The Unijunction TransistorThe Unijunction Transistor

Base 2 B2

B2

nE i E E

r’B2

pEmitter E

r’B1

B 1B1

B1

B1

Base 1

ConstructionSymbol Equivalent

Circuit

Notes on UJTNotes on UJT

UJT has only one pn junction.It has an emitter and two bases B and BIt has an emitter and two bases, B1 and B2.

r’B1 and r’B2 are internal dynamic resistances.

The interbase resistance, r’BB = r’B1 + r’B2.r’B varies inversely with emitter current IEr B1 varies inversely with emitter current, IEr’B1 can range from several thousand ohms to t f h d di Itens of ohms depending on IE.

Basic UJT BiasingBasic UJT Biasing

B

Vr’B1 = ηVBBη = r’B1/r’BB is the standoff ratio.

B2

r’B2

If VEB1 < Vr’B1 + Vpn,IE = 0 since pn junction is notforward biased (V = barrier

E+ VBB

+_

forward biased (Vpn = barrierpotential of pn junction)At VP = ηVBB + Vpn, the UJT

B

r’B1 ηVBBVEB1

_

At VP ηVBB Vpn, the UJTturns on and operates in a negative resistance region up to

B1 a certain value of IE.It then becomes saturated andI increases rapidly with VIE increases rapidly with VE.

UJT Characteristic CurveUJT Characteristic Curve

VE

Saturation

NegativeResistanceCutoff

VPPeak

VV

Valley

IP IVIE

P V

Applications of UJTApplications of UJT

UJT can be used as trigger device for SCRs and triacs. Otherapplications include nonsinusoidal oscillators, sawtoothgenerators phase control and timing circuitsgenerators, phase control, and timing circuits.

+VBB VP

VE

RE

P

VV t

C VB1R

VE VB1

R1 tWaveforms for UJT relaxation oscillator

RelaxationRelaxationoscillator

Conditions For UJT Oscillator Conditions For UJT Oscillator Operation

In the relaxation oscillator, RE must not limit IE at the peak point to less than IP at turn‐on, i.e., VBB ‐VP > IPRE.

To ensure turn‐off of the UJT at the valley point, RE must be large enough that IE can decrease below IV, i.e., VBB ‐VV < IVRE.

So, for proper operation:BB VBB P V VV V R −−

> > BB VBB PE

P V

RI I

> >

11

ln BB Vo E

BB P

V Vf R CV V

−⎡ ⎤⎛ ⎞−

= ⎢ ⎥⎜ ⎟−⎝ ⎠⎣ ⎦

R1 is usually << RE, and the frequency of oscillations is

BB PV V⎝ ⎠⎣ ⎦

UJT Frequency of OscillationUJT Frequency of Oscillation

Assuming VV ≅0 and VP≅ ηVBB

111ln

1o Ef R Cη

−⎡ ⎤⎛ ⎞

= ⎢ ⎥⎜ ⎟−⎝ ⎠⎣ ⎦

For most UJT η ≅ 0.63

1 η⎝ ⎠⎣ ⎦

[ ] 1o Ef R C −= [ ]o Ef

UJT Oscillator ExampleUJT Oscillator Example

For the shown oscillator:η=0.63 rBB=9.2 kΩη BBVv=1.5V IV=3.5mA IP=5µA

Find:Find:(a) VP(b) Oscillation Frequency( ) q y(c) REmin and REmax(d) Sketch VB1

UJT Relay TimerUJT Relay Timer

R3 is chosen to maintain CR relaymaintain CR relay coil energized but not to pick the coilp

The delay is set by (REF+REV)CE

UJT as SCR TriggerUJT as SCR Trigger

For component sizing refer to the text book

The Programmable UJTThe Programmable UJT

The PUT is actually a type of thyristorIt can replace the UJT in some oscillator It can replace the UJT in some oscillator applications.It is more similar to an SCR (four‐layer device) except that its anode‐to‐gate voltage can be used to both turn on and turn off the device.

PUT Construction & SymbolPUT Construction & Symbol

Anode (A) +V

R2R1

pn

Gate (G)G

AR2

pn

K

Vin R3

Cathode (K)

K

Basic Construction PUT Symbol and Biasing

Notes On PUTNotes On PUT

Notice that the gate is connected to the nregion adjacent to the anoderegion adjacent to the anode.The gate is always biased positive with

t t th th drespect to the cathode.When VA ‐VG > 0.7 V, the PUT turns on.A G

The standoff ratio is replaced by external elements to program VPelements to program VP

The characteristic plot of VAK versus IA is i il t th V I l t f th UJTsimilar to the VE versus IE plot of the UJT.

PUT Relaxation OscillatorPUT Relaxation Oscillator

•RG1V can control theRG1V can control the frequency of oscillation with a constant charging rate•Can operate from lower supply voltages compared to UJTUJT•Can be used for very low frequencies thanks to thefrequencies thanks to the availability of lower IP

Constant Slope Ramp Constant Slope Ramp Generator

VG determines theVG determines the ramp height