Chapter 17Chapter 17
Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall
Chapter 17
Chapter 17Chapter 17
Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall
Summary
Bipolar junction transistors (BJTs)
The BJT is a transistor with three regions and two pn junctions. The regions are named the emitter, the base, and the collector and each is connected to a lead.There are two types of BJTs – npn and pnp.
n
np
p
pn
E (Emitter)
B (Base)
C (Collector)
E
B
C
Separating the regions
are two junctions.
Base-Collector junction
Base-Emitter junction
Chapter 17Chapter 17
Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall
Summary
BJT biasing
For normal operation, the base-emitter junction is forward-biased and the base collector junction is reverse-biased.
npn
BE forward- biased
BC reverse- biased
For the npn transistor, this condition requires that the base is more positive than the emitter and the collector is more positive than the base.
+
+
-
-
Chapter 17Chapter 17
Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall
Summary
BJT currents
A small base current (IB) is able to control a larger collector current (IC). Some important current relationships for a BJT are:
C DC EαI I
C DC BβI II
I
I
IB
IE
IC
Chapter 17Chapter 17
Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall
Summary
Voltage-divider bias
Because the base current is small, the approximation2
B CC1 2
RV V
R R
is useful for calculating the base voltage.
R1
R2
RC
RE
VB
VE
After calculating VB, you can find VE by subtracting 0.7 V for VBE.
Next, calculate IE by applying Ohm’s law to RE:
C EI IThen apply the approximation
Finally, you can find the collector voltage from C CC C CV V I R
VC
EE
E
VI
R
Chapter 17Chapter 17
Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall
Summary
Voltage-divider bias
Calculate VB, VE, and VC for the circuit.
2B CC
1 2
6.8 k15 V =
27 k + 6.8 k
RV V
R R
R1
R2
RC
RE
VE = VB 0.7 V =
C E 2.32 mAI I
C CC C C 15 V 2.32 mA 2.2 kV V I R
EE
E
2.32 V 2.32 mA
1.0 k
VI
R
27 k
6.8 k 1.0 k
2.2 k
+15 V
2N3904
3.02 V
2.32 V
9.90 V
Chapter 17Chapter 17
Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall
Summary
The BJT as a switch
BJTs are used in switching applications when it is necessary to provide current drive to a load.
In cutoff, the input voltage is too small to forward-bias the transistor. The output (collector) voltage will be equal to VCC.
In switching applications, the transistor is either in cutoff or in saturation. RC
VCC VCC
RC
VOUTIIN = 0 = VCC
IIN > IC(sat)/DC When IIN is sufficient to saturate the transistor, the transistor acts like a closed switch. The output is near 0 V.
Chapter 17Chapter 17
Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall
Bipolar junction transistor (BJT)
Class A amplifier
Saturation
Selected Key Terms
An amplifier that conducts for the entire input cycle and produces an output signal that is a replica of the input signal in terms of its waveshape.
A transistor with three doped semiconductor regions separated by two pn junctions.
The state of a transistor in which the output current is maximum and further increases of the input variable have no effect on the output.
Chapter 17Chapter 17
Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall
Cutoff
Q-point
Amplification
Common-emitter (CE)
Class B amplifier
The dc operating (bias) point of an amplifier.
Selected Key Terms
A BJT amplifier configuration in which the emitter is the common terminal.
The non-conducting state of a transistor.
An amplifier that conducts for half the input cycle.
The process of producing a larger voltage, current or power using a smaller input signal as a pattern.
Chapter 17Chapter 17
Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall
Junction field-effect transistor
(JFET)
MOSFET
Depletion mode
Enhancement mode
Metal-oxide semiconductor field-effect transistor.
A type of FET that operates with a reverse-biased junction to control current in a channel.
Selected Key Terms
The condition in a FET when the channel is depleted of majority carriers.
The condition in a FET when the channel has an abundance of majority carriers.
Chapter 17Chapter 17
Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall
Common-source
Oscillator
Feedback
A circuit that produces a repetitive waveform on its output with only a dc supply voltage as an input.
An FET amplifier configuration in which the source is the common terminal.
Selected Key Terms
The process of returning a portion of a circuit’s output signal to the input in such a way as to create certain specified operating conditions.
Chapter 17Chapter 17
Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall
Quiz
1. The Thevenin circuit shown has a load line that crosses the y-axis at
a. +10 V.
b. +5 V.
c. 2 mA.
d. the origin.
+10 V
5.0 k
Chapter 17Chapter 17
Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall
Quiz
2. In a common-emitter amplifier, the output ac signal will normally
a. have greater voltage than the input.
b. have greater power than the input.
c. be inverted.
d. all of the above.
Chapter 17Chapter 17
Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall
Quiz
3. In a common-collector amplifier, the output ac signal will normally
a. have greater voltage than the input.
b. have greater power than the input.
c. be inverted.
d. have all of the above.
Chapter 17Chapter 17
Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall
Quiz
4. The type of amplifier shown is a
a. common-collector.
b. common-emitter.
c. common-drain.
d. none of the above.R1
R2RE
C1
VCC
Chapter 17Chapter 17
Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall
Quiz
5. A major advantage of FET amplifiers over BJT amplifiers is that generally they have
a. higher gain.
b. greater linearity.
c. higher input resistance.
d. all of the above.
Chapter 17Chapter 17
Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall
Quiz
6. A type of field effect transistor that can operate in either depletion or enhancement mode is an
a. D-MOSFET.
b. E-MOSFET.
c. JFET.
d. none of the above.
Chapter 17Chapter 17
Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall
Quiz
7. For an FET, transconductance is the ratio of
a. drain voltage to drain current.
b. gate-source voltage to drain current.
c. gate-source current to drain voltage.
d. drain current to gate-source voltage.
Chapter 17Chapter 17
Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall
Quiz
8. A transistor circuit shown is a
a. D-MOSFET with voltage-divider bias.
b. E-MOSFET with voltage-divider bias.
c. D-MOSFETwith self-bias.
d. E-MOSFET with self bias.RD
+VDD
R1
R2
Chapter 17Chapter 17
Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall
Quiz
9. A Colpitts or Hartley oscillator both have
a. positive feedback.
b. amplification.
c. a closed loop gain of 1.
d. all of the above.
Chapter 17Chapter 17
Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall
Quiz
10. If you were troubleshooting the circuit shown here, you would expect the gate voltage to be
a. more positive than the drain voltage.
b. more positive than the source voltage.
c. equal to zero volts.
d. equal to +VDD
RD
+VDD
R1
R2
Chapter 17Chapter 17
Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall
Quiz
Answers:
1. c
2. d
3. b
4. a
5. c
6. a
7. d
8. b
9. d
10. b
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