Bjt and its differnet parameters
- 1. Group member :- Fahid Bin Tariq (15f-8389) Hassan Tariq (14f-8355)
- 2. 2 Transistors •Transistors is a semiconductor which is used to control the flow of voltage or current. • The switch current can be controlled by either current or voltage • Bipolar Junction Transistors (BJT) control current by voltage • Field Effect Transistors (FET) control voltage by current •They can be used either as switches or as amplifiers
- 3. 3 NPN Bipolar Junction Transistor •One N-P (Base Collector) diode one P-N (Base Emitter) diode
- 4. 4 PNP Bipolar Junction Transistor •One P-N (Base Collector) diode one N-P (Base Emitter) diode
- 5. 5 NPN BJT Current flow
- 6. 6 BJT α and β •From the previous figure iE = iB + iC •Define α = iC / iE •Define β = iC / iB •Then β = iC / (iE –iC) = α /(1- α) •Then iC = α iE ; iB = (1-α) iE •Typically β ≈ 100 for small signal BJTs (BJTs that handle low power) operating in active region (region where BJTs work as amplifiers)
- 7. 7 BJT in Active Region Common Emitter(CE) Connection • Called CE because emitter is common to both VBB and VCC
- 8. 8 BJT in Active Region •Base Emitter junction is forward biased •Base Collector junction is reverse biased •For a particular iB, iC is independent of RCC ⇒transistor is acting as current controlled voltage source (iC is controlled by iB, and iC = β iB) • Since the base emitter junction is forward biased, from Shockley equation − = 1exp T BE CSC V V Ii
- 9. 9 BJT in Active Region •Normally the above equation is never used to calculate iC, iB Since for all small signal transistors vBE ≈ 0.7. It is only useful for deriving the small signal characteristics of the BJT. •For example, for the CE connection, iB can be simply calculated as, BB BEBB B R VV i − = or by drawing load line on the base –emitter side
- 10. 10 BJT in Cutoff Region •Under this condition iB= 0 •As a result iC becomes negligibly small •Both base-emitter as well base-collector junctions may be reverse biased •Under this condition the BJT can be treated as an off switch
- 11. 11 BJT in Saturation Region •Under this condition iC / iB < β in active region •Both base emitter as well as base collector junctions are forward biased •VCE ≈ 0.2 V •Under this condition the BJT can be treated as an on switch
- 12. 12 •A BJT can enter saturation in the following ways (refer to the CE circuit) •For a particular value of iB,if we keep on increasing RCC •For a particular value of RCC,if we keep on increasing iB •For a particular value of iB,if we replace the transistor with one with higher β BJT in Saturation Region
- 13. Early Effect and Early Voltage As reverse-bias across collector-base junction increases, width of the collector-base depletion layer increases and width of the base decreases (base-width modulation). In a practical BJT, output characteristics have a positive slope in forward- active region; collector current is not independent of vCE. Early effect: When output characteristics are extrapolated back to point of zero iC, curves intersect (approximately) at a common point vCE = -VA which lies between 15 V and 150 V. (VA is named the Early voltage) Simplified equations (including Early effect) 13 iC =IS exp vBE VT 1+ vCE VA βF =βFO 1+ vCE VA iB = IS βFO exp vBE VT
- 15. 15 BJT ‘Q’ Point (Bias Point) •Q point means Quiescent or Operating point • Very important for amplifiers because wrong ‘Q’ point selection increases amplifier distortion •Need to have a stable ‘Q’ point, meaning the the operating point should not be sensitive to variation to temperature or BJT β, which can vary widely
- 16. . 16 Input Characteristics • Plot IB as f(VBE, VCE) • As VCE increases, more VBE required to turn the BE on so that IB>0. • Looks like a pn junction volt-ampere characteristic.
- 17. . 17 Output Characteristics • Plot IC as f(VCE, IB) • Cutoff region (off) both BE and BC reverse biased • Active region BE Forward biased BC Reverse biased • Saturation region (on) both BE and BC forward biased VCC/RC VCC