Unit 10 Common Collector

What is the same as a Common Emitter?

• The input to the circuit is still as the base and the same DC bias is required for the base, so R1 and R2 will set this.

• We need to allow AC to pass into and out of the circuit while not allowing DC to pass, so C1 and C2 will do this as well

What is different?

• the varying current through the collector does not need to be converted into a varying voltage because the output is no longer connected there so there doesn’t need to be a resistor.

• The output is now connected to the emitter so R3 will now be there to convert the changing current into a changing voltage

• There is no thermal runaway issue so C3 and R4 are not needed

What would happen if C3 was left in the circuit?

There would be no varying voltage at the emitter which would mean no audio output

VB is made more positive with NPN

• current through the transistor increases

• current through R3 increases

• Voltage drop of R3 increases

• The emitter moves farther away from ground (it becomes more positive

VB is made more positive with a PNP

• Current through the transistor decreases

• Current through R3 decreases

• Voltage drop of R3 decreases

• The emitter moves closer to ground (it becomes more positive, or less negative)

Voltage Gain

Low

Current Gain

High

Input Impedance (AC)

High

Output Impedance

Low

R1 and R2 purpose

Acts as a voltage divider to set the DC bias of the base and R2 affects the input impedance of the circuit

• Allows amplification of entire waveform

• Sets 0V at input to halfway between saturation and cutoff

R3 Purpose

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

C1

• 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

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

• C1 is a part of the input impedance

• Typically a large enough value that all audio passes without voltage drop

C2

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

• Typically a large enoguh value that all audio passes without voltage drop

• C2 is a part of the output impedance

What is apart of the input impedance?

Forward biased base-emitter junction, R2, R3, C1

What connected to R3 (the resistor that converts a changing current into a changing voltage)

C2, emitter, ground

In a common collector circuit utilizing an NPN transistor, the collector is connected to 

V+

the input connects to the

base

the output connects to the

emitter

V-/+ connects to the

collector

what is the difference between common emitter and collector?

There is no extra resistor at the V+/V-. The collector is connected directly to the voltage source. The output is also connected to the emitter so there is a resistor from the emitter to ground to covert the changing current into a changing voltage. There is no thermal runway so C3 is not needed and if it is included, it can cause problems with the audio at the output.