1 © unitec new zealand de4401 b ipolar j unction t ransistor bjt
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DE4401
BIPOLAR JUNCTION TRANSISTOR
BJT
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Remember PN Diode
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Now we combine 2 diodes
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Principles of BJT amplifier
• Base-Emitter junction is forward Biased and a small Base current flows
• This injects charge-carriers into the base area, which enhances (‘controls’) high current flow from Collector to Emitter through the reverse-biased Collector-Base junction
• This creates “current gain”, where a small base current variation controls a large current variation from C to E
• We “DC-BIAS” the transistor to operate in the desired region by choosing the correct resistors around it
• Then we superimpose a small AC signal on the B-E circuit, which produces a large replica on the CE circuit
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3 Ways to Configure BJT –CE most common
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Summary of Basic Config
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Amplify AC signal or Switch (On/Off)
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NPN construction, Symbol & Connection
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NPN ‘biasing’ and DC current gain
• Rb must be calculated to limit max base current when Vb is applied between B & E
• Vbe must be >0.7V (BE junction forward biased) for BJT to turn on and conduct between C & E. (Vb=Ve+0.7V)
• Small input current Ib provides charge carriers to control large current Ic (from the more +ve C to –ve E)
• Input Impedance low (Forward Biased)
• Output Impedance High (Rev Biased)
• limits Max output current
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Current Controlled Amplifier
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Current Gain examples
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Common Emitter Amplifier Class A
• First ‘Bias’ base voltage to operate BJT within it’s Linear active region, then it will accurately reproduce (amplify) the +ve & -ve halves of small AC signal put into Base
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Characteristic Curves for typical BJT
• Choose resistor values to set the DC load line and Q point to centralize Vce for Max + & - swing of Vout
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Choosing to set load line
• NPN transistors conduct when Vc>>Vce and Vb>Ve+0.7
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PNP opposite to NPN Vb is Ve-0.7V
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PNP circuit – opposite polarity voltages
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NPN & PNP complementary Class B Amp
• NPN conducts + half of input AC, PNP does – half
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Distinguishing PNP & NPN with meter
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Summary
• Bipolar PNP transistor will only conduct if both Base and collector terminals are NEGATIVE with respect to emitter
• Bipolar NPN transistor will only conduct if both Base and collector terminals are POSITIVE with respect to emitter
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Transistor Biasing
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Transistor Biasing
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Transistor Biasing
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Transistor Biasing
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Transistor Biasing
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Use BJT as SWITCH instead of Amplifier
• Instead of Biasing in linear active region, stick to shaded areas
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Just use Saturation (ON) or Cutoff (OFF)
• Use in Logic Circuits or control high power devices like motors, solenoids, lamps
• Avoid the high power dissipation in BJT itself = (VxI) in the active linear region, by forcing it into either Cutoff (Ic=0) or Saturation (Vce=0) only
• Cutoff Vbe<0.7V, Ic=0, hence Pdiss (=VxI) =0
• Saturation Vbe>0.7V, Vce=0, hence Pdiss (=VxI) =0
• Avoids wasting power in transistor and heating it up
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Summary CUTOFF & SATURATION
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Basic NPN Switch Circuit
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Ex1
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Ex2
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Digital Logic to switch Loads
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PNP logic switch
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Higher Current Gain needed - Darlington
• If DC current gain too low to switch load, and multiply current gains of 2 BJTs in ‘Darlington’ configuration
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Switching Transistor Summary
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