Negative impedance circuit
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This document discusses negative differential resistance, which is a characteristic of some electronic devices where an increase in current causes a decrease in voltage. It can be exhibited by tunnel diodes and Gunn diodes. Negative differential resistance adds energy to a circuit unlike a regular resistor which consumes energy. It has applications in oscillators where it can produce sustained oscillations by counteracting energy losses in a resonant circuit. It can also function as a signal amplifier at microwave frequencies when used with a bandpass filter.
Negative impedance circuit
- 2. Definition: It is a characteristic of some electronic instruments in which an increase in the current (I) gives rise to a voltage (V) drop across the circuit. It is also called negative differential resistance or negative differential conductance.
- 3. Properties: Some electric circuits show a negative current–voltage characteristic, which signifies that current flow is inversely proportional to the potential difference. Tunnel diodes and Gunn diodes usually display a negative current–voltage curve. In simple words, these are non-linear devices connected to a two terminal-circuit. The property is also exhibited by double-base diodes in which the emitter current increases until the power supply restricts it. This generally occurs when various active components are used to design a circuit in order to provide a source of energy. In fact, any current passing through a negative resistance adds energy to circuits, unlike a positive resistor that consumes the same energy. In a typical voltage vs. current graph, a true negative resistor has a negative curve that passes through the origin of the coordinate system that makes it act like a current-to-voltage converter. For example, if the same current I flows through a positive resistor as well as an S-shaped negative resistor with the same resistance R, the former subtracts a voltage drop V = R.I from the circuit while the latter adds the same amount of voltage. Negative resistors are usually auxiliary sources that cannot operate independently and rely heavily on the main source of current.
- 4. History: In the past, negative differential resistance was observed in arc discharge equipments and few vacuum tubes such as the dynatron. With the advent of solid state technology, it became possible to manufacture devices that could display this characteristic. John G. Linvill and Leo Esaki were the pioneers of negative impedance converter and tunnel diode, respectively.
- 5. Equation:
- 9. Oscillators Regions of amplifying devices such as vacuum tubes exhibiting negative differential resistance are referred to as “oscillators”. One-port negative resistors like tunnel diodes, Gunn diodes and magnetron tubes are used in a number of electronic oscillator circuits. A DC voltage is applied to a circuit comprising of a resonator like LC circuit, cavity conductor or quartz crystal, connected across a negative resistor. An active negative resistor produces sustained oscillations after counteracting the effective loss resistance of the resonator. The circuits are highly applicable for oscillators at microwave frequencies.
- 10. Amplifiers At frequencies between 300 MHz (0.3 GHz) and 300 GHz, a negative resistor can function as a signal amplifier. However, these devices do not completely exhibit the property at microwave frequencies and usually demand the use of a filter. A circuit can be subjected to pure resistance if the reactive components of the device’s equivalent circuit are absorbed into the filter design in the presence of a bandpass filter that passes frequencies in a desired range, and attenuates the ones outside the range. The output terminal is attached to one port of a 3-port circulator. The reflection coefficients at two ends of the filter for a positive value of R0 are given below: Г1= Z1- R0/ Z1+R0 and, Г2= Z2-R1/ Z2+R1 In the absence of resistive elements, energy dissipation is nil and the two reflection coefficients are equal i.e.│Г1│=│Г2│