The UJT is often used in the timing and triggering circuits. Figure A shows the circuit diagram for the UJT relaxation oscillator. When the switch. In electronics a relaxation oscillator is a nonlinear electronic oscillator circuit that produces a .. Unijunction transistor – A transistor capable of relaxation oscillations. . Multivibrator · ring oscillator · Pearson–Anson oscillator · basic Royer. stage oscillator circuits using a particular device like unijunction transistor. Such a oscillator which uses UJT is called UJT relaxation oscillator. The basic circuit.

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UJT is an excellent switch with switching times in the order of nano seconds. It baasic a negative resistance region in the characteristics and hasic be easily employed in relaxation oscillators.

The UJT relaxation oscillator is called so because the timing interval is set up by the charging of a capacitor and the timing interval is ceased by the the rapid discharge of the same capacitor. From the name itself, the UJT or uni junction transistor is a semiconductor device that has only one junction.

These end terminals are called B1 and Jut. A heavily doped P-type region is constructed on one side of the bar close to the B2 region. This heavily doped P region is called emitter and it is designated as E.

The internal block diagram, simplified internal circuit model and circuit symbol of a UJT is given in the figure below. The diode symbol shown in the internal circuit model represents the P-N junction formed between the heavily doped P-region E and the lightly doped N-Type bar.

Relaxation oscillator – Wikipedia

It is the resistance between the terminals B1 and B2. In simple words, it is the resistance of the N-Type bar when measured lengthwise. The typical range of intrinsic standoff ratio is from 0. For ease of understanding, the internal model of the UJT is used in the circuit Fig 1. Current starts flowing into the emitter only when the bias voltage Ve has exceeded the forward drop of the internal diode Vd plus the voltage drop across RB1 Vrb1.


This condition can be expressed using the following equation. A typical silicon diode has a forward voltage drop of 0.

UJT as Relaxation Oscillator

When this factor is considered, the equation can be re written as. This minimum value of the emitter voltage Ve for which the emitter current starts to flow is called the firing voltage of UJT. But the Ve can be only increased up to a particular point called Vp peak voltage. These holes are repelled by B2 and attracted by B1. As a result, the region between emitter E and B1 terminal starts saturating by holes and the conductivity of this region starts to increase.

This phenomenon of increasing conductivity by the insertion of holes is called conductivity modulation.

This results in a condition where emitter current Ie increases oscilltaor the emitter voltage Ve decreases. This situation is similar to a negative resistance scenario. In the graph Fig: This negative resistance region in the UJT characteristics is employed in relaxation oscillators.

At last the emitter current Ie will be increased to a point that no more increase in conductivity is possible. The emitter current corresponding to valley point is denoted as Iv and the corresponding emitter voltage is denoted as Vv. Beyond the valley point, the UJT is fully saturated and the junction behaves like a fully saturated P-N junction. The circuit diagram of a UJT relaxation oscillator is given shown above.


R1 and R2 are current limiting resistors. Resistor R and capacitor C determines the frequency of the oscillator.

When power supply is switched ON the capacitor C starts charging through resistor R. The capacitor keeps on charging until the voltage across it becomes equal to 0. After this point the emitter to RB1 resistance drops drastically and the capacitors starts discharging through this path.

This cycle is repeated and results in a sort of sawtooth waveform across the capacitor. Practical circuit diagram and testing of oscillayor UJT relaxation oscillator will basc added soon. Tutorials Previous post Next relaaxation You may also like: Do you know how RFID wallets work and how to make one yourself?

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UJT Relaxation oscillator circuit

Your email address will not be published. UJT uni junction transistor From the name itself, the UJT or uni junction transistor is a semiconductor device that has only one junction. Applications of UJT Relaxation oscillators.

Voltage or current limiting circuit. Voltage or current regulators. Previous post Next post. You may also like: Where do you get the period of oscillation Formula. Leave a Reply Cancel reply Your email address will not be published.