Lecture 07 DC and AC Load Line. DC biasing circuits DC and AC equivalent circuit Q-point (Static operation point) DC and AC load line Saturation Cutoff Condition Compliance. Book Reference. Electronic Devices and Circuit Theory by Robert Boylestad & Louis Nashelsky ( Prentice Hall )
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DC biasing circuits
DC and AC equivalent circuit
Q-point (Static operation point)
DC and AC load line
Saturation Cutoff Condition
Compliance
and AC Load Line
Electronic Devices and Circuit Theory by Robert Boylestad & Louis Nashelsky ( Prentice Hall )
Electronic Devices by Thomas L. Floyd ( Prentice Hall )
DC and AC Load Line
DC and AC Load Line
To turn the device“ON”
To place it in operation in the region of its characteristic where the device operates most linearly, i.e. to set up the initial dc values of IB, IC, and VCE
DC and AC Load Line
The base-emitter junction of the transistor is forward biased through the voltage divider circuit set up by RB1 and RB2
(RE stabilizes the DC signals against variations in HFE)
Therefore RE is essential for DC biasing purposes
But the inclusion of RE limits the available AC voltage swing (max possible voltage swing will be limited to VCC-VE)
The bypass capacitor C3 is used to shorten RE in front of the AC signal while not affecting the DC bias conditions since it appears as an open circuit in front of DC bias
DC and AC Load Line
(1)
DC and AC Load Line
What is IC(sat) and VCE(off) ?
DC and AC Load Line
EE3110 DC and AC Load Line
DC and AC Load Line
When an ac signal is applied to the base of the transistor, IC and VCE will both vary around their Q-point values.
When the Q-point is centered, IC and VCE can both make the maximum possible transitions above and below their initial dc values.
When the Q-point is above the center on the load line, the input signal may cause the transistor to saturate. When this happens, a part of the output signal will be clipped off.
When the Q-point is below midpoint on the load line, the input signal may cause the transistor to cutoff. This can also cause a portion of the output signal to be clipped.
DC and AC Load Line
DC and AC Load Line
The ac load line of a given amplifier will not follow the plot of the dc load line.
This is due to the dc load of an amplifier is different from the ac load.
DC and AC Load Line
What does the ac load line tell you?
The ac load line is used to tell you the maximum possible output voltage swing for a given common-emitter amplifier.
In other words, the ac load line will tell you the maximum possible peak-to-peak output voltage (Vpp ) from a given amplifier.
(AC Saturation Current Ic(sat) , AC Cutoff Voltage VCE(off) )
DC and AC Load Line
DC and AC Load Line
Bias Circuit
DC equivalent circuit
AC equivalent circuit
DC and AC Load Line
On the other hand the AC load line can be obtained by writing KVL for the circuit shown
ic(ac)rc+vce(ac)=0
Or alternatively
ic(ac)=-vce(ac)/(rc)(2)
DC and AC Load Line
The total collector current in the circuit is composed from the DC and AC components
Ic(total)=ICQ+iC(ac)(3)
The total collector emitter voltage is composed from DC and AC components as well
vce(total)=vCEQ+vce(AC)(4)
If we rearrange (3) and combine it with (2) and (4) we get
Ic(total)-ICQ=-1/RC*[vce(total)-vCEQ](5)
DC and AC Load Line
Equation (5) represent the a linear relation between ic(total) and vce(total) which is the AC load line
When vce(total)=0, then the max collector current
Ic(total)max=ICQ+vCEQ/RC(6)
On the other hand when ic(total)=0, then
vce(total)=vCEQ+RC*ICQ(7)
The AC load line will have a slope of -1/RCwhere RC here is the AC resistance of the amplifier
DC and AC Load Line
In order to achieve the maximum possible AC swing then the Q point has to be in the middle of the AC load line
The above condition is based on the assumption that RB1 and RB2 are adjustable to meet the corresponding IB and VBE bias conditions
If we assume that the Q point is in the center of the AC load line then the max collector current is twice than the DC collector current
Ic(max)=2iCQ(8)
If we replace Ic(max) by Ic(total)max=ICQ+vCEQ/RC Then we may have the following relation
2ICQ=ICQ+VCEQ/RC or
ICQ=VCEQ/rC(9)
DC and AC Load Line
Analytical solution for both ICQ and VCEQ in terms of VCC, rCandREcan be obtained by solving the DC load line equation(10) and (9) i.e.
VCC=VCEQ+ICQ(RC+RE)(10)
VCC=VCEQ+VCEQ(RC+RE)/RC(11)
or
VCC=VCEQ(1+(RC+RE)/RC))(12)
Rearranging (12) we get
VCEQ=VCC(RC/(2RC+RE))(13)
If RC is the AC equivalent resistance and RC+RE is the DC equivalent resistance then
VCEQ=VCC(RAC/(RAC+RDC))
DC and AC Load Line
If we take RAC as a common factor the denominator and the numerator, then
VCEQ=VCC/(1+RDC/RAC)
DC and AC Load Line
Now the ICQ current can be found from (9) and (13) as
ICEQ=VCC (RC/(2RC+RE))*1/RC(14)
OR
ICEQ=VCC/(RAC+RDC)(15)
Graphically the Q point for maximum optimum swing can be found by following these steps
Constructing the DC load line
Drawing the AC load line by determining the points of IC(max)and VCE when IC=0
The operating conditions should be chosen so as not to exceed the maximum collector dissipation
DC and AC Load Line
Refer to the examples in your note book
DC and AC Load Line
When a capacitor is used to connect the load the AC equivalent resistance became as
RAC=RC//RL
The maximum load current can be found from the current divider rule according to
ILmax=Icmax*RC/RL
DC and AC Load Line
When determining the output compliance for a given amplifier, solve both equation (A) and (B). The lower of the two results is the compliance of the amplifier.
DC and AC Load Line
For the voltage-divider bias amplifier shown in the figure, what is the ac and dc load line. Determine the maximum output compliance.
DC and AC Load Line