The article examines the diverse applications of tunnel diodes, emphasizing their unique negative resistance property. It explores their role in high-frequency oscillators for signal generation and control, as well as their utility in high-speed switching circuits.
When the tunneling principle of a highly doped P–N junction was first discovered, the tunnel diode was recognized as an important high-speed switching device. State switching could essentially take place at the speed of light. The response time of the switching action is primarily limited by diode capacitance. As a rule, this is in the order of 1−10 pF. This means that switching can occur from the zero point to the peak point with a very short rise time.
The negative resistance characteristic of a tunnel diode has also opened the door for a number of other applications. High-frequency signal generation or oscillation can now be achieved by using this characteristic. The negative resistance characteristic also permits the tunnel diode to be used as a high-frequency amplifying device. High-frequency signal control is an application of the tunnel diode.
High-Frequency Oscillator
The negative resistance characteristic of a tunnel diode can be used to a unique advantage when it is connected across an LC tank circuit. An inductor and capacitor connected in parallel form an LC tank circuit. Figure 1 shows a tank circuit connected to a DC source. When the switch of the circuit is closed momentarily, it causes the tank circuit to be “shocked” into oscillation. The resulting damped oscillatory waveform is shown in Figure 1. Note that the amplitude of the wave decreases with each succeeding wave. This is caused by the effective resistance of the tank circuit. The resistance essentially dissipates the power so that it causes the waves to die out after a few cycles of operation. If the tank circuit had only pure inductance and capacitance, the oscillations would be continuous after the initial voltage was applied. In practice, the circuit will always have some resistance. This means that the resulting oscillations will die out after a few cycles of operation.
Figure 1. LC tank circuit and damped oscillatory waveform.
The negative resistance effect of a tunnel diode can be used to cancel or overcome the effective resistance of a tank circuit. This permits the tank circuit to behave as if it has no resistance. As a result, when the tank is shock-excited with voltage, oscillations will occur without a change in amplitude. The frequency of the oscillating wave is determined by the capacitance and inductance of the tank circuit. This oscillator is very simple in operational theory and construction.
To use a tunnel diode for the tank circuit of an oscillator, the diode must be forward biased. The amount of forward-bias voltage needed to reach the negative resistance area is very critical. Ideally, this operating point is located between the peak and valley voltage points. This part of the characteristic curve is in the order of 60−350 mV. A representative operating voltage would be somewhere near the center of this voltage range or approximately 145 mV.
Figure 2 shows a tunnel diode oscillator with a variable capacitor that permits frequency changes. Tunnel diode biasing is achieved by a voltage-divider network consisting of resistors R1 and R2. The generated output signal developed by the circuit appears across the tank. An oscillator of this type has good stability and can generate signals in the microwave range. It is, however, rather sensitive to changes in temperature and bias voltage.
Figure 2. A tunnel Diode Oscillator
Switching
A very common application of the tunnel diode is in high-speed switching circuits. A tunnel diode can perform logic functions and memory. The tunnel diode offers the following advantages:
- Small size
- Low operating power
- High speed
- Low cost
- High reliability
It is possible to form a simple two-state, or bistable, switching circuit by connecting a tunnel diode in series with a voltage source and a single resistor. Figure 3 shows this type of circuit and an I–V curve for the diode. To see how the circuit responds as a switch, the operating range of the circuit must be plotted. A solid line extending across the I–V curve shows the range of the anticipated operation. If a source voltage of 0.5 V is applied to a 100 Ω resistor, it will cause a forward current of 5 mA. Note the location of these two points (source voltage and forward current) on the solid line of the I–V curve which represent the end points of the line. Note also that the operating line crosses the I–V curve at three places: point A, point B, and point C. Point A occurs near the peak current point or IP. Point B occurs in the negative resistance region of the curve. This is considered to be an unstable operating point in a switching circuit. Point C is located slightly above the valley voltage point, VV. Points A and C are considered to be the stable operating points of the switching circuit.
Figure 3. Bistable switching circuit and operating characteristics.
To see how a tunnel diode is used as a bistable switch, we must consider some specific operating conditions for the circuit in Figure 3. Point A is located at an IF of 4.75 mA and a VF of 25 mV. Point C is located at an IF of 1.0 mA and a VF of 400 mV. The circuit can be switched to either of these positions, and it will remain there until a state change occurs. The operational state of the circuit is determined by the value of the source voltage. Before the circuit is energized, both IF and VF are zero. When the circuit is energized by the 500-mV source, its operation will change to point A. For the circuit to operate at point C, the source voltage must be increased to 550 mV. This causes the operation of the circuit to shift to the dotted line where it intersects the I–V curve at stable point D. The circuit will remain at this operating point as long as the source remains at 550 mV. However, reducing the source to a value of 500 mV causes the circuit to remain in its stable condition by moving to point C on the curve. To change the state of operation back to that of point A, the source voltage would have to drop in value below 25 mV. When the source voltage is changed back to 500 mV, the circuit will remain in its stable state at point A. Thus, the circuit has two stable states of operation depending on the value of the source voltage. This type of circuit is called a bistable voltage switch.
Tunnel Diode Applications Key Takeaways
Tunnel diodes, with their unique negative resistance properties, have a wide range of applications, particularly in high-frequency oscillators and high-speed switching circuits. Their ability to generate stable high-frequency signals and perform fast switching operations at low power consumption and high reliability makes them invaluable in modern electronics. Tunnel diodes enable efficient signal control and amplification, contributing to advancements in microwave technology and high-speed logic circuits. Furthermore, their use in bistable switching circuits allows for fast, reliable memory and logic functions in compact, cost-effective designs. These characteristics make tunnel diodes essential components in various fields, including telecommunications, computing, and signal processing, where speed and efficiency are critical.
Tunnel Diode Applications Review Questions
1. When a tunnel diode is used as a switch, it has _____stabilized conditions of operation.
2. When a tunnel diode is used as an oscillator, it is biased to operate in the _____ region.
3. A(n) _____ will conduct in either the forward or reverse direction when properly biased.
4. A tunnel diode generally operates in the (forward, reverse) direction for most of its applications.
5. When a tunnel diode is used as an oscillator, the _____ cancels the effective resistance of a tank circuit.
6. The two stable operating points of a bistable tunnel diode switch are to the left of the _____ point and to the right of the _____ point.
Answers
- two
- negative resistance
- tunnel diode
- forward
- negative resistance
- Voltage peak or VP , valley voltage or VV