BJT AC and DC Analysis
This slide condenses the two ways analysis of BJT (AC and DC).
At the end of the slide, it has review question answer with answer key as providing.
3. Transistor
ο Developed in December 23, 1947 in
Bell Laboratories
ο By John Bardeen, William Shockley, and
Walter Brattain
ο Basically a reSISTOR that amplifies
electrical impulses as they are
TRANsferred from its input to its
output terminals
4.
5. ο½ E β Emitter
ο½ B β Base
ο½ C - Collector
Two pn Junctions:
ο½ Emitter-Base Junction
ο½ Collector-Base Junction
6. ο It is a three layer semiconductor
device consisting of either two N-
type and one P-type layers of
materials or two P-type and one N-
type layers of semiconductor
materials.
8. Base
ο½ The middle portion which forms two PN junctions between the
emitter and the collector is called the base.
ο½ The base of transistor is thin, as compared to the emitter and is
a lightly doped portion.
ο½
ο½ The function of base is to control the flow of charge carrier.
ο½ 1017 dopants/ cm3
ο½ Moderately doped
9. Emitter
οThe portion on one side of transistor that supplies
charge carriers (i.e electron or holes) to the other
two portions
ο1019 dopants/ cm3
οHeavily doped
Collector
ο the portion on the other side of the transistor (i.e. the side
opposite to the emitter) that collects the charge carriers
(i.e. electrons or holes).
ο 1015 dopants/ cm3
ο Lightly doped
ο½ The doping level of the collector is in between the heavily
doping of emitter and the light doping of the base.
18. Alpha (Ξ±)
β¦ In the dc mode, the levels of IC and IE due to majority
carriers are related by a quantity called alpha and
defined by the following equation:
19.
20. ο Common Emitter Configuration
V
i
V
o
I
c
I
e
I
b
R
E
R
B
VB
B
VCC
ο The emitter is common
to both input (base-
emitter) and output
(collector-emitter).
ο The input is on the base
and the output is on the
collector.
21.
22.
23. Beta (Ξ²)
ο§ the ratio of collector current to the base current .
ο§ represents the amplification factor of a transistor. ( is
sometimes referred to as hfe, a term used in transistor
modeling calculations)
28. ο½ Common Collector Configuration
Vi
Vo
Ic
Ie
Ib
RE
RB
VBB
VCC
The input is on the base and the
output is on the emitter.
29.
30.
31. Transistor Testing
Checking the forward-
biased base-to-emitter
junction of an npn
transistor
Checking the reverse-
biased base-to-
collector junction of an
npn transistor.
What will happen to our
resistance?
Lower
Resistance
Higher
Resistance
32. Gamma (Ξ΄)
β¦ the ratio of collector current to the base current .
Ξ΄ =
Ie
Ib
33. Characteristic Common Base
Common
Emitter
Common
Collector
Power Gain moderate highest moderate
Voltage Gain highest moderate
Lowest (less
than 1)
Current Gain
lowest (less
than1)
moderate highest
Input
Impedance
lowest moderate highest
Output
Impedance
highest moderate lowest
Phase
Inversion
none
180o out of
phase
none
Application RF amplifier universal isolation
34. Bias
ο§ An electrical, mechanical or magnetic
force applied to a device to establish a
desired electrical or mechanical
reference level for its operation.
ο§ Is a DC voltage or current that sets the
operating point for amplifying the AC
signal
36. The DC input establishes an
operating or quiescent point
called the Q-point.
37. The Three States of Operation
Active or Linear Region Operation
ο BaseβEmitter junction is forward biased
ο BaseβCollector junction is reverse biased
Cutoff Region Operation
ο BaseβEmitter junction is reverse biased
Saturation Region Operation
ο BaseβEmitter junction is forward biased
ο BaseβCollector junction is forward biased
38. DC Biasing Circuits
ο Fixed-bias circuit
ο Emitter-stabilized bias circuit
ο Collector-emitter loop
ο Voltage divider bias circuit
ο DC bias with voltage feedback
44. Self Bias
ο The amplifier produces its own DC voltage
from an IR drop across a resistor in the return
circuit of the common terminal.
ο Self bias is probably the type of bias used
most often because it is economical and has
stabilizing effect on the DC level of the output
current.
ο Can be emitter stabilized or collector
stabilized.
45. What is the purpose of adding Re β Resistor @
Emitter?
Adding a resistor (Re) to the emitter circuit
stabilizes the bias circuit
46. Improved Biased Stability
Stability refers to a circuit condition in which the currents
and voltages will remain fairly constant over a wide range
of temperatures and transistor Beta values.
61. Active region
ο§ Base-emitter junction is forward biased
and the collector-base junction is reversed
biased.
ο§ Transistorβs active operation as an
amplifier.
Saturation region
ο§ Both junctions are forward biased.
ο§ Switch on operation for the transistor.
Cut off region
ο§ Both junctions are reverse biased.
ο§ Switch off operation for the transistor.
62. Loadline
- Is a straight line drawn on the collector
curves between the cut-off and
saturation points of the transistor.
Q-point (Quiescent point )
- Is the operating point of the transistor
with the time varying sources out of the
circuit.
64. ο½ Re model
ο½ Hybrid π πππππ
ο½ Hybrid equivalent model
65. ο½ an equivalent circuit that represents the
AC characteristics of the transistor
ο½ uses circuit elements that approximate the
behaviour of the transistor.
66.
67.
68.
69. New parameters to be considered:
1. Input impedance β defined from the base to ground
2. Input current β as the base current of the transistor
3. Output current β as the current through the load Rc
4. Output voltage β as the voltage from the collector to
ground
5. Output impedance β across your voltage output
70.
71.
72.
73. ππ input impedance is βlooking intoβ the system whereas
π πoutput impedance is the impedance βlooking back intoβ
the system from the output side.
NOTES:
ππ =
π½π
π°π
π π =
π½ π
π° π
SYSTEM
74. The purpose of identifying the important
quantities/parameters is to indicates how output voltage
Vo related to input voltage Vi (Voltage Gain) and Current
Gain
75. 1. Setting all DC sources to zero and
replacing them by a short circuit
equivalent
2. Replacing all capacitors by a short circuit
equivalent
3. Removing all elements by passed by the
short circuit introduced by step 1 and 2
4. Redrawing the network in a more
convenient and logical form
89. ο½ Common Emitter Fixed Bias
ο½ Voltage Divider Bias
ο½ CE Emitter Bias (Unbypassed)
ο½ Emitter Follower
ο½ Common Base
ο½ Collector Feedback
107. ο This is also known as the common-collector configuration.
ο The input is applied to the base and the output is taken from
the emitter.
ο There is no phase shift between input and output.
ο The output voltage is always slightly less than the input signal
due to the drop from base to emitter.
ο Approx. Av = 1
ο Frequently used for IMPEDANCE MATCHING PURPOSES.
- It presents a high impedance at the input and a low impedance
at the output, which is opposite of the standard fixed bias
configuration
116. ο The re model employs a diode
and controlled current source to
duplicate the behavior of a
transistor in the region of
interest.
οNote: BJT transistor amplifiers
are current controlled devices.
117. ο½ βππ, β ππ, β ππ, β ππ
ο½ Term βhybridβ was chose due to its mixture of
variables in each equation
118.
119. ο½ We can determine it by isolating each parameters
and examined the relationship!
120. Set ππ¨ = π(short circuit the
output terminals, Solve for
π‘ ππ)
π‘ ππ is the ratio of the input voltage to the input current with the
output terminals shorted, it is called short-circuit input-
impedance parameter
Isolation:
The subscript 11 defines the fact that
the parameter is determined by a ratio
of quantities measured at the input
terminals.
121. Set ππ’ = π(short circuit the
output terminals, Solve for
π‘ ππ)
π‘ ππ the ratio of the input voltage to the output voltage with the
input current equal to zero., it is open-circuit reverse transfer
voltage ratio parameter
Isolation:
The subscript 12 reveals that the
parameter is a transfer quantity
determined by a ratio of input to
output measurements
122. Set π½ π = π(short circuit the
output terminals, Solve for
π‘ ππ)
π‘ ππ is the ratio of the output current to the input current with the
output terminals shorted, it is short-circuit forward transfer
current ratio parameter
Isolation:
123. Set π° π = π(short circuit the
output terminals, Solve for
π‘ ππ)
π‘ ππ the ratio of the output current to the output voltage, it is the
output conductance parameter and is measured in siemens (S),
it is open-circuit output admittance parameter
Isolation:
130. 1. Draw the DC, re model and h-model analysis in
each config below; and write the parameters of the
approximate hybrid equivalent circuit of the
following:
a. Fixed Bias Configuration
b. Voltage Divider Configuration
c. Emitter follower Configuration
2. The voltage gain, current gain, input impedance and
Output Impedance of a complete hybrid equivalent
model.
133. 1. Which of the following is necessary for a
transistor action
a. the base region must be very wide
b. the base region must be very narrow
c. the base region must be made from insulating
materials
d. the collector region must be heavily doped
134. 2. It is the most stable type of circuit biasing
a. self-bias
b. signal bias
c. voltage-divider bias
d. fixed bias
135. 3. The quiescent state of a transistor implies
a. zero bias
b. no output
c. no distortion
d. no input signal
136. 4. Which of the following device is unipolar?
a. FET
b. BJT
c. Zener diode
d. LED
137. 5. The parameter HFE corresponds to
a. Ξ²DC
b. Ξ²AC
c. rβe
d. rβc
139. 7. What is the current gain of a common base
circuit called?
a. gamma
b. delta
c. bravo
d. alpha
140. 8. The name of the very first transistor
a. diode
b. junction transistor
c. point contact transistor
d. triode
141. 9. Region in a transistor that is heavily doped
a. collector
b. emitter
c. base
d. gate
142. 10. Which are the three terminals of a bipolar
transistor?
a. Cathode, plate and grid
b. Base, collector and emitter
c. Input, output and ground
d. Gate, source and sink
143. 11. A transistor in which n-type and p-type
materials are used is called
a. Unijunction
b. TTL
c. Bipolar
d. FET
144. 12. The region in an electronic transistor that is
lightly doped and very thin is referred to the
a. Collector-base
b. Collector
c. Base
d. Emitter
145. 13. In the BJT schematic symbol, the arrow
a. Points from p-type to n-type
b. Points from north to south
c. Points from n-type to p-type
d. Points from south to north
146. 14. _____ is the region in the transistor that is
heavily doped
a. Collector
b. Ground
c. Base
d. Emitter
147. 15. The arrow in the symbol of a transistor
indicates the direction of
a. Electron current in the collector
b. Donor ion current
c. Electron current in the emitter
d. Hole current in the emitter
148. 16. ____ is the term used to express the ratio of
change in the DC collector current to a
change in base current in a bipolar
transistor
a. Gamma
b. Beta
c. Alpha
d. Delta
149. 17. Solve the collector current if the base current is
200mA and the current gain is 20
a. 10 A
b. 4 A
c. 1 A
d. 40 A
150. 18. The flow of electrons in an NPN transistor
when used in electronic circuits is from
a. Collector to emitter
b. Collector to base
c. Emitter to collector
d. Base to emitter
151. 19. A transistor acts as _____ when saturated
a. Open circuit
b. Very low resistance
c. Very high resistance
d. Variable resistance
152. 20. For a BJT, the BE junction is reverse biased
and BC forward biased. The BJT is in what
operating mode?
a. Forward active
b. Cut-off
c. Reverse active
d. Saturation
153. 21. Line representing all the DC operating
points of the BJT.
a. DC loadline
b. Collector curve
c. AC loadline
d. Operating line
154. 22. What is another name for base bias?
a. Fixed bias
b. Gate bias
c. Emitter bias
d. Beta bias
155. 23. What is the most stable type of biasing
a. Current feedback
b. Fixed bias
c. Voltage divider
d. Voltage feedback
156. 24. The h-parameter hf is a
a. Resistance
b. Reverse voltage gain
c. Conductance
d. Forward current gain
157. 25. Among the common emitter h-parameters,
which is the smallest?
a. hie
b. hre
c. hfe
d. hoe
158. 26. Which of the BJT amplifier configuration has
the highest power gain?
a. CE
b. CC
c. CB
d. Emitter follower
159. 27. Another name for common collector
a. Collector follower
b. Base follower
c. Emitter follower
d. Collector divider
160. 28. Which of the BJT amplifier configuration can
be used as a buffer?
a. CB
b. CS
c. CC
d. CE
161. 29. Which transistor configuration has the
highest input resistance?
a. Common base
b. Common emitter
c. Common collector
d. Common transistor
162. 30. Capacitor used to established an ac ground
at a specific point in a circuit
a. Electrolytic
b. Coupling
c. Bypass
d. Choke
163. 31. ____ is a unipolar semiconductor device
which the current is carried by the majority
carriers only
a. Field-effect transistor
b. Point-contact transistor
c. Zener diode
d. Junction transistor
164. 32. The two types of bipolar transistor are:
a. PN and NP
b. PNP and NPN
c. PPN and NNP
d. N and P
165. 33. The three terminals of a bipolar junction
transistor are called
a. p, n, p
b. n, p, n
c. Input, output and ground
d. Base, emitter and collector
166. 36. The largest region of a bipolar transistor is
the
a. Base
b. Emitter
c. Collector
d. N-region
167. 37. The emitter of the transistor is generally
doped the heaviest because it
a. Has to dissipate maximum power
b. Has to supply the charge carriers
c. Is the first region of the transistor
d. Must posses low resistance
168. 38. In a PNP transistor, the p-regions are
a. Base and emitter
b. Base and collector
c. Emitter and collector
d. None of these
169. 39. During normal operation, the highest
percentage of electrons leaves a NPN
transistor from which region?
a. Base
b. Emitter
c. Collector
d. N-region
170. 40. For operation as an amplifier, the base of an
NPN transistor must be
a. Positive with respect to the emitter
b. Negative with respect to the emitter
c. Positive with respect to the collector
d. 0 V
171. 41. A bipolar transistorβs majority current
carriers are:
a. Electrons
b. Holes
c. Dependent upon the type of transistor
d. Always both electrons and holes
172. 42. In which region is a bipolar transistor
normally operated
a. Saturation
b. Cut-off
c. Linear
d. Beta
173. 43. A transistor has a common base forward
circuit gain hFE=0.98 the DC forward current
gain hFE is
a. 49
b. 50
c. 98
d. Not determinable from the data given
174. 44. When the transistor is fully switched on, it
is to be
a. Shorted
b. Open
c. Saturated
d. Cut-off
175. 45. In which operating region should normal
figures calculated
a. Saturation
b. Breakdown
c. Cut-off
d. Active
176. 46. Which transistor circuit arrangement
produces the highest power gain?
a. Common base
b. Common collector
c. Common emitter
d. A transistorβs power gain is the same in any
circuit
177. 47. The DC loadline of a transistor circuit
a. Has a negative slope
b. Is a curved line
c. Gives graphic relation between IC and IB
d. Does not contain the Q-point
178. 48. The Ξ²DC of a transistor is its
a. Current gain
b. Voltage gain
c. Power gain
d. Internal resistance
179. 49. If in a bipolar junction transistor, Ib = 100
ΞΌA and Ic = 10 mA, what is the value of its
beta?
a. 0.1
b. 10
c. 100
d. None of these
180. 50. If Ic is 50 times larger than Ib then Ξ²DC is
a. 0.02
b. 100
c. 50
d. 500
181. 51. If Ξ²DC is 100, the value of Ξ±dc is
a. 99
b. 0.99
c. 101
d. 0.01
182. 52. The approximate voltage across the
forward-biased base-emitter junction of a
silicon BJT is
a. 0 V
b. 0.7 V
c. 0.3 V
d. Vbb
183. 53. If the output of a transistor amplifier is 5
Vrms and the input is 100 Vrms, the voltage
gain is
a. 5
b. 500
c. 50
d. 100
184. 54. When operated in cut-off and saturation, the
transistor acts like
a. Linear amplifier
b. A switch
c. A variable capacitor
d. A variable resistor
185. 55. The unit of measurement for transconductance
is
a. Ohm
b. Mho
c. Siemens
d. B or C
186. 56. Which circuit produces a voltage gain that is
always less than unity?
a. Common-drain
b. Common-gate
c. Common-source
d. All of the above
187. 57. Which of the following circuits generally has
the greatest gain?
a. Common-source
b. Common-gate
c. Common-drain
d. None of the above
188. 58. In a PNP transistor, the p-regions are
a. Base and emitter
b. Base and collector
c. Emitter and collector
d. Wala lang
189. 59. In cut-off, VCE is
a. 0 V
b. Minimum
c. Maximum
d. Equal to VCC
190. 60. In saturation VCE is
a. 0.7 V
b. Equal to VCC
c. Minimum
d. Maximum
191. 61. To saturate a BJT
a. IB = IC
b. IB > IC(SAT) / Ξ²DC
c. VCC must be at least 10 V
d. The emitter must be grounded
192. 62. Once in saturation, a further increase in
base current will
a. Cause the collector current to increase
b. Not affect the collector current
c. Cause the collector current to decrease
d. Turn the transistor off
193. 63. If the base-emitter junction is open, the
collector voltage is
a. VCC
b. 0 V
c. Floating
d. 0.2 V
194. 64. The maximum value of collector current in a
biased transistor is
a. BDCIB
b. IC(SAT)
c. Greater than IE
d. IE - IB
195. 65. Ideally, a dc loadline is a straight line drawn
on the collector characteristic curves
between
a. The Q-point and cut-off
b. The Q-point and saturation
c. VCE(CUT-OFF)
d. IB = 0 and IB = IC / Ξ²DC
196. 66. If a sinusoidal voltage is applied to the base of
a biased npn transistor and the resulting
sinusoidal collector voltage is clipped near zero
volts, the transistor is
a. Being driven into saturation
b. Being driven into cutoff
c. Operating nonlinearly
d. Answers A and C
197. 67. The disadvantage of a base bias is that
a. It is very complex
b. It produces voltage gain
c. It is too beta dependent
d. It produces high leakage current
198. 68. Emitter bias is
a. Essentially independent of Ξ²DC
b. Very dependent on Ξ²DC
c. Provides a stable bias point
d. Answers A and C
199. 69. The input resistance at the base of a biased
transistor depends mainly on
a. Ξ²DC
b. RB
c. RE
d. Ξ²DC and RE
200. 70. In a voltage-divider biased transistor
circuit, RIN(BASE) can generally be neglected in
calculations when
a. RIN(BASE) > R2
b. R2 > 10RIN(BASE)
c. RIN(BASE) > 10R2
d. R1 << R2
201. 71. In a certain voltage-divider biased npn
transistor, VB is 2.95 V. The dc emitter
voltage is approximately
a. 2.25 V
b. 2.95 V
c. 3.65 V
d. 0.7 V
202. 72. voltage-divider bias
a. Cannot be independent of Ξ²DC
b. Can be essentially independent of Ξ²DC
c. Is not widely used
d. Requires fewer components than all the other
methods
203. 73. In a voltage-divider biased npn transistor, if
the upper voltage-divider resistor (the one
connected to VCC) opens,
a. The transistor goes into cut-off
b. The transistor goes into saturation
c. The transistor burns out
d. The supply voltage is too high
204. 74. A small signal amplifier
a. Uses only a small portion of its loadline
b. Always has an output signal in the mV range
c. Goes into saturation once on each input cycle
d. Is always a common-emitter amplifier
209. 79. The following relationships between alpha
and beta are true except
a. Beta = alpha/ (1- alpha)
b. Alpha = beta/ (beta - 1)
c. Alpha = beta/ (beta + 1)
d. (1- alpha) = 1/ (1 + beta)
210. 80. The circuit that provides the best
stabilization of operating point is
a. Base bias
b. Collector feedback bias
c. Voltage divider bias
d. Emitter feedback bias
211. 81. The smallest of the four h-parameters of
the transistor is
a. Hr and Ho
b. Hi and Ho
c. Hr and Hf
d. Hi and Hf
212. 1. Why does base bias produce such
an unstable Q point?
2. When a transistor is operating in the
active region, why is the collector
considered a current source?