Negative Resistance Oscillator Circuit

Key Takeaways

• The negative differential resistance property generates oscillations in oscillator circuits utilizing elements such as tunnel diodes, IMPATT diodes, TEDs, and negative impedance converters. 

• The negative resistance component cancels the positive resistance of the resonant circuit and generates steady oscillations.

• The transistor in the negative resistance oscillator circuit should be in an unstable condition to utilize its negative resistance region. 

The oscillator uses a tunnel diode to form a negative resistance oscillator circuit

Most engineers know the famous Ohm’s law, but few are aware of its exceptions; cases in which voltage and current are inversely proportional to each other and can be considered an instance of negative resistance property. 

The negative resistance property is of great use in communication systems, especially in oscillator circuits employed in radio receivers, transmitters, signal generators, frequency counters, network analyzers, etc. The oscillators that utilize the negative resistance property of electronic components can be generalized as negative resistance oscillator circuits, and these circuits can be implemented in different ways. 

Negative Resistance Explained

As the voltage applied across the element in positive resistance circuits increases, there is an increase in current and vice versa. However, certain electronic elements exhibit negative resistance. In such elements, the current tends to decrease when voltage increases and vice versa. 

Common Elements Showcasing Negative Resistance Property

Negative resistance region in the V-I curve

In the voltage-current curve of negative resistance devices (shown above), there is a negative resistance region where the current decreases with increasing applied voltage. Before and after the negative resistance region, devices behave as positive resistance. By operating in the negative resistance region, it is possible to build oscillator circuits using these devices even without a positive feedback loop.

The negative resistance oscillator circuit generates sustained oscillations without a feedback loop. The negative differential resistance property generates oscillations in the oscillator circuits utilizing elements such as a tunnel diode, IMPATT diode, TED, negative impedance converter, etc. 

The product of voltage and current in the negative resistance oscillator circuit is negative when the tunnel diode, IMPATT diode, TED, etc. operates in the negative resistance region and leads to power generation. The damping limitation of conventional oscillators is also eliminated by utilizing negative resistance oscillator circuits.

Block Diagram of Negative Resistance Oscillator Circuit

 Block diagram of a negative resistance oscillator circuit

1. Negative resistance component - The negative resistance element can be two-terminal devices such as tunnel diodes, IMPATT, Gunn diodes, UJT, or three-terminal transistors like MESFETs. Two NPN transistors connected in a push-pull configuration or as differential pairs form a negative resistance network. The two-terminal devices are polarized to operate as negative resistance, whereas a feedback network is required for three-terminal devices to operate in the negative resistance mode. 

2. Resonant circuit - Frequency selection is made with the help of a resonant circuit. A parallel combination of resistor, capacitor, and inductor is an example of a commonly used resonant circuit. 

3. Load - Oscillation appears across the load. There can be transition circuits included between the source and the load to improve the performance of the oscillator.

In short, negative resistance oscillators operate without positive feedback. The negative resistance component cancels the positive resistance of the resonant circuit and generates steady oscillations.

In a negative resistance oscillator circuit, the quiescent point is located at the center of the negative resistance region to obtain maximum output. The oscillation is limited in amplitude when the operating point leaves the negative resistance region given in the V-I curve of the negative resistance component. 

Designing a Two-Port Negative Resistance Oscillator Circuit Using Transistors

Block diagram of two-port negative resistance oscillator circuit

Let’s modify the block diagram of the negative resistance oscillator given in the previous section into a two-port circuit by incorporating a generator tuning network and terminating network. The generator tuning network can be a resonant circuit or an impedance-matched circuit. The negative resistance component chosen in this design is a transistor. While selecting the transistor, generator tuning network, and terminating network for a negative resistance oscillator circuit, three conditions must be met:

1. The transistor should be in an unstable condition. For sustained oscillations, the slope of the load line on the output characteristics of the transistor represented by parameter ‘K’ should be less than 1. The K value less than unity indicates the transistor is operating in the negative resistance region. 

2. For sustained oscillations, the product of the reflection coefficients of the input port and generator tuning network should be greater than or equal to unity.

3. For sustained oscillations, the product of the reflection coefficients of the output port and terminating network should be greater than or equal to unity.