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 ComplianceRef:080314HKNEE3110 DC and AC Load Line1

2. Book Reference Electronic Devices and Circuit Theory byRobert Boylestad & Louis Nashelsky( Prentice Hall ) Electronic Devices by Thomas L. Floyd( Prentice Hall )Ref:080314HKN EE3110 DC and AC Load Line 2 3. DC Biasing Circuits The ac operation of an +VCCamplifier depends on theinitial dc values of IB, IC, andVCE.RC RB By varying IB around an v outinitial dc value, IC and VCEare made to vary around theirinitial dc values. v invce DC biasing is a staticiboperation since it deals withsetting a fixed (steady) level icof current (through thedevice) with a desired fixedvoltage drop across thedevice.Ref:080314HKN EE3110 DC and AC Load Line 3 4. Purpose of the DC biasing circuit To turn the device ON To place it in operation in the region of itscharacteristic where the device operatesmost linearly, i.e. to set up the initial dcvalues of IB, IC, and VCERef:080314HKN EE3110 DC and AC Load Line 4 5. Voltage-Divider Bias The voltage divider (or +VCCpotentiometer) bias circuit is byfar the most commonly used. RC RB1, RB2 R1 voltage-divider to set thev outC1C2value of VB , IB v in C3 R2 to short circuit ac signals to ground, while not effect the DC REC3 operating (or biasing) of a circuit(RE stabilizes the ac signals) Bypass CapacitorRef:080314HKN EE3110 DC and AC Load Line 5 6. Graphical DC Bias Analysis +VCCVCC ICRC VCE IERE = 0 for I C I E 1 VCCICRCIC = VCE +RC + RERC + RER1Point - slope form of straight line equation :y = mx + c IC(sat) = VCC/(RC+RE) R2DC Load LineIEICRE (mA)VCE(off) = VCC VCERef:080314HKNEE3110 DC and AC Load Line6 7. DC Load LineThe straight line is know as the DC load line IC(sat) = VCC/(RC+RE)Its significance is that regardless of the behavior DC Load Lineof the transistor, the collector current IC and the IC (mA)collector-emitter voltage VCE must always lie onVCE(off) = VCCthe load line, depends ONLY on the VCC, RC and VCERE(i.e. The dc load line is a graph that representsall the possible combinations of IC and VCE fora given amplifier. For every possible value ofIC, and amplifier will have a corresponding valueof VCE.)It must be true at the same time as the transistorcharacteristic. Solve two condition usingWhat is IC(sat) and VCE(off) ?simultaneous equationRef:080314HKN Q-point !! and AC Load Line graphically EE3110 DC 7 8. Q-Point (Static Operation Point) When a transistor does not have an ac input, itwill have specific dc values of IC and VCE. These values correspond to a specific point on thedc 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 signalapplied and therefore has constant dc values of ICand VCE.Ref:080314HKNEE3110 DC and AC Load Line8 9. Q-Point (Static Operation Point) The intersection of the dc biasvalue of IB with the dc load linedetermines the Q-point. It is desirable to have the Q-point centered on the load line.Why? When a circuit is designed tohave a centered Q-point, theamplifier is said to be midpointbiased. Midpoint biasing allowsoptimum ac operation of theamplifier. Ref:080314HKN EE3110 DC and AC Load Line9 10. DC Biasing + AC signal When an ac signal is applied to the base ofthe transistor, IC and VCE will both varyaround their Q-point values. When the Q-point is centered, IC and VCEcan both make the maximum possibletransitions above and below their initial dcvalues. When the Q-point is above the center onthe load line, the input signal may causethe transistor to saturate. When thishappens, a part of the output signal will beclipped off. When the Q-point is below midpoint on theload line, the input signal may cause thetransistor to cutoff. This can also cause aportion of the output signal to be clipped. Ref:080314HKNEE3110 DC and AC Load Line 10 11. DC Biasing + AC signalRef:080314HKN EE3110 DC and AC Load Line 11 12. DC and AC Equivalent Circuits +VCC +VCCRCICRCR1 R1RLrC vin vcevinR2 R1//R2 R2IERERErC = RC//RL Bias Circuit DC equivalent AC equivalentcircuit circuitRef:080314HKNEE3110 DC and AC Load Line12 13. AC Load LineIC(sat) = VCC/(RC+RE) The ac load line of a givenamplifier will not follow theDC Load Line ICplot of the dc load line.(mA) VCE(off) = VCC This is due to the dc load ofVCE an amplifier is different fromthe ac load.IC(sat) = ICQ + (VCEQ/rC)ac load line ac load line ICIC Q - point dc load lineVCE(off) = VCEQ + ICQrCVCEVCERef:080314HKNEE3110 DC and AC Load Line13 14. AC Load LineWhat does the ac load line tell you? The ac load line is used to tell you the maximumpossible output voltage swing for a given common-emitter amplifier. In other words, the ac load line will tell you themaximum possible peak-to-peak output voltage (Vpp )from a given amplifier. This maximum Vpp is referred to as the compliance of theamplifier.(AC Saturation Current Ic(sat) , AC Cutoff Voltage VCE(off) )Ref:080314HKNEE3110 DC and AC Load Line14 15. AC Saturation Current and ACCutoff Voltage IC(sat) = ICQ + (VCEQ/rC)ac load line rC ICvin vceR1//R2 VCE(off) = VCEQ + ICQrC VCE rC = RC//RLRef:080314HKN EE3110 DC and AC Load Line 15 16. Amplifier Compliance The ac load line is used to tell the maximumpossible output voltage swing for a givencommon-emitter amplifier. In another words, theac load line will tell the maximum possible peak-to-peak output voltage (VPP) from a givenamplifier. This maximum VPP is referred to as thecompliance of the amplifier. The compliance of an amplifier is found bydetermine the maximum possible of IC and VCE fromtheir respective values of ICQ and VCEQ.Ref:080314HKN EE3110 DC and AC Load Line 16 17. Maximum Possible ComplianceRef:080314HKN EE3110 DC and AC Load Line 17 18. ComplianceThe maximum possible transition for VCE is equal to thedifference between VCE(off) and VCEQ. Since this transition isequal to ICQrC, the maximum peak output voltage from theamplifier is equal to ICQrC. Two times this value will givethe maximum peak-to-peak transition of the outputvoltage:VPP = 2ICQrC(A) VPP = the output compliance, in peak-to-peak voltage ICQ = the quiescent value of IC rC = the ac load resistance in the circuitRef:080314HKN EE3110 DC and AC Load Line 18 19. ComplianceWhen IC = IC(sat)-, VCE is ideally equal to 0V. When IC = ICQ, VCE is atVCEQ. Note that when IC makes its maximum possible transition(from ICQ to IC(sat)), the output voltage changes by an amount equalto VCEQ. Thus the maximum peak-to-peak transition would beequal to twice this value:VPP = 2VCEQ(B) Equation (A) sets the limit in terms of VCE(off). If the value obtainedby this equation is exceed, the output voltage will try to exceedVCE(off), which is not possible. This is called cutoff clipping,because the output voltage is clipped off at the value of VCE(off). Equation (B) sets of the limit in terms of IC(sat). If the valueobtained by this equation is exceed, the output will experiencesaturation clipping.Ref:080314HKN EE3110 DC and AC Load Line 19 20. Cutoff and Saturation Clipping When determining the output compliance for a givenamplifier, solve both equation (A) and (B). The lower ofthe two results is the compliance of the amplifier.Ref:080314HKNEE3110 DC and AC Load Line20 21. Example For the voltage-divider bias amplifiershown in the figure, what is the ac and dcload line. Determine the maximum outputcompliance.+12V R C R 1 4.7k 33 k R L10k = 200 R 210k R E 2.2k Ref:080314HKN EE3110 DC and AC Load Line 21