1. Bipolar Junction Transistors
A gentle intro
Bipolar Junction Transistors

2. The invention that changed everything…
The real thing
Circuit diagram symbol

3. BJT’s animated

4. What transistors are good for
~731M transistors in Intel core i7

5. EWH kit

6. Transistor action: BJT
BJT = Bipolar Junction Transistor
Goal: control current flow from collector to emitter by input
signal at base
Fluid Analogy:
Small signal controls a much larger signal

7. Another transistor analogy
The gate has to swing open for horses to start running around the track.
Base (B) has to ‘open’ the electrons to flow from E to C = conventional current flow from C to E

8. Transistor action: electron flow-
Emitter - P
For conventional current flow (tracking
positive charge movement), flip the arrows! -
Base - - -
Collector N

9. Bipolar Junction Transistor
BE: fwd bias to get e- flowing
BC: rev bias to vacuum up e- making the journey through BJT
𝐼 𝐶 =𝛽 𝐼 𝐵
Current amplification:

10. Base-Emitter (BE) Characteristic (just a pn diode!)IB
Base-Emitter (BE) Characteristic (just a pn diode!)
DIB
Transistor ON  Fwd biased diode
Working on the steep part of the curve
(blue dotted box highlights normal operating range)
DVBE small
DIB relatively large
* Rule of thumb: use VBE ~ 0.7V when designing BJT circuits
DVBE

11. Transistor as Switch
Collector-Emitter (CE) Characteristic Curves

12. BJT LED switch design example
1. KVL BE “small loop”
0 = VBB – IBRB – VBE
2. KVL CE “big loop”:
0 = VCC – VLED – ICRL – VCE
𝐼 𝐶 = (𝑉 𝐶𝐶 − 𝑉 𝐿𝐸𝐷 ) 𝑅 𝐿
𝑉 𝐶𝐸 = 𝑉 𝐶𝐶 − 𝑉 𝐿𝐸𝐷
VCC = 9 V VBB = 5V RL = ? W RB = ? W VLED = 3.5 V (blue) Design: ILED ~ 20 mA

13. Output Characteristic
Saturation
Output Characteristic
Active region (light blue):
Ic = bIB
Ic ~independent of VCE
Saturation (yellowish):
Ic depends on VCE and IB
Digital switch applications
0.1 – 0.3V

14. Field Effect Transitors (FETs)
Field Effect Transistor
JFETs (Junction Field Effect Transistors
MOSFETs (Metal Oxide Semiconductor FET)

15. What’s the difference between an Ohm and Coulomb?

16. FET Gallery
Image credit:

17. Image credits
Prasad: EIS paper: