1 © Unitec New Zealand DE4401 B IPOLAR J UNCTION T RANSISTOR BJT

1© Unitec New Zealand

DE4401

BIPOLAR JUNCTION TRANSISTOR

BJT

Remember PN Diode


Now we combine 2 diodes


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


3 Ways to Configure BJT –CE most common


Summary of Basic Config


Amplify AC signal or Switch (On/Off)


NPN construction, Symbol & Connection


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


Current Controlled Amplifier


Current Gain examples


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


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


Choosing to set load line

• NPN transistors conduct when Vc>>Vce and Vb>Ve+0.7


PNP opposite to NPN Vb is Ve-0.7V


PNP circuit – opposite polarity voltages


NPN & PNP complementary Class B Amp

• NPN conducts + half of input AC, PNP does – half


Distinguishing PNP & NPN with meter


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


Transistor Biasing


Transistor Biasing


Transistor Biasing


Transistor Biasing


Transistor Biasing


Use BJT as SWITCH instead of Amplifier

• Instead of Biasing in linear active region, stick to shaded areas


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


Summary CUTOFF & SATURATION


Basic NPN Switch Circuit


Ex1


Ex2


Digital Logic to switch Loads


PNP logic switch


Higher Current Gain needed - Darlington

• If DC current gain too low to switch load, and multiply current gains of 2 BJTs in ‘Darlington’ configuration


Switching Transistor Summary


Documents

1 © Unitec New Zealand DE4401 B IPOLAR J UNCTION T RANSISTOR BJT