Tunnel Diode as amplifier and oscillator
The tunnel diode exhibits special feature known as negative resistance. This feature makes it useful to develop microwave amplifier and oscillator. Tunnel is normally constructed using germanium or gallium arsenide. Here p region and n regions are much more heavily dopped in comparison to conventional diode used as rectifier. Let us explore tunnel diode equivalent circuit, amplifier circuit and oscillator circuit diagrams.
The figure-1 depicts equivalent circuit. Rj is diode junction resistance (about 100 Ohm) , Cj is junction capacitance (in pF), Ls is lead inductance (in nH) and Rs (few Ohms) is the lead resistance/bulk resistance of the semiconductor.
Impedance of the tunnel circuit mentioned is expressed as follows:
Z= Rs- (Rj/(1+(w*Rj*Cj)2)) + (j*w*Rj*Cj)*[(Ls/(Rj*Cj))-(Rj/(1+(w*Rj*Cj)2))]
From oscillation condition ( Re(Z) = 0 and Im(Z) = 0 ) of tunnel diode following two frequencies viz.
resistive cutoff frequency and self resonant frequency are derived.
Resistive cutoff frequency (fr) = ( 1 / 2*pi*Rj*Cj ) * ( (Rj/Rs)-1 )0.5
Self resonant frequency (fo) = (1/2*pi) * [ ( 1/Ls*Cj ) - (1/(Rj*Cj )2 ) ]0.5
Tunnel Diode Amplifier circuit
Fig:2 Tunnel Diode Amplifier
The figure-2 depicts tunnel diode circuit used as amplifier. Low noise makes this device used as first stage of amplification in communication receivers. In this circuit, characteristic impedance of circulator must be greater than negative resistance of the tunnel diode.
As we know in circulator RF energy travels only in one direction. As shown in the figure-2, tunnel diode is connected across tuned input and output. Normally this circuit produces oscillations when feedback is allowed to be reflected back. In this configuration, feedback is prevented as all the energy is absorbed in the load (RL) due to circulator. Here tunnel diode does not oscillate but amplifies the input signal to produce amplified output. Bandpass filter is used to provide impedance matching anf selection of desired frequencies.
Tunnel diode amplify the signal fed at port-2 and will provide amplified output at port-3.
Tunnel Diode Oscillator circuit
Fig:3 Tunnel Diode Oscillator
The figure-3 depicts tunnel diode based oscillator circuit. The circuit helps generate microwave frequencies upto 100 GHz.
As shown in the figure-3, tunnel diode is connected in series with the tank circuit. When power is applied surge current produces oscillation in the tank. R and C values will make DC bias at the center of negative resistance curve of tunnel diode. Sustained oscillation results when magnitude of negative resistance of tunnel diode is greater or equal to positive resistance of the tank circuit.
Conclusion: The unique negative resistance behavior of tunnel diodes is the key factor that allows them to serve as amplifiers and oscillators. Due to its operational range limitation, careful circuit design and biasing is required to achieve stable and reliable performance.
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