The UJT output voltage, obtained over the 47 ohm resistor R3, switches the bipolar transistor between a couple of thresholds: saturation and cutoff, generating horizontal-topped output pulses. Depending on the off time t of the pulse, the output waveform could be sometimes narrow rectangular pulses or as indicated across the output terminals in Fig. The frequency, or cycling frequency, is determined by the adjustment of a 50 k pot resistance and the capacitor value of C1. When resistance setting is at minimum, probably with only R1 at 1. The dc output coupling can be seen in schematic, but ac coupling could be configured by placing a capacitor C2 within the high output lead, as demonstrated through the dotted image.

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UJT is an excellent switch with switching times in the order of nano seconds. It has a negative resistance region in the characteristics and can 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.

UJT uni junction transistor 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 B2. 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. 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. 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. 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 and the emitter voltage Ve decreases. This situation is similar to a negative resistance scenario. In the graph Fig:2 you can see that the regions between Vp peak voltage point and Vv valley voltage have a negative slope.

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.

Magnetic flux sensors. Voltage or current limiting circuit. Bistable oscillators. Phase control circuits. 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 the UJT relaxation oscillator will be added soon.


UJT Relaxation Oscillator

Next Page An oscillator is a device that produces a waveform by its own, without any input. Though some dc voltage is applied for the device to work, it will not produce any waveform as input. A relaxation oscillator is a device that produces a non-sinusoidal waveform on its own. This waveform depends generally upon the charging and discharging time constants of a capacitor in the circuit. The RC time constant determines the timings of the output waveform of the relaxation oscillator. Both the bases are connected with a resistor each. The dc voltage supply VBB is given.


UJT as Relaxation Oscillator

It resembles to that of the diode with a single junction of the P-N. But the operation is completely different in comparison with it. As the name suggesting it is a single junction transistor but it is widely used in the circuits of timing, triggering circuits and so on… it is a device that consists of dual layers along with three terminals present in it. It is having very different characteristics in comparison with the other transistors. Its three terminals are named as base1, base2 and the emitter. The current at the terminal emitter tends to increase as the input gets triggered. These are used during switching of the devices other than amplification.

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