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

Remember PN Diode 2 © Unitec New Zealand

Now we combine 2 diodes 3 © Unitec New Zealand

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 4 © Unitec New Zealand

3 Ways to Configure BJT –CE most common 5 © Unitec New Zealand

Summary of Basic Config 6 © Unitec New Zealand

Amplify AC signal or Switch (On/Off) 7 © Unitec New Zealand

NPN construction, Symbol & Connection 8 © Unitec New Zealand

NPN ‘biasing’ and DC current gain 9 © Unitec New Zealand

Current Controlled Amplifier 10 © Unitec New Zealand

Current Gain examples 11 © Unitec New Zealand

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 12 © Unitec New Zealand

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 13 © Unitec New Zealand

NPN transistors conduct when Vc>>Vce and Vb>Ve © Unitec New Zealand

PNP opposite to NPN Vb is Ve-0.7V 15 © Unitec New Zealand

PNP circuit – opposite polarity voltages 16 © Unitec New Zealand

NPN & PNP complementary Class B Amp NPN conducts + half of input AC, PNP does – half 17 © Unitec New Zealand

Distinguishing PNP & NPN with meter 18 © Unitec New Zealand

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 19 © Unitec New Zealand

Transistor Biasing 20 © Unitec New Zealand

Transistor Biasing 21 © Unitec New Zealand

Transistor Biasing 22 © Unitec New Zealand

Transistor Biasing 23 © Unitec New Zealand

Transistor Biasing 24 © Unitec New Zealand

Use BJT as SWITCH instead of Amplifier Instead of Biasing in linear active region, stick to shaded areas 25 © Unitec New Zealand

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 26 © Unitec New Zealand

Summary CUTOFF & SATURATION 27 © Unitec New Zealand

Basic NPN Switch Circuit 28 © Unitec New Zealand

Ex1 29 © Unitec New Zealand

Ex2 30 © Unitec New Zealand

Digital Logic to switch Loads 31 © Unitec New Zealand

PNP logic switch 32 © Unitec New Zealand

Higher Current Gain needed - Darlington If DC current gain too low to switch load, and multiply current gains of 2 BJTs in ‘Darlington’ configuration 33 © Unitec New Zealand

Switching Transistor Summary 34 © Unitec New Zealand