The purpose of this article is to pay tribute to one of the most popular transistors of the 70s - 90s - KT315. Availability, small size and fairly good parameters allowed radio amateurs to use the KT315 transistor in various schemes, from simple to microcomputers. The tables below indicate the main parameters of the KT315 line.
Limit parameters of KT315 transistors at T=25°C
| I K, max mA | U KER max (U KE0 max), V | U EB0 max , V | P K max , (P max), mW | T, °C | Tp max, °C | Tmax, °C | |
| 100 | 25 | 6 | 150 | 25 | 120 | 100 | |
| 100 | 20 | 6 | 150 | 25 | 120 | 100 | |
| 100 | 40 | 6 | 150 | 25 | 120 | 100 | |
| 100 | 35 | 6 | 150 | 25 | 120 | 100 | |
| 100 | 40 | 6 | 150 | 25 | 120 | 100 | |
| 100 | 35 | 6 | 150 | 25 | 120 | 100 | |
| 50 | 15 | 6 | 100 | 25 | 120 | 100 | |
| 50 | 60 | 6 | 100 | 25 | 120 | 100 |
Parameters of KT315 transistors at T=25°C
| h 21E (h 21E) | U KB (U KE), V | I E (I K), mA | U CE us, V | I KB0 , (I KER), µA | f gr (f h21), MHz | C K, pF | |
| 20...90 | (10) | 1 | 0,4 | 1 | 250 | 7 | |
| 50...350 | (10) | 1 | 0,4 | 1 | 250 | 7 | |
| 20...90 | (10) | 1 | 0,4 | 1 | 250 | 7 | |
| 50...350 | (10) | 1 | 0,4 | 1 | 250 | 7 | |
| 20...90 | (10) | (1) | 1 | 1 | 250 | 7 | |
| 50...350 | (10) | (1) | 1 | 1 | 250 | 7 | |
| 30...250 | (10) | (1) | 0,5 | 1 | 150 | 10 | |
| 30 | (10) | (1) | 1 | 250 | 7 |
A little background: - the first planar-epitaxial transistor of the late 60s, that is, when during the manufacturing process the emitter, collector and base were manufactured sequentially on one silicon wafer. 
To do this, it is necessary to dope a silicon wafer doped into type n (collector) to a certain depth into type p (base), and then dope it again to a lesser depth into type n (emitter). Next, using a scriber, the plate must be cut into parts, and each part must be packaged in a plastic case.
This manufacturing process was much cheaper than alloy technology, and made it possible to obtain previously unimaginable transistor parameters (in particular, an operating frequency of up to 300 MHz).
And of course, mounting the crystal not in a metal case, but on a metal strip with leads led to cheaper production - a crystal, on the bottom side of which the collector was soldered to the central terminal, and the base and emitter were connected with a welded wire, filled with plastic, excess parts of the tape were cut off - and KT315 turned out like this.
Let's give a couple of examples of circuits using the KT315 transistor.
1. Headphone amplifier.
While the loop is intact, the base of the transistor is connected to ground and the transistor is closed. When entering a protected area, the attacker breaks the wire, a positive bias is applied to the base of the transistor and the transistor opens, which ultimately leads to the activation of the electromagnetic relay. The relay contact circuit may contain a siren, radio transmitter, or something else.
3. ULF output power indicator.
C1, C2 - 10 uF x 16V
D11 - KD510A
Rx - 300 Ohm - 100 Kom (must be selected for each stage.)
D1 - D10 - LEDs of different colors.
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A headphone amplifier is a very justified solution, as evidenced by many publications on this site. Moreover, their simplicity is a good guide for beginners. This design uses a solution that has been known for more than 30 years. And, if it is often remembered, then this indicates how successful it is.
Amplifier for low impedance ears.
The idea used in this amplifier is not new. Probably many remember the book by V.A. Vasiliev “Foreign amateur radio designs"(it was published twice in sometime in the late 70s and early 80s). There, at one time, a very simple, not very powerful UMZCH cash register “A” was published. Many who repeated it were very pleased with the results. Once, having reduced all currents and voltages, I used it for headphone output in one of my designs, published in the magazine “Radio” (“UMZCH with a non-polar power source” (“Radio”, No. 6, 1999, p. .16).
This is what happened:
Since there were already two options for a laptop (they are awaiting publication in the journal “Radio”), I only tested the circuit from a nine-volt battery similar to the “Krona” that was stuck in a radio microphone (they are at my work). Everything was assembled on a breadboard (raw foil Gitenax is not found in Israel).
Don’t blame me for being trashy, I brought with me from Soyuz a lot of KT315B transistors (I once disassembled into parts one instrument called “FAEMI-M”). It was important to test the idea. And you know - it worked, and quite well, both with high-impedance ears and with low-impedance ones, for example with the good old TDS-3, which are probably many more years old than many of the citizens of this site (someone gave them to me as unnecessary).
In principle, no adjustment is needed. The quiescent current is set around 20 – 20 mA per channel. The voltage between the output transistors is automatically set to 2.4 V (initially it was calculated on power supply from the “USB” socket), but it can be changed by selecting (reducing) resistors R5 and R6. The transistors do not seem to heat up, so there is no need to think about cooling.
Everything looks like this when assembled:
Of course, I didn’t measure anything, but it’s on those same TDS-3s that I listen to the sound from the DVD in the evenings, when I watch a movie on the roof, so as not to scare the neighbors. Believe me, it sounds much better from dead batteries than from a mains power supply. And after each performance I have at least a dozen of these batteries left.
--
Thank you for your attention!
Igor Kotov, founder of Datagor magazine
P.S. I dream of finding and listening to the good old Soviet TDS-7 Amphiton, at least for a while.
KT315 is a legendary domestic transistor, copies of which are present in large quantities in every radio amateur. Not surprising - after all, this is the very first mass-produced silicon transistor; you can find it in almost any Soviet device. By the beginning of the 90s, more than 7 billion of them were manufactured. By modern standards, the KT315 is far from an ideal transistor in terms of its parameters, because new, cheaper and more advanced ones have been invented and have been produced for a long time semiconductor devices. But, nevertheless, sometimes you want to take a handful of old transistors out of the back drawer and assemble something simple with them, for example, an amplifier.
Scheme
The circuit is special in that it does not contain any other active elements except KT315 transistors. This scheme will be an excellent choice not only for antique lovers, but also for those who do not have the opportunity to get other transistors. Resistor values are not very critical and can vary within 20-30%, the same with capacitors. It is advisable to select transistors for this circuit with a high gain, in this case the maximum volume of the amplifier will increase. In this case, it is necessary to comply with the condition - both transistors of the output stage must have the same letter index. The circuit starts working with a voltage of 5 volts, the most optimal power supply is 9 volts. The current consumption is approximately 20 mA and is almost independent of the volume level. It should also be taken into account that to reproduce a stereo signal, the circuit must be repeated twice.
Amplifier assembly
(downloads: 245)
The circuit is assembled on printed circuit board dimensions 50x40 mm, which already contains both channels. First of all, using laser-iron technology we produce the board itself. Below are some photos of the process.
Once the board is ready, you can start soldering the parts. First of all, resistors are installed on the board, then capacitors with transistors. The terminals of KT315 transistors, unlike the terminals of modern parts, are thin flat strips that come off very easily from the body, so you should not apply too much force to them.
After installing the parts on the board, you need to check the adjacent tracks for short circuits and check that the transistors are installed correctly - after all, they can easily be soldered on the wrong side. The base terminal of the KT315 is on the right when looking at the front side of the transistor. Now all that remains is to connect the board with the speakers and sound source using wires, apply power and the amplifier is ready.
First launch and tests
The amplifier can work with speakers with a resistance of 4-8 Ohms, and you can also connect headphones to its output, which do not have enough power from the standard signal source. The signal source can be, for example, a telephone, player or computer. Before connecting one of the supply wires for the first time, you need to turn on the milliammeter and measure the current consumed; it should not exceed 100 mA in total for both channels. If it exceeds, then it is necessary to reduce the supply voltage. Due to its low consumption, this amplifier can even be powered from the crown. The power of the resulting amplifier is approximately 0.1 watt - not much, but quite enough for quiet listening to music indoors. Happy building!Figure 1 shows the circuit of the inverting amplifier DC, the transistor is connected according to a common emitter circuit:
Figure 1 - Circuit of the DC amplifier on the KT315B.
Let's consider the calculation of circuit elements. Let's say the circuit is powered from a source with a voltage of 5V (this could be, for example network adapter), we select the collector current Ik of transistor VT1 so that it does not exceed the maximum permissible current for the selected transistor (for KT315B the maximum collector current Ikmax = 100 mA). Let's choose Ik=5mA. To calculate the resistance of the resistor Rk, divide the supply voltage Up by the collector current:
If the resistance does not fall into the standard series of resistances, then you need to select the closest value and recalculate the collector current.
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Using the family of output current-voltage characteristics, we will construct a load line along the points Up and Ik (shown in red). On the load line, select the operating point (shown in blue) in the middle.
Figure 2 - Output current-voltage characteristics, load line and operating point
In Figure 2, the operating point does not fall on any of the available characteristics but is slightly below the characteristic for the base current Ib = 0.05 mA, so we will choose the base current a little less, for example Ib = 0.03 mA. Using the selected base current Ib and the input characteristic for a temperature of 25°C and voltage Uke = 0, we find the voltage Ube:
Figure 3 - Input characteristics of the transistor for selecting voltage Ube
For the base current Ib = 0.03 mA, we will find the voltage Ube but choose a little more since Uke>0 and the characteristic will be located to the right, for example, choose Ube = 0.8V. Next, we select the resistor current Rd1, this current should be more current base but not so large that most of the power is lost in it, we choose this current three times greater than the base current:
Using Kirchhoff's first law, we find the resistor current Rd2:
Let us designate the found currents and voltages in the diagram:
Figure 4 - Amplifier circuit with found branch currents and node voltages
Let's calculate the resistance of resistor Rd1 and select its closest value from the standard series of resistances:
Let's calculate the resistance of the resistor Rd2 and select its closest value from the standard series of resistances:
Let's designate the resistor resistances in the diagram:
Figure 5 - DC amplifier on KT315B.
Since the calculation is approximate, it may be necessary to select elements after assembling the circuit and checking the output voltage, elements Rd1 and/or Rd2 in this case must be selected so that the output voltage is close to the selected voltage Ube.
To amplify the alternating current, capacitors must be placed at the input and output to pass only the variable component of the amplified signal, since the constant component changes the operating mode of the transistor. The capacitors at the input and output should not create much resistance for the flow of alternating current. For thermal stabilization, you can place a resistor with a small resistance in the emitter circuit and a capacitor in parallel with it to weaken feedback By alternating current. The resistor in the emitter circuit, along with the divider resistors, will set the operating mode of the transistor.
The photo below shows an amplifier assembled according to the circuit in Figure 2:
There is no voltage applied to the amplifier input; a voltmeter connected to the output shows 2.6V, which is close to the selected value. If you apply a voltage of normal polarity to the input (such as in Figure 5), then the output voltage will decrease (the amplifier inverts the signal):
If you apply a voltage of reverse polarity to the input, the output voltage will increase but not more than the supply voltage:
The decrease in voltage at the input, when connected to the input of a source, is less than the increase in voltage at the output, which indicates that the input signal is being amplified with inversion. The common-emitter circuit produces greater power amplification than the common-base-common-emitter circuit, but unlike the other two, it produces signal inversion. If it is necessary to amplify DC power without inversion, then you can cascade connect two circuits in Figure 5, but it is necessary to take into account that the first stage will change the operating mode of the transistor of the second stage, so the resistance of the resistors in the second stage will need to be selected so that this change is as possible less. Also, with a cascade connection, the gain of the entire amplifier will increase (it will be equal to the product of the gain of the first stage and the gain of the second).
