Common Emitter Amplifier Flashcards
This is a Common Emitter amplifier because the input is applied to the _____, the output is taken from the _____, and the _____ is common.
This is a Common Emitter amplifier because the input is applied to the base, the output is taken from the collector, and the emitter is common.
What is common about the emitter in the Common Emitter Amplifier?
The amplifier has two basic circuits:
- Input - Base to Emitter
- Output - Emitter to Collector
The emitter is common to both circuits.
Would change in one circuit affect the other circuit? Why or why not?
Yes
A changing input signal changes the bias voltage on the base of the transistor. When the input signal increases, the bias voltage increases, causing current flow in the emitter collector circuit to increase.
Reversed if input signal decreases.
What effects does a changing input signal have?
An increasing input signal produces a decreasing output signal.
A decreasing input signal produces an increasing output signal.
Transistor bias is determined by _____?
R1, R2, R3, R4, and C1.
When an input signal is applied to the base of Q1, it’s combined with the fixed bias voltage established by R1 and R2. R1 and R2 form a voltage divider that is operated by +Vcc.
Why is this voltage divider important?
A voltage divider reduces the voltage applied to the base of Q1.
Without R2 when Vcc is first applied, there is no current flow yet and no voltage drop across R1. This allows the FULL VALUE of Vcc to be applied to the base of Q1, which could damage the transistor.
With R2, current flows through R2 and R1 even when Q1 is in cutoff. Base voltage (VB) will be significantly less than Vcc, thus protecting the B-E junction from high voltage and stabilizing VB.
What is the purpose of R4 and C1?
Resistor R4 and bypass capacitor C1 provide self biasing or stability for the circuit.
Normal operation of the amplifier produces heat. Heat causes an unpredictable change at the output. R4 provides temperature stability by decreasing emitter current when current increases due to heat.
C1 provides a path for any unwanted AC that is produced. This ensures that the output signal is not affected.
What is the purpose of R3?
R3 determines output signal.
When the emitter to collector current increases, more voltage drops across R3, and the output signal decreases.
When current decreases, less voltage is dropped across R3, and the output signal increases.
What is the purpose of C2 and C3 when added to the amplifier circuit?
C2 is an input coupling capacitor that prevents any DC component of the input from reaching the base of the transistor.
C3 is an output coupling capacitor that prevents the DC bias on the collector from reaching the output.
What are needed to identify normal operation of a common emitter amplifier?
Class of Operation
Voltage Gain (Av)
What determines Class of Operation?
Class of operation is determined by the bias placed on the base of Q1 by R1 and R2.
What determines the Gain?
The gain is determined by emitter resistor R4 and collector resistor R3.
If class of operation and voltage gain are known, normal operation is determined by what?
Determined by comparing input and output signals.
For example, this is a class A common emitter with a gain of 100. The input to output comparison shows normal operation.
Which component(s) establish base bias?
R1 and R2
Which component(s) establish emitter bias?
R4 and C2
In common emitter circuits, as collector current increases, the voltage on the collector _______.
decreases
Which component(s) establish collector bias?
R3
