chapter 17 electronics fundamentals circuits, devices and applications - floyd © copyright 2007...

Chapter 17Chapter 17

Electronics FundamentalsCircuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall

Chapter 17



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



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.

+

+

-

-



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



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



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



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.



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.



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.



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.



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.



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



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.



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.



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



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.




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.



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.



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



Quiz

9. A Colpitts or Hartley oscillator both have

a. positive feedback.

b. amplification.

c. a closed loop gain of 1.




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



Quiz

Answers:

1. c

2. d

3. b

4. a

5. c

6. a

7. d

8. b

9. d

10. b

chapter 17 electronics fundamentals circuits, devices and applications - floyd © copyright 2007...

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