ELT136 Transistor Basics
HO-03 Loadlines
Several types of amplifiers can be designed when the transistor is biased correctly. A
class A amplifier is one such type. Class C and class B amplifiers will be discussed
later. A Load Line graph is a GREAT illustration tool used for design and analysis of
amplifiers. This handout will discuss the process of establishing a load line graph.
Definitions:
1. Vcc is the potential difference of the power source. In this example, 10 volts =
Vcc
2. Vc is the potential difference of the collector with respect to ground. One way to
find this is Vc = Vcc – IcRc
3. Ve is the potential difference of the emitter with respect to ground. This is the
voltage drop across RE.
4. Vce is the voltage drop across the transistor from the collector and the emitter.
This is found by the equation Vcc - Vc – Ve.
Steps in solving for VCE.
1. Solve for (I) through R1 and R2.
2.
3.
4.
5.
6.
Vcc
=I
( R1  R 2)
Solve for VB = I x R2. (Voltage Divider)
Solve for VE using VB – VBE = VE. (Note that VBE is equal to the junction diode d
rop across the PN junction of the base and emitter.)
Find IE using Ohms Law . IE = VE/RE
Understand that IE is equal is equal to IC (more or less), find VC using VC = ICRC .
Now, understanding that Vcc = VRE + VRC + VCE, you can solve for VCE.
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ELT136 Transistor Basics
HO-03 Loadlines
A “Class A” amplifier is designed to bias the Q-point to a point on the load line
that is midway between 0V and Vce(cutoff).
IC
Isat
Q
IB
VCE
Vcutoff
Finding Saturation (When Vce = 0 volts).
1. The process is simple. Analyze the circuit when CE part of the transistor is
represented as a short. Find the current through Rc and Re by taking:
Vcc
 I ( sat )
( Rc  Re)
note: when we apply RLoad things may change.
Finding Cutoff (When Ic = 0 amps)
1. The process is also simple. Analyze the circuit when CE part of the transistor is
represented as an open. In this circuit Vce(cutoff) is Vcc-V(Re).
note: when we apply RLoad things may change.
At this point, Ic(sat) and Vce(cutoff) can now be plotted on the load line graph
and the load line drawn between them (see graph above).
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