Download Operating point of a transistor

Monday 11/1/2010
Electronics circuit I lab
Section : A
Fourth report.
Written By:
Raed Suleiman
Group:
Abbass Taher
Raed Suleiman
Tarek Fawaz.
Instructor Dr: Nahida Abdallah
Fall 2009-2010
Equipments:
-Com3lab.
-Pc.
-Connecting wires.
Objectives:
-To know the characteristics of circuits containing transistors.
- To know the control characteristics and power dissipation with various
types of configuration in AC or DC.
-To know the load line characteristics.
-To know the voltage and current gains along with power dissipation and
phase difference in AC analysis using the common emitter
I)Operating point of a transistor :
To determine the operating point of a transistor consider the following circuit
configuration:
When we draw the load line where Q is moving here (having R2 as a variable resistor)
, the load line in this voltage divider circuit will look like this :
Here the drawing shows Ic vs Uce where Q is located , the best place for Q is the
middle of the line , as i moves towards Ic it is reaching saturation and towards Uce it
is reaching cutoff.
Transistor in common emitter circuit \ emitter
amplifier :
The general configuration of the common emitter circuit is simple as follows :
The emitter is at the ground and there is no resistor at the emitter here in this case.
The emitter is grounded with respect to the alternating voltage.
-Now we study the AC analysis of common emitter circuit , the capacitors will act as
short.
Consider the following voltage divider circuit with common emitter :
The main aspects of transistor circuits is :
1) Input and output impedance
2) Current and voltage gain
3) Power dissipation
This circuit (with AC generator and voltmeters set as shown) will allow us to
measure the voltage gain :
Here we have the values of U2 and U2 as shown and we can calculate the voltage
gain of this transistor by dividing them.
The input impedance is R2 and the output is R4.
-Now for the current gain ,we do the following circuit:
This configuration allows us to measure the current gain .
NB : when one of the bypass capacitor is connected , R4 will have no effect and so
affecting the whole current and voltage gains of the circuit.
-For power dissipation calculation we connect the same circuit to an oscilloscope as :
Here we can calculate the power dissipation and observe the phase shift under AC
current.
As a result, the AC power dissipation is negative and reduces the DC power
dissipation.
Conclusion:
-In DC analysis of common emitter circuits , the operating point Q is fixed
somewhere on the load line and its best to be in the middle.
-In AC analysis , Q will oscillate along the load line so the circuit components must
be good chosen to capture Q in good oscillation region away from cutoff or
saturation.
-At the operating point, the alternating current is prevented from lowering the base
emitter voltage beyond the forward threshold.
-The operating point is located on the slop of the collector and/or emitter resistance in
the output characteristic quadrant.
-to prevent a rise in temperature from shifting the operating point, this point is
stabilized by the emitter resistance.
-What important in transistor circuits is the voltage and current gains in it and its
power dissipation inside the transistor , they determine the function of the circuit .
-Similar techniques are used to analyze the common base circuit and other types of
transistor circuits.
- The AC power dissipation is negative and reduces the DC power dissipation.