5 transistor applications
- 1. MECHATRONICS APPLICATION OF TRANSISTORS PUSHPARAJ MANI PATHAK MECHANICAL & INDUSTRIAL ENGINEEING, IIT ROORKEE 1
- 2. Introduction • Because of the Bipolar Junction Transistor (BJT) base-collector current characteristics, it can be used to – amplify current – simply switch current on and off. • Transistor as a switch is basis for most digital computers because it allows easy implementation of a two state binary representation. • We focus on switch design in our mechatronic applications 2
- 3. Transistor Connections • Common base connection • Common emitter connection • Common collector connection 3
- 4. Common Emitter Connection • Input current IB • Output current IC • Base current amplification factor = ∆𝐼𝐶 ∆𝐼𝐵 = 𝛽 • ∆𝐼𝐵 < 5% ∆𝐼𝐸 • 𝛽 > 20 (IC ≈ IE) • 𝛽 ≈ 100 4
- 5. • If a BJT’s emitter is grounded and we apply an input voltage to the base, we get the common emitter circuit. • Input characteristics • VBE v/s IB for VCE=constant 5 VBE IB VCE=1V
- 6. Output Characteristic • VCE v/s IC for constant base current (IB) • As IB increases, base to emitter conducts when VBE = 0.6V, IC= 𝛽IB • As IB is further increased, VBE slowly increases to 0.7V, IC rises exponentially. 6
- 7. • As IC increases, voltage drop across RC increases, so VCE drops towards ground. • Transistor is said to go to saturation i.e., collector current is determined by RC and linear relation between IC and IB no longer holds. 7
- 8. • When designing a transistor switch, we need to guarantee that the transistor is in fully saturation condition. • VCE minimum is about 0.2 V for BJT. • Power dissipated is smallest= IC VCE for given collector current when it is fully saturated. • If transistor is not fully saturated it gets hot faster and can fail. 8
- 9. Bipolar Transistor Switch • If VBE<0.7V, BE junction is not forward bias, so IC=0, IB=0 9 Off
- 10. • If VBE=0.7V, BE junction is forward biased. Current passes through CE circuit • Vo is close to ground potential (0.2V) for saturated BJT. 10 ON
- 11. • RB required to limit base current 𝐼𝐵 = 𝑉𝑖 − 𝑉𝐵𝐸 𝑅𝐵 • When Vi < 0.7 V, IB = 0 and VBE=Vi • Transistors used for power application are called power transistor. 11
- 12. Bipolar Transistor Packages 12 Signal transistor Power transistor Used in Printed Circuit Boards
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- 14. Darlington Transistor • Pair of transistors • Current gain is product of two individual transistor gains. • Can be of the order of 10000 • Used in power circuit of mechatronic system. 14
- 15. Phototransistor • A special class of transistor whose junction between base and emitter acts as photo diode are phototransistor. • LED’s and Phototransistor are used in pairs, where the LED is used to create the light, and this light in turn biases the phototransistor. • The pair can be used to detect the presence of an object that may partially or completely interrupt the light beam between the LED and phototransistor. 15
- 16. Optoisolator • Comprised of a LED and a phototransistor separated by a small gap. • The light emitted by LED causes the current to flow in the phototransistor circuit. • Output circuit for different ground reference and supply voltage VS can be chosen to establish a desired output voltage range. • The opto isolator creates a start of electrical isolation between the input and output circuit by transmitting the signal optically. 16
- 17. Angular Position of a Robotic Scanner • Problem statement • In design of autonomous robot, include a laser scanning device to sweep the environment to detect the obstacles. Head of the scanner is rotated through 360˚ by a DC motor. Find – Angular position of scan head. – What should be done, if on board computer to use this scanned value. 17
- 18. • Solution • Design a sensor that provides digital output. • Use LED-photo transistor pair called photo-interrupter. • The device produce a light that can be broken or interrupted. • Provide a disk attached to motor passing through slot. • Each slot will provide digital pulse as it interrupts the light beam during rotation. 18
- 19. • R1- current limiting resistor • R2- pull up resistor to provide output. • As slotted disk rotates • When light passes, transistor conducts – 0 o/p. • Slot interrupted, 5V o/p returned. • No of pulses is measure of rotation. • If 360 slots, 1 pulse = 1˚ 19
- 20. Field Effect Transistors • Drawback of bipolar transistors – Low input impedance due to forward bias emitter junction. – Considerable noise level. • In FET: current conduction is by one type of carrier i.e, electrons or holes. • With a FET, the electric field produced by a voltage on one electrode controls the availability of charge carriers in a narrow region, called a channel, through which a current can be made to flow. • Therefore, a FET the output current is controlled by an input voltage. 20
- 21. Construction Details • A p-type or n type silicon bar containing two pn junctions at the sides. • Bar forms the conducting channel for the charges carriers. • If bar is n type, it is called n channel FET and if it is p type it is p channel FET. 21 Source(S) Gate(G) n Drain(D) n p p Source(S) Gate(G) p Drain(D) p n n n-Channel FET p-Channel FET
- 22. • The two pn junctions forming diodes are connected internally and common terminal is called Gate. • Thus FET has three terminals i.e., Gate (G), Source (S) and Drain (D). 22 Source(S) Gate(G) n Drain(D) n p p n-Channel FET
- 23. FET Polarities • Voltage between Gate and Source is such that it is reverse biased. • Drain and Source terminals are interchangeable. 23 Source(S) Gate(G) n Drain(D) n p p VDS VGS n-Channel FET
- 24. Working principle of FET • (i) As VDS>0, VGS=0 • pn junction establishes depletion layer. • Electrons flow from source to drain through channel between depletion layer. • Size of layer determines width of channel and current through the bar. 24 Source(S) Gate(G) n Drain(D) n p p VDS VGS n-Channel FET
- 25. • Reverse voltage at VGS • Width of depletion layer increases • Width of conducting channel decreases • Resistance increases of n type bar • Current flow decreases. • Current can be controlled by reverse voltage. 25 Source(S) Gate(G) n Drain(D) n p p VDS VGS n-Channel FET
- 27. Difference between FET and Bipolar Transistor Bipolar • Both n and p type carrier • Low input impedance • Current driven device • Characterise by current gain 27 FET • Unipolar(either n or p type carrier) • High input impedance (isolation possible) • Voltage driven device • Characterise by transconductance (ratio of change in o/p current to i/p (gate) voltage.
- 28. Output Characteristic of FET • Keep VGS at some constant value • ID rapidly increases, as VDB increases. • After pinch off voltage, channel width becomes narrow so constant current is obtained. 28 𝑉𝐷𝑆 O A 𝐼𝐷 𝑉𝐺𝑆 = 1𝑉 𝑉𝐺𝑆 = 2𝑉 𝑉𝐺𝑆 = 3𝑉
- 29. Metal Oxide Semiconductor FET (MOSFET) • The input impedance of MOSFET is much more than that of a FET because of very small gate leakage current. • The same equations apply as used for FET 29
- 30. MOSFET Construction Details • Only a single p region • A thin layer of metal oxide (usually silicon dioxide) is deposited over the left side of the channel. • A metallic gate is deposited over the oxide layer. • As silicon dioxide is an insulator , so gate is insulated from channel 30 Source(S) Gate(G) n Drain(D) n p Oxide layer Subtrate S G D n-Channel FET Subtrate
- 31. Working of MOSFET • Here gate is formed as capacitor • One plate of capacitor is gate, other is channel and metal oxide is dielectric. • When negative voltage is applied at gate, electrons accumulated on it. • These electrons repel the conduction band electrons in the n channel. • So less no of electrons are available for conduction through channel. • If gate is given positive voltage , more electrons are made available in the n channel. So current from source to drain increases. 31 Source(S) Gate(G) n Drain(D) n p Oxide layer Subtrate
- 32. References • W. Bolton, Mechatronics: Electronic Control Systems in Mechanical and Electrical Engineering (6th Edition), Pearson, 2015 • D.G. Alciatore and Michael B. Histand, Introduction to Mechatronics, Tata Mc Graw Hill, 2012. 32
- 33. 33 Thank You