Transistor keys. Scheme, working principle

When working with complex schemes it is useful to use various technical tricks, which allow achieving the set goal by small efforts. One of them is the creation of transistor keys. 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 be paid attention to?

What are transistor wrenches made of?

They are performed using field or bipolar transistors. The former are further divided into MDPs and keys that have a control pn-junction. Among bipolar, non-saturated ones are distinguished. Transistor wrench 12 Volt can satisfy basic requests from the radio amateur.

Static mode of operation

It analyzes the closed and open state of the key. In the first one, there is a low voltage level at the input, which indicates a logical zero signal. In this mode, both transitions are in the opposite direction (cutoff is obtained). And the collector current can only be affected by the thermal current. In the open state, a high voltage level corresponds to the signal of the logical unit at the input of the key. It is possible to work in two modes simultaneously. Such a function can be in the saturation region or the linear region of the output characteristic. We will dwell on them in more detail.

Saturation of the key

In such cases, transistor transitions are shifted in the forward direction. Therefore, if the base current changes, the value on the collector does not change. In silicon transistors, approximately 0.8 V is needed to obtain a bias, whereas for germanium ones, the voltage fluctuates within 0.2-0.4 V. And how is the key saturation achieved in general? 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 saturate the key? There is a special coefficient that reflects the state of affairs. With its increase, the load capacity that transistor keys have, the destabilizing factors begin to influence with less force, but the performance degrades. Therefore, the value of the saturation coefficient is chosen from compromise considerations, guided by the task that will need to be performed.

Disadvantages of an unsaturated key

And what will happen if the optimal value has not been reached? Then there will be such shortcomings:

1. The voltage of the public key drops to about 0.5 V.
2. The noise immunity will worsen. This is due to the increased input impedance that is observed in the keys when they are open. Therefore, interference such as voltage surges will also lead to a change in the parameters of the transistors.
3. The saturated key has significant temperature stability.

As you can see, this process is still better to conduct, in the end, to get a more perfect device.

Speed

This parameter depends on the maximum allowable 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 parasitic parameters. To measure the speed of a logical element, the mean time that occurs when the signal is delayed, when it is transmitted to a transistor key, is often indicated. The scheme that displays it, usually just such an average response range and shows.

Interaction with other keys

For this, the communication elements are used. So, if the first key on the output has a high voltage level, then the opening of the second one opens and operates in the preset mode. And vice versa. Such a chain of communication significantly affects the transients that arise during the switching and speed of the keys. Here's how the transistor key works. The most common are the circuits in which the interaction takes place only between two transistors. But this does not mean that it can not be made a device in which three, four or even more elements will be applied. But in practice it is difficult to find such an application, so the operation of a transistor key of this type is not used.

What to choose

With what it is better to work? Let's imagine that we have a simple transistor switch with a supply voltage of 0.5 V. Then with the oscilloscope you can record all the changes. If the collector current is set to 0.5 mA, the voltage drops by 40 mV (the base will be approximately 0.8 V). By the standards of the problem, it can be said that this is a rather significant deviation, which imposes a restriction on the use in whole series of circuits, for example, in analog signal switches . Therefore, they use special field-effect transistors, where there is a control pn-junction. Their advantages over bipolar cousins are:

1. Insignificant value of the residual voltage on the key in the wiring state.
2. High resistance and, as a result, a small current that flows through the closed element.
3. Low power is consumed, so a significant source of control voltage is not needed.
4. You can switch low-level electrical signals, which are a few microvolts.

The transistor switch of the relay is the ideal application for the field. Of course, this message is posted here solely for the readers to have an idea of their application. A bit of knowledge and savvy - and the possibilities of realizations, in which there are transistor keys, will be invented a great many.

Example of work

Let's take a closer look at how a simple transistor switch works. The switched signal is transmitted from one input and is removed from the other output. To lock the key, a voltage supply is applied to the gate of the transistor, which exceeds the source and drain values by a value greater than 2-3 V. However, care must be taken not to exceed 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 of the pn junction additionally affects. In the same state, the capacitance between the circuit of the switched signal and the control electrode varies in the range 3-30 pF. Now open the transistor wrench. The circuit and practice will show that then the voltage of the control electrode will be close to zero, and depends strongly on the load resistance and the switched voltage characteristic. This is due to the whole system of gate, drain and source interactions of the transistor. This creates certain problems for the breaker operation.

As a solution to this problem, various schemes have been developed that ensure the stabilization of the voltage that flows between the channel and the gate. And thanks to the physical properties, even a diode can be used in this capacity. To do this, it must be included in the direct direction of the closing voltage. If the necessary situation is created, the diode will close and the pn-junction will open. To change the switched voltage, it remains open and the resistance of its channel does not change, a high-resistance resistor can be connected between the source and the key input. And the presence of a capacitor will greatly accelerate the process of recharging the tanks.

Calculation of transistor wrench

For understanding I give an example of calculation, you can substitute your data:

1) Collector-emitter - 45 V. Total power dissipation - 500 mw. The collector-emitter is 0.2 V. The boundary frequency of operation is 100 MHz. The base-emitter - 0,9 V. Collector current - 100 mA. The statistical coefficient of current transfer is 200.

2) Resistor for current 60 mA: 5-1,35-0,2 = 3,45.

3) Collector resistance rating: 3.45 \ 0.06 = 57.5 Ohm.

4) For convenience, we take the nominal value of 62 ohms: 3.45 \ 62 = 0.0556 mA.

5) Consider the base current: 56 \ 200 = 0.28 mA (0.00028 A).

6) How many will be on the resistor of the base: 5 - 0,9 = 4,1V.

7) Determine the resistance of the base resistor: 4.1 \ 0.00028 = 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 is he like? True, the collection of electronic charges. This is the second name. That's all in all. As you can see, the principle of operation and the scheme of the device of transistor keys is not something complicated, so to understand this - it is feasible. It should be noted that even the author of this article needed a bit of reference for refreshing his own memory. Therefore, when questions arise to terminology, I suggest that you recall the availability of technical dictionaries and conduct a search for new information about transistor keys there.