EC8452 Electronic Circuits II - UJT Relaxation Oscillator

UJT Relaxation Oscillator

1. Academic Year 2018-19 EC8452 Electronic Circuits II UJT Relaxation Oscillator Mrs.R.Chitra , AP/ECE, Ramco Institute of Technology, Rajapalayam

2. Physical structure It is a three terminal semiconductor device One lightly-doped (high resistivity) N-type silicon slab. Two base contacts ( B 1 and B 2 ) at both end of the slab. The p-n junction is form by alloying an aluminum rod (p-type)to the slab. The aluminum rod is closer to B 2 contact than B 1 contact. B 2 is made positive with respect to B 1 . UNIJUNCTION TRANSISTOR (UJT)

3. Physical structure

4. Equivalent circuit The equivalent circuit comprised of two resistors, one fixed ( R B 2 ) and one variable ( R B 1 ) and a single diode with V D as voltage drop across it.

5. When emitter is not conducting then resistance between two bases B1 and B2 is called R BB is the interbase resistance Typical range of R BB : 4 k - 10 k The position of the aluminum rod determine the ralative values of R B 1 and R B 2 . Contd...

6. with emitter terminal open, V BB is applied between two bases. with emitter terminal open, VBB is applied between two bases. Then, voltage drop across RB1 Is obtained by using potential divider rule is given by This voltage reverse biases the PN junction and emitter current is cut-off UNIJUNCTION TRANSISTOR (UJT)

7. UNIJUNCTION TRANSISTOR (UJT)

8. For V E > V RB 1 by V D (0.35  0.70 V), the diode will fire and I E will begin to flow through R B 1 . The emitter firing potential V P is given by: UNIJUNCTION TRANSISTOR (UJT)

9. Characteristics of representative UJT:

10. Cut-off Region : VE < VP and PN junction is reverse biased. A small amount of reverse saturation current flows through the device. Negative resistance region : When emitter voltage become equal to V P , PN junction becomes forward biased and I E start flowing and holes are injected into N layer Hence, resistance decreases thereby decreasing V E for increase in I E . So, there is a negative resistance region Saturation Region: Increase in I E further valley point drives the device in the saturation region and behaves like a conventional forward biased PN junction.

11. Basic UJT relaxation oscilator UJT RELAXATION OSCILLATORS

12. Assume that the initial capacitor voltage, V C is zero. When the supply voltage V BB is first applied, the UJT is in the OFF state. I E is zero and C charges exponentially through R 1 towards V BB . The operation UJT RELAXATION OSCILLATORS

13. When the supply voltage V C ( = V E ) reaches the firing potential, V P , the UJT fires and C discharges exponentially through R 2 until V E reaches the valley potential V V . UJT RELAXATION OSCILLATORS

14. When V E reaches the valley potential V V the UJT turns OFF, I E goes to zero and the capacitor is recharged. This process repeats itself to produce the waveforms for v C and v R 2 as shown below; UJT RELAXATION OSCILLATORS

15. UJT RELAXATION OSCILLATORS

16. UJT RELAXATION OSCILLATORS

17. The frequency; UJT RELAXATION OSCILLATORS

18. References: Sedra and Smith, “Micro Electronic Circuits”; Sixth Edition, Oxford University Press, 2011. Jacob Millman , ‘Microelectronics ‘, McGraw Hill, 2nd Edition, Reprinted, 2009.