UJT Working And UJT As a Relaxation Oscillator

Ujt Working And Operation 

Ujt work­ing and oper­a­tion Equiv­a­lent circuit 

UJT Working

UJT is a three ter­mi­nal semi­con­duc­tor switch­ing device. It has only one PN junc­tion. The basic struc­ture and sym­bol of UJT is above the picture. 

It con­sists of a light­ly doped N‑type sil­i­con bar with a heav­i­ly doped P‑type mate­r­i­al is dif­fused into the bar, forms a PN junction 

The P type region is called emit­ter. Two ohmic con­tacts are pro­vid­ed at the ends of N‑type sil­i­con bar called base‑1 and base‑2 the resis­tance between B1 and B2 is called inter­base resistance. 

Equivalent circuit of UJT 

UJT Working
Equiv­a­lent Cir­cuit Of UJT

The diode D rep­re­sents the PN junc­tion. Accord­ing to the emit­ter ter­mi­nal a inter­base resis­tance is sep­a­rat­ed into two resis­tances RB1 and RB2 

Hence RBB = RB1 + RB2. The resis­tance RB1 is vari­able because it’s val­ue can be var­ied accord­ing to the bias volt­age between P and N junction 

From the equiv­a­lent cir­cuit the Volt­age across the resistor

RB1=V1= \frac {\vphantom{\big(}RB1} {\vphantom{\big(}RB1+RB2}×VBB 
=\frac {\vphantom{\big(}RB1} {\vphantom{\big(}RBB}×VBB
V1=η  VBB(∵η=\frac {\vphantom{\big(}RB1} {\vphantom{\big(}RBB})

where η is known as intrin­sic stand off ratio 

∵ is Before Resoaning 


UJT Working

The ter­mi­nal B2 is always pos­i­tive volt­age (VBB) With respect to B1. Usu­al­ly a pos­i­tive volt­age (VEE ) is applied to the ter­mi­nal E with respect to B1. The oper­at­ing con­di­tion of UJT depends upon their emit­ter voltage 

Accord­ing to the base volt­age VBB Volt­age V1 is always devel­oped across the resis­tor RB1 which is applied to the cath­ode ter­mi­nal of the PN junc­tion diode. When the emit­ter volt­age is zero the diode works in reverse bias so no emit­ter cur­rent flows. Now the UJT is in OFF state. Now a small leak­age cur­rent only flows from B2 to E 

When a pos­i­tive volt­age is applied to the emit­ter the PN junc­tion is also in reverse biased. If the emit­ter Volt­age is lin­ear­ly increased which reduces the amount of reverse bias of the PN junction 

When the applied volt­age is increased greater than V1 the PN junc­tion will be for­ward biased 

Now the holes are inject­ed from P‑region into N‑region. The holes are repelled by the ter­mi­nal B2 and are attract­ed by the ter­mi­nal B1. Now the accu­mu­la­tion of holes in E to B1 region reduces the resis­tance in this section. 

Hence emit­ter cur­rent IE is increased and the volt­age VE is decreased. Now the device is in the ON state 

When a neg­a­tive volt­age is applied to the emit­ter the PN junc­tion is reverse biased and the emit­ter cur­rent is very low. The device is now in OFF state 

char­ac­ter­is­tics of UJT 

UJT Working
Cir­cuit Dia­gram Of UJT

The curve plot­ted between emit­ter Volt­age and the emit­ter cur­rent at a giv­en VBB is called emit­ter char­ac­ter­is­tics of UJT 

Here upto the peak point the diode is reverse biased. Hence the region to the left of the peak point is called cut-off region. The UJT has a sta­ble fir­ing volt­age which depends on VBB after the fir­ing volt­age the diode starts conduction. 

UJT Working
Emit­ter Char­ac­ter­is­tics Of UJT

Now the cur­rent IE is lin­ear­ly increased and VE is lin­ear­ly decreased which pro­duce a neg­a­tive resis­tance region. 

This region lies in between peak point and val­ley point . After the val­ley point cur­rent becomes sat­u­ra­tion. The region beyond the val­ley point is called sat­u­ra­tion region. In this region the UJT is ON state 

Appli­ca­tions of UJT 

  • It is used in switch­ing circuits 
  • It is used in saw tooth generator 
  • It is used in pulse generator 
  • It is used to trig­ger SCR and TRIAC 
  • it is used in phase con­trol circuits
  • It is used in tim­ing circuit

UJT as a relaxation Oscillator 

UJT Working
UJT as a relax­ation Oscillator

Uni­junc­tion tran­sis­tor can be used to gen­er­ate saw tooth sig­nal. In saw­tooth sig­nal the volt­age increas­es slow­ly and falls down to zero in short time 

A capac­i­tor C is con­nect­ed in between the ter­mi­nal E and B1. This capac­i­tor Volt­age is used to trig­ger the UJT and also acts as the out­put sig­nal of the relax­ation Oscillator. 

The charges of the capac­i­tor depend upon the con­duc­tion lev­el of UJT. Before con­duc­tion the capac­i­tor charges through R1 and then after con­duc­tion the capac­i­tor dis­charges through the emit­ter to base 1 path of UJT 

UJT Working

When the sup­ply volt­age is giv­en the capac­i­tor charges through R1. As long as the capac­i­tor Volt­age is less than the base to emit­ter 1 volt­age VEB1 the junc­tion is reverse biased and the UJT will not con­duct. When the capac­i­tor Volt­age exceeds the reverse biased volt­age the UJT starts conducting. 

Dur­ing con­duc­tion the resis­tance between emit­ter and base 1 falls to a low­er val­ue. Hence the capac­i­tor dis­charges the volt­age in a low time and the UJT again goes to non con­duc­tion level. 

The capac­i­tor repeats the charge and dis­charges func­tions con­tin­u­ous­ly. By this cumu­la­tive action a con­tin­u­ous saw­tooth sig­nal is pro­duced at its output. 


  • Used in television 
  • Used in oscilloscope 
  • Used in radar equipment 
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