Unit 10 Common Emitter

Common Emitter Output polarity

is inverted compared to input

Common Emitter VB made more positive on an NPN

current through the transistor increases, current through R3 increases, Voltage drop of R3 decreases, the collector moves farther away from V+

Common Emitter VB made more positive on an PNP

Curren through the transistor decreases, Curren through R3 decreases, Voltage drop of R3 decreases, the collector moves closer to V-

Voltage Gain of Common Emitter

Medium

Current gain of Common Emitter

Medium

Input Impedance of Common Emitter

Medium

Output Impedance of Common Emitter

Medium

Common Emitter R1 and R2 purpose

Act as a voltage divider to set the DC bias of the base. R2 also affects the input impedance of the circuit

• Allows amplification of entire waveform

• Sets 0V at input halfway between saturation and cutoff so that there is equal room for amplification of the positive and negative halves of the input waveform

Common Emitter R3 Purpose

Converts the changing current through the transistor into a changing voltage for the output

Common Emitter R4 Purpose

Solves thermal Runaway and affects the input impedance.

• If current were to increase, then R4 causes voltage at emitter to increase which would decrease current through the transistor

Common Emitter C1 Purpose

• Allows AC (input signal) to pass into circuit

• Blocks DC from getting into circuit and changing DC bias of base set by R1 and R2

• Block DC bias set by R1 and R2 from getting out to what is connected to the input

• is part of input impedance

Common Emitter C2 Purpose

• Blocks DC that is in AC+DC at collector from going out the output

• C2 is part of the output impedanc

when can C1 and/or C2 be left out

if DC coupling is desired or other coupling method is used, such as a transformer

C3

Gives varying current (AC) a separate path from R3

• low reactance path creates no voltage drop, which means no change in emitter voltage

• Should be a large enough value to provide low reactance path from all audio frequencies

What if C3 was missing?

If it is missing emitter voltage will change with varying current through R4 causing a reduction in amplification

All amplifiers in unit 10 are

Class A amplifiers

What is input impedance

a measurement of the path from the + input to ground through the circuit to which the signal is being sent

what is output impedance

a measurement of the path from the + output of the device from which signal is being sent

Both the input and output must only allow AC to pass, not DC.
Describe how we ensure that this happens.  Make sure to describe how it will be done and what components will be used.

A capacitor in series with the input and one in series with the output (C1 and C2). C1 will allow AC to pass into the circuit and block DC from the input. C2 blocks DC that is in the AC+DC at the collector from going out the output. 

What is the purpose of the resistor going from collector to -V in a a common emitter circuit utilizing a PNP transistor?

The purpose of the resistor is to convert the changing current through the transistor into a changing voltage for the output 

When a component with a positive thermal coefficient heats up            current flows.

more

thermal runaway

An increase of temperature on the transistor causes an increase in current flow which causes more heat which then causes more current flow and the cycle repeats itself. 

what are all the components that are apart of input impedance of a common emitter amplifier

forward biased base-emitter junction, R2, R4, C1, C3

C3 should be a            value capacitor.

large

In a common emitter circuit utilizing a transistor, the collector is connected to               .

V+/-, Output

In a common emitter circuit utilizing a transistor, the base is connected to  

V+/-, ground, input

If a common emitter circuit has C3 connected with the + side to ground, a         transistor is being used.

PNP

the input connects to

the base

the output connects to

the collector

ground connects to

the emitter