In modern electronics, tunnel diodes have been superseded by components that are more convenient for solving the same problems. But why not experiment with an active element that was once considered one of the fastest?
Tunnel diodes are divided into intended for amplifiers, pulse generators and key circuits. According to datasheet
, the 3I306 series diodes are designed for use in switching devices. The graph shows the dependence of the voltage drop across the diode from the current through it in the straight section of the IVC:
The author’s character is improvised; it consists of a signal generator, a 10-ohm resistor and an oscilloscope. An error occurs: one channel of the oscilloscope measures the total voltage across the entire series circuit from the diode and the resistor, and the other one only on the resistor (the current can be indirectly determined from the second of these voltages). You can calculate the voltage drop only on the diode by exporting the curves to a CSV file, and then generating graphics in Python with matplotlib.
Example of IVC tunnel diode on the oscilloscope screen:
Initially, the current through the diode rises to about 11 mA until the voltage rises to 150 mV, then drops sharply to 500 μA and rises again. This is the area of negative differential resistance where the current drops with increasing voltage.
To study the operation of the diode in the switching device, the author connected it to two BNC connectors. Their cases are connected together, and a diode is connected between the central contacts. The signal from the generator with an output impedance of 50 ohms goes through a diode to an oscilloscope with the same input impedance:
The behavior of the diode does not depend on the waveform. When the voltage exceeds the threshold, switching occurs. The author gave a triangular signal with a frequency of about 100 kHz. The fall in current occurs in 900 picoseconds, and the increase in 1.1 nanoseconds. Impressive, especially when you consider that the circuit consists of one part, not counting the signal generator. The rectangular pulse generator on timer 555 switches about 100 nanoseconds.
But the output swing is small, because tunnel diodes operate at low voltages and currents.
Next, the author tries to apply a switching diode to the wrong purpose - in the generator. Here he will maintain a continuous oscillation in the circuit:
The oscillatory circuit initially consisted of one coil with a diameter of 9 mm and a 2 pF capacitor. A 10 nF capacitor closes the generated oscillations to itself, not passing them into the power circuit. The supply voltage is 700 mV, after starting the generator continues to operate when the voltage drops to 330 mV.
First, the generator operated at a frequency of 295 MHz. When replacing a capacitor in a circuit with another one, with capacitance in pF, the frequency increased only to 300 MHz, which means that the diode's own capacity further lowered the frequency.Having calculated the inductance of the coil, the author further calculated the diode's own capacitance - 18 pF. The datasheet says that it does not exceed 30 pF, and this turned out to be the case.
When observing vibrations, it is important not to add extra capacity to the circuit. The 10-fold oscilloscope probe has a capacitance of 10 pF, which is enough to further reduce the frequency. Therefore, the author has closed the oscilloscope entrance to the case, having received one more turn - the measuring one. Bringing it to the loop circuit, you can get a transformer without a core. The amplitude of the oscillations is not recognized, but you can see how it depends on the supply voltage.
To increase the generation frequency, the author shortened the diode leads and connected a capacitor with an axial pin arrangement directly to them. The coil is no longer needed, the inductance provide the outputs of the components. After the supply voltage of 700 mV was applied to the circuit, the generation started at a frequency of 581 MHz. How else to increase it? Take volume resonator?
Probably, it was not easy for designers to work with tunnel diodes: the rule “build an amplifier - it turns out a generator” here was hard to follow. Therefore, the author has not yet tried to make an amplifier on such a diode.
The author shot the output signal in the same way, and although it looks like perfectly sinusoidal, it may be distorted, just at a frequency of 581 MHz for a 1 GHz oscilloscope to detect distortion is not enough resolution. Just as in the previous case, it is impossible to measure the amplitude accurately, which means that we cannot compare this generator with the previous one.
Tunnel diodes are very “gentle”: one of them failed the author when removing the IVC due to too much amplitude of the signal from the generator, the other from overheating when soldering. With the remaining eight, the author treated much more delicately. It is necessary to solder the diode at a temperature of no more than 260 ° C for no longer than 3 seconds and with a heat sink. The author recommended for such purposes copper tweezers with a thickness of 2 mm, the author does not have, but an aluminum clip, originally acquired for soldering germanium components, came up:
Diodes are also afraid of static, moreover, "testing diodes with a tester is not allowed." The author survived the diode after such an experience, but during the test did not ring in one direction. You need to determine the polarity by the illustration in the datasheet.
If you are going to experiment with tunnel diodes, get them just in case with a margin, but follow these simple rules and start right away. And then you will not lose any.