BJT AC MODELS
CONTENT
a. BASE BIASED AMPLIFIER
b. EMMITER BIASED AMPLIFIER
c. SMALL SIGNAL OPERATION
d. AC BETA & AC RESISTANCE OF EMMITER DIODE
e. 2 TRANSISTOR MODELS & ANALYZING AN AMPLIFIER
f. H- PARAMETERS AND RELATIONS BETWEEN R AND H PARAMETERS
COUPLING CAPACITOR
• Block DC and allows AC.
• Allows ac signal to be coupled to the transistor without disturbing Q-point
• CC to work properly Xc should be least at lowest freq of the ac signal
• 10:1 rule to follow
CONCLUSION
• For DC analysis the capacitor is open circuit
• For AC analysis the capacitor is short circuit
Q.1. FOR THE CIRCUIT FIGURE SHOWN FIGURE FIND THE VALUE OF C TO ACT
AS A GOOD COUPLING CAPACITOR IF R=2KΩ AND THE FREQ RANGES
FROM 20 HZ TO 20 KHZ.
Q.2. CALCULATE THE VALUE OF C IF THE LOWEST FREQ IS
1KHZ AND R = 1.6 KΩ
EXPLANATION WRT CC AND Q-POINT (DC CIRCUIT)
• DC CIRCUIT:
• 𝐼𝐵 = 30uA
• WITH 𝛽𝐷𝐶 = 100
• 𝐼𝐶 = 3mA
• 𝑉𝐶 = 15 V
• Q POINT (𝐼𝐶 = 3mA , 𝑉𝐶 = 15 V)
AMPLIFIER CIRCUIT
• C BLOCKS DC SO 𝐼𝐵 = 30uA
• C BLOCKS DC SO 𝑉𝐶 = 15 V
• AC VTG SOURCE OF 100uV
• ADD AN AC CURRENT TO THE DC BASE CURRENT
• AC 𝐼𝐵 INCREASE THE AC 𝐼𝐶 DUE TO 𝛽
• SO THIS IS SUPERIMPOSED ON DC 𝐼𝐶
• THIS FLOWS THROUGH 𝑅𝐶 PRODUCING A VARYING VOLTAGE
OUT OF
PHASE
VOLTAGE WAVEFORM OF BB AMPLIFIER
VTG GAIN OF AMPLIFIER
𝑣𝑜𝑢𝑡
• 𝐴𝑣 = 𝑣𝑖𝑛
EMITTER BAISED AMPLIFIER
• DUE TO UNSTABLE Q POINT THE BASE BIASED AMPLIFIER IS NOT USED
• INSTEAD EMITTER BIASED AMPLIFIER WITH TSEB OR VDB IS PREFERRED
BYPASS CAPACITOR
• WHEN FREQ IS HIGH C SHORT CIRCUITS AND CONNECTS
THE POINT E TO THE GROUND.
• THIS ALLOWS AN AC GROUND TO BE CREATED WITHOUT
DISTURBING Q POINT OF THE AMPLIFIER NOTE : R IS THEVENIN
Q 1.WHAT IS THE VALUE OF “C” TO EFFECTIVLY SHORT
POINT “E” TO THE GROUND. FREQ OF THE V IS 1 KHZ.
Q 2. REPEAT FOR R= 50 Ω
VDB AMPLIFIER
TSEB AMPLIFIER
SMALL SIGNAL OPERATION
• Small signal refers to input signal (Vbe)whose magnitude is much less than the
thermal voltage Vt
• When a small signal is coupled to the base of the transistor an ac signal appears
across the base emitter diode as shown in the graph.
• INSTANTANEOUS OPERATING POINT
• DISTORTION
• REDUCING DISTORTION WHY IS IT IMPORATNT ?
THE 10 % RULE
• Total emitter current =
• AMPLIERS THAT SATISFY THE 10 % RULE ARE
REFERED TO AS SMALL SIGNAL AMPLIFIER
Q. FOR THE CIRCUIT SHOWN IN FIGURE FIND THE
MAX SMALL SIGNAL EMITTER CURRENT.
AC BETA
• 𝛽𝐷𝐶 = 𝐼𝐼𝐶 DC CURRENT GAIN
𝐵
𝑖𝑐
• β= 𝑖 ac current gain
𝑏
• ℎ𝐹𝐸 Data sheet notation for dc current gain
• ℎ𝑓𝑒 Data sheet notation for ac current gain
AC RESISTANCE OF EMITTER
DIODE
• Fig shows a graph of emitter current vs emitter voltage
• Applying of a small ac voltage across emitter diode produces an emitter
current
• The size of the emitter current depends on the Q-point on the graph
• Total emitter current
• Total base emitter voltage
• The ac resistance of emitter diode is
• “ ‘ ” is an indicator that the resisitance is inside the transistor.
Q. What is the value of re’”for the circuit shown.
Q. What is the value of re’”for the circuit shown.
Q. What is the value of re’ for the circuit shown.
2 TRANSISTOR MODELS
& ANALYZING AN AMPLIFIER
• To analyse the ac operation of a transistor its ac equivalent circuit is required
T MODEL:
• One of the earliest ac model known as Ebers Moll model.
• W.r.t small ac signal the emitter diode acts as an ac resistance re’
• Collector diode acts as a current source “𝑖𝑐 ”
• As the Ebers Moll model like a T the equivalent
circuit is called as T MODEL
INPUT IMPEDEANCE AT BASE :
• Ac input signal drives the transistor
• This create the ac base emmiter voltage at the
emitter diode
• It generate ac base current 𝑖𝑏
• This causes the amplifier to work properly
• Looking at the base of the transistor the ac voltage source see
• At lower freq the impedance is purely resistive
𝜋 MODEL OF TRANSISTOR
• More easy to use
• Both are ac model of the transistor and can be use for
analysis
• Mostly the 𝜋 model will be used.
ANALYZING TRANSISTOR
• Transistor analysis is lttle difficult as both ac and dc sources are in the same
circuit.
• So SUPERPOSITION THEOREM is used to analyze
• First the effect of dc source is calculated and then the effect of ac source is
calculated
• Result of both is then added
1. DC CIRCUIT ANALYSIS (DC EQUIVALENT CIRCUIT)
• Analysis as per the analysis of the dc circuit
• Most imp is the emitter current as it is needed for ac ananlysis
2. AC EFFECT OF A DC VOLTAGE SOURCE
AC EQUIVALENT CIRCUIT
• For analysis of ac equivalent circuit all the capacitor and voltage source are
replaced by short circuit
• Transistor is replace by its 𝜋 or T model
BASE BIASED AMPLIFIER
• VDB AMPLIFIER
• TSEB AMPLIFIER
