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1. and AC Load Line 1 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
2. DC and AC Load Line 2 Book Reference Electronic Devices and Circuit Theory by Robert Boylestad & Louis Nashelsky ( Prentice Hall )
Electronic Devices by Thomas L. Floyd ( Prentice Hall )
3. DC and AC Load Line 3 DC Biasing Circuits
4. DC and AC Load Line 4 Purpose of the DC biasing circuit 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
5. DC and AC Load Line 5 Voltage-Divider Bias 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
6. DC and AC Load Line 6 Graphical DC Bias Analysis
7. DC and AC Load Line 7 DC Load Line
8. Ref:080314HKN EE3110 DC and AC Load Line 8 Q-Point (Static Operation Point) When a transistor does not have an ac input, it will have specific dc values of IC and VCE.
These values correspond to a specific point on the dc load line. This point is called the Q-point.
The letter Q corresponds to the word (Latent) quiescent, meaning at rest.
A quiescent amplifier is one that has no ac signal applied and therefore has constant dc values of IC and VCE.
9. DC and AC Load Line 9 Q-Point (Static Operation Point) The intersection of the dc bias value of IB with the dc load line determines the Q-point.
It is desirable to have the Q-point centered on the load line. Why?
When a circuit is designed to have a centered Q-point, the amplifier is said to be midpoint biased.
Midpoint biasing allows optimum ac operation of the amplifier.
10. DC and AC Load Line 10 DC Biasing + AC signal 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.
11. DC and AC Load Line 11 DC Biasing + AC signal
12. DC and AC Load Line 12 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.
13. DC and AC Load Line 13 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) )
14. DC and AC Load Line 14 AC Saturation Current and AC Cutoff Voltage
15. DC and AC Load Line 15 DC and AC Equivalent Circuits
16. The AC load line equation derivation On the other hand the AC load line can be obtained by writing KVL for the circuit shown
DC and AC Load Line 16
17. The AC load line The total collector current in the circuit is composed from the DC and AC components
The total collector emitter voltage is composed from DC and AC components as well
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 17
18. The 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
On the other hand when ic(total)=0, then
The AC load line will have a slope of -1/RC where RC here is the AC resistance of the amplifier DC and AC Load Line 18
19. DC and AC Load Line 19 The maximum optimum possible swing 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
If we replace Ic(max) by Ic(total)max=ICQ+vCEQ/RC Then we may have the following relation
20. DC and AC Load Line 20 The maximum optimum possible swing Analytical solution for both ICQ and VCEQ in terms of VCC, rC and RE can be obtained by solving the DC load line equation(10) and (9) i.e.
Rearranging (12) we get
If RC is the AC equivalent resistance and RC+RE is the DC equivalent resistance then
21. DC and AC Load Line 21 The maximum optimum possible swing If we take RAC as a common factor the denominator and the numerator, then
22. DC and AC Load Line 22 The maximum optimum possible swing Now the ICQ current can be found from (9) and (13) as
ICEQ=VCC (RC/(2RC+RE))*1/RC (14)
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
23. DC and AC Load Line 23 AC- load line example Refer to the examples in your note book
24. DC and AC Load Line 24 Using an infinte coupling capacitor When a capacitor is used to connect the load the AC equivalent resistance became as
The maximum load current can be found from the current divider rule according to
25. DC and AC Load Line 25 Cutoff and Saturation Clipping 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.
26. DC and AC Load Line 26 Example For the voltage-divider bias amplifier shown in the figure, what is the ac and dc load line. Determine the maximum output compliance.