current value is reached, it stops increasing. Why do 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 affect with less force, but performance deterioration occurs. Therefore, the value of the saturation coefficient is chosen from compromise considerations, guided by the task that will need to be performed.
The disadvantages of the unsaturated key
And what will happen if the optimum value has not been reached? Then there will be such disadvantages:
- The voltage of the public key drops will drop to about 0.5 V.
- Noise immunity will worsen. This is due to the increased input resistance, which is observed in the keys when they are in the open state. Therefore, interference such as power surges will lead to a change in the parameters of the 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 advanced device.
Performance
This parameter depends on the maximum permissible frequency when signal switching can be performed. This, in turn, depends on the duration of the transient process, which is determined by the inertia of the transistor, as well as the influence of spurious parameters. To characterize the speed of a logical element, the average time that occurs when the signal is delayed when it is transmitted to the transistor switch is often indicated. The circuit that displays it, usually just such an average response range, shows.
Interaction with other keys
For this, communication elements are used. So, if the first switch at the output has a high voltage level, then at the input of the second, it opens and works in the specified mode. And vice versa. Such a communication chain significantly affects the transients that occur during switching and key performance. This is how the transistor switch works. The most common are circuits in which the interaction occurs 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 application, 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 the oscilloscope, it will be possible to record all changes. If the collector current is set at 0.5 mA, then the voltage will drop by 40 mV (at the base it will be about 0.8 V). By the standards of the problem, we can say that this is a rather significant deviation, which imposes a restriction on the use of
analogue signals in a whole series of circuits, for example
. Therefore, they use special
field-effect transistors, where there is a control p – n junction. Their advantages over bipolar counterparts are:
- An insignificant value of the residual voltage on the key in the wiring state.
- High resistance and, as a result, a small current that flows through a closed element.
- Low power is consumed, so a significant source of control voltage is not needed.
- You can switch low-level electrical signals, which are units of microvolts.
A transistor relay key is an ideal field application. Of course, this post is posted solely so that readers have an idea of their application. A lot of knowledge and ingenuity - and the possibilities of implementations in which there are transistor switches, will be invented by a great many.
Work example
Let's take a closer look at 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 supply is used to the transistor gate, which exceeds the source and drain values by an amount greater than 2-3 V. But at the same time, care must be taken and not to go beyond the permissible range. When the key is closed, its resistance is relatively large - exceeds 10 ohms. This value is obtained due to the fact that the reverse bias current pn of the junction also influences. In the same state, the capacitance between the switching signal circuit and the control electrode fluctuates in the range of 3-30 pF. Now open the transistor switch. The circuit and practice will show that then the voltage of the control electrode will approach zero, and depends heavily on the load resistance and the switched voltage characteristics. This is due to a whole system of interactions of the gate, drain and source of the transistor. This creates certain problems for operating in breaker mode.
As a solution to this problem, various schemes have been developed that provide voltage stabilization that flows between the channel and the gate. Moreover, due to the physical properties, even a diode can be used as such. 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 pn junction will open. So that when the switched voltage changes, it remains open, and the resistance of its channel does not change, a high-resistance resistor can be turned on between the source and the input of the switch. And the presence of a capacitor will significantly accelerate the process of recharging containers.
Transistor Key Calculation
For understanding, I give an example of calculation, you can substitute your data:
1) The collector-emitter is 45 V. The total power dissipation is 500 mw. The collector-emitter is 0.2 V. The cutoff frequency is 100 MHz. The emitter base is 0.9 V. The collector current is 100 mA. The statistical current transfer coefficient is 200.
2) Resistor for 60 mA current: 5-1.35-0.2 = 3.45.
3) The collector resistance rating: 3.45 \ 0.06 = 57.5 Ohms.
4) For convenience, we take a nominal value of 62 Ohms: 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) We determine the resistance of the base resistor : 4.1 \ 0.00028 = 14.642.9 Ohms.
Conclusion
And finally, about the name "electronic keys". The fact is that the state changes under the influence of current. And what is he like? True, a collection of electronic charges. From this comes the second name. That's all. As you can see, the principle of operation and the circuit diagram of the device transistor switches is not something complicated, so to understand this is feasible. It should be noted that even the author of this article needed a little reference literature to refresh his own memory. Therefore, if you have questions about terminology, I suggest that you recall the availability of technical dictionaries and search for new information about transistor switches there.