Design Common Emitter Amp

 Use NPN (Not Pointing iN)  transistor to build a single stage amp

• Voltage Divider  Rb1 28K and Rb2  20K  
• Emitter Degeneration Resistor Re it megates the non-leanarities of the transistor and it works with the bias circuit. 
• Rc and Re are Collector Resistor and Emitter Resistors
• Bias at the middle of the AC load line (Vce =12v   half of Vcc; Ic saturate = 8ma (2x 4ma)
• Common Emitter Amp - when the input goes +ve positive the output goes negative -ve
• Electrolytic Capacitor the +ve positive lead is longer than the -ve negative lead. Also the Stripe painted on the body usually denotes the nagative lead 
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Small Signal Common Emitter Amplifier 

The voltage at the collector will be 1/2 of Vcc  to give the proper headroom for the waveform, if the bias is too high,  it will clip the top of the waveform and if it is too low, it will clip the bottom of the waveform. you will not have headroom for the swing of the output waveform voltage. This is the Q point when the signal is at crossover or the amp is idle. No signal.

Example  (see Diagram below)

Rc  = 1/2 Vcc , the lower the value of Rc the higher the current. eg 4.7K ohm resistor.  so Ic = 1/2 Vcc/4.7K ohms  = 1.37mA

So approx 1.37 mA will flow thru the emitter circuit also. 

Gain =  Rc /Re, so if your gain is 100, and Rc = 4.7K, then 1/10 of Rc = 470 ohms. 

Calculate the Ib = Ic/beta = 1.3mA /100 = 13uA

Voltage divider current is typically 20 x current at Ib, so 20 x 13uA =260uA 

So the sum of the 2 Resistors Rb1 and Rb2 of Voltage divider  = 24volts/260uA = 46 K Ohms

Rb2 = Voltage Drop across Re (470 ohms x 1.3mA)  + Diode junction drop of Base Emitter voltage  (.65volts)  = .61Volts + .65volts = 1.26Volts. 

So Base Emitter Junction voltage is 1.26Volts.

From Ohm's law you can determine the Rb2 Resistor value = 1.26volts/13uA =4.8K Ohms

Now calculate Rb1 = 39K ohms

Capacitor  input is a high pass filter equation

Note Common Emitter Amp - when the input goes +ve positive the output goes negative -ve

Ref: https://www.youtube.com/watch?v=9325TKD4dfY

Transistor: 2N2222A, NPN 

Ref: https://www.youtube.com/watch?v=EVekdy2_Dyw&t=10s

For DC Analysis - 

• Take out all the AC components - Short the AC sources
• Open Circuit all Capacitors - Disconnect source and disconnect load and bypass capacitor
• Simplify Circuit
• Find the operating Points 
• What is the Collector Current Ic   and 
• What is Collector Emitter voltage Vce  

Lets say Beta is 150 for the NPN Transistor 

Rb2 is  < 0.1 x Beta x Re

For AC Analysis

Short ALL DC voltages

Short ALL capacitors

Replace Transistor with an AC modle of Transistor

Simplify Circuit - Combine Resistors

 

Deternine AC Gain, Input Impedeance and Output impedence.

https://www.youtube.com/watch?v=DVLO4YBURSo