09/16/2010© 2010 NTUST Today Course overview and information.

09/16/2010 © 2010 NTUST

Today

• Course overview and information

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

Bipolar Junction Transistor (BJT)

BJT

BJT

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.

+

+

For the pnp transistor, this condition requires that the base is more negative than the emitter and the collector is more negative than the base.

pnp

+

+

BJT Biasing

Transistor Biasing

A small base current (IB) is able to control a larger collector current (IC). Some important current relationships for a BJT are:

E C BI I I

C DC EαI I

C DC BβI II

I

I

IB

IE

IC

BJT Currents

Transistor Currents

Transistor Voltages

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

Voltage-divider Bias

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

Voltage-divider Bias

Examples

Amplifier

Collector Characteristic Curves

Collector Characteristic Curves

Collector Characteristic Curves

The collector characteristic curves are a family of curves that show how collector current varies with the collector-emittervoltage for a given IB.

IC

VCE0

IB6

IB5

IB4

IB3

IB2

IB1

IB = 0

The saturation region occurs when the base-emitter and the base-collector junctions are both forward biased.

The curves are divided into three regions:The active regionactive region is after the saturation region. This is the region for operation of class-A operation.

The breakdown regionbreakdown region is after the active region and is is characterized by rapid increase in collector current. Operation in this region may destroy the transistor.

Collector Characteristic Curves

Amplifier

A load line is an IV curve that represents the response of a circuit that is external to a specified load. For example, the load line for the Thevenin circuit can be found by calculating the two end points: the current with a shorted load, and the output voltage with no load.

+12 V

2.0 k

00

4 8 12

2

4

6

I (mA)

V (V)

ISL = 6.0 mA

VSL = 0 V

INL = 0 mA

VNL = 12 V

Load line

Load Lines

The IV response for any load will intersect the load line and enables you to read the load current and load voltage directly from the graph.

+12 V

2.0 k

00

4 8 12

2

4

6

I (mA)

V (V)

Read the load current and load voltage from the graph if a 3.0 k resistor is the load.

3.0 kRL =

IV curve for 3.0 k resistor

VL = 7.2 V IL = 2.4 mA

Q-point

Load Lines

The load line concept can be extended to a transistor circuit. For example, if the transistor is connected as a load, the transistor characteristic

+12 V

2.0 k

00

4 8 12

2

4

6

I (mA)

V (V)

curve and the base current establish the Q-point.

Load Lines

Load lines can illustrate the operating conditions for a transistor circuit.

00

4 8 12

2

4

6

I (mA)

V (V)

If you add a transistor load to the last circuit, the base current will establish the Q-point. Assume the base current is represented by the blue line.

+12 V

2.0 k

For this base current, the Q-point is:

Assume the IV curves are as shown:

The load voltage (VCE) and current (IC) can be read from the graph.

Load Lines

For the transistor, assume the base current is established at 10 A by the bias circuit. Show the Q-point and read the value of VCE and IC.

00

4 8 12

2

4

6

IC (mA)

VCE (V)

+12 V

2.0 k

Bias circuitIB = 5.0 A

IB = 10 A

IB = 15 A

IB = 20 A

IB = 25 A

The Q-point is the intersection of the load line with the 10 A base current.

VCE = 7.0 V; IC = 2.4 mA

Load Lines

When a signal is applied to a transistor circuit, the output can have a larger amplitude because the small base current controls a larger collector current.

00

4 8 12

2

4

6

IC (mA)

VCE (V)

IB = 5.0 A

IB = 10 A

IB = 15 A

IB = 20 A

IB = 25 A For the load line and characteristic curves from the last example (Q-point shown) assume IB varies between 5.0 A and 15 A due to the input signal. What is the change in the collector current?

Reading the collector current, IC varies from 1.2 mA to 3.8 mA.

The operation along the load line is shown in red.

Signal Operation

Signal Operation

The BJT as a switchBJTs 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

= 0 V

When IIN is sufficient to saturate the transistor, the transistor acts like a closed switch. The output is near 0 V.

BJT as a Switch

Examples

Examples

The field-effect transistor (FET) is a voltage controlled device where gate voltage controls drain current. There are two types of FETs – the JFET and the MOSFET.

JFETs have a conductive channel with a source and drain connection on the ends. Channel current is controlled by the gate voltage.

p

n

S (Source)

G (Gate)

D (Drain)

S

G

D

n

n

p p

The gate is always operated with reverse bias on the pn junction formed between the gate and the channel. As the reverse bias is increased, the channel current decreases. n-channel JFET p-channel JFET

The FET

The MOSFET differs from the JFET in that it has an insulated gate instead of a pn junction between the gate and channel.Like JFETs, MOSFETs have a conductive channel with the source and drain connections on it.

p

S (Source)

G (Gate)

D (Drain)

S

G

D

n

n

p

p-channel MOSFET

n-channel MOSFET

ChannelSubstrate

Channel current is controlled by the gate voltage. The required gate voltage depends on the type of MOSFET.

The FET

Bipolar junction transistor (BJT)

Class A amplifier

Saturation

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.

Selected Key Terms

Cutoff

Q-point

Amplification

Common-emitter (CE)

Class B amplifier

The dc operating (bias) point of an amplifier.

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.

Selected Key Terms

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.

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.

Selected Key Terms

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.

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.

Selected Key Terms

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.

d. all of the above.

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-MOSFET with 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.

d. all of the above.

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

Quiz