Lecture 14-1

dc Bias Point Calculations

• Instead of supplying two dc voltages, the VBE is generally established by a voltage divider using the VCC supply

• But then the ac input cannot be attached as shown. Why?

VCC

RL

R2

R1 RE

RC

vin

Lecture 14-2

Decoupling

• What is the advantage of adding decoupling to this circuit?

VCC

RL

R2

R1

RC

vin

Lecture 14-3

Output Decoupling

• What would be the ideal values for these capacitors?

• How do we select the capacitor values?

VCC

RL

R2

R1

RC

vin

Lecture 14-4

PNP Amplifiers

• dc bias voltages are negative

-10V

RL

R2

R1

RC

vin

Lecture 14-5

PNP Amplifiers

• Which can also be designed by shifting all of the dc voltages by the same potential:

10V

RL R2

R1

vin RC

Lecture 14-6

Decoupling and Biasing

• Decoupling also allows us to specify the effective base resistance, RBB, independent of Vin

VCC

RL

R2

R1

RC

vin

VCC

RL

RBB

RC

vin

VBB

Lecture 14-7

Decoupling and Biasing

• We’d like to have VBB >> VBE so that the currents are not overly dependent on VBE. Why is this dependency undesirable?

• But what is the problem with a large VBB?

VCC

RL

RBB

RC

vin

VBB

Lecture 14-8

SPICE

• Discrete amplifier design rules of thumb: select bias point so that the expected ac signal can swing from max to min without clipping at the supply or forcing the transistor into saturation

dc solution

Q1Custom

R2333E3Ω

C2

1E-6F

+ -

RC1400Ω

+ 10VVCC

Ib

0.000 pA +

-

VOUTAC

0.000 pV

C111E-6F+ -

IC

2.209 mA

+ SINVSSIN

+

-

VIN

794.702 mV

R1143E3Ω

R1950E3Ω

+

-

VOUT

6.908 V

10k Hz10mV

Lecture 14-9

SPICE

• The 1µF capacitors are nearly perfect at decoupling the input and output dc signal components at this 10kHz frequency

time0.0 0.1 0.2 0.3 0.4 0.5 0.6 ms

-2

0

2

4

6

8

V

VOUTAC VOUT VINAC*20

Lecture 14-

SPICE

• Decoupling is evident from the frequency response

• Decoupling would cause distortion at low frequency

frequencye2 e3 e4 e5 e6 e7 e8 e9

36

37

38

39

40

41

42

DB(VOUTAC)

Lecture 14-

SPICE

• For smaller decoupling C’s (10nF) we will see some distortion even at 10kHz

time0.0 0.1 0.2 0.3 0.4 0.5 0.6 ms

-1

0

1

2

3

4

5

6

7

8

V

VOUTAC VOUT VINAC*20

Lecture 14-

SPICE

• Frequency response with 10nF decoupling C’s

frequencye2 e3 e4 e5 e6 e7 e8 e9

-20

-10

0

10

20

30

40

50

DB(VOUTAC)

Lecture 14-

Controlling Parameter Variations

• The current gain, β,varies with temperature

• Recombination greatly depends on temperature

• Recombination also depends on the injection level (magnitude of current)

• An emitter resistor, RE is used as negative feedback to reduce the β-variation effect on IC

VCC

RL

R2

R1

RC

vin RE

Lecture 14-

Controlling Parameter Variations

• Make RBB small enough that the base voltage is roughly independent of base current so that RE feedback can be most effective

• But why not too small?

VCC

RL

R2

R1

RC

vin RE

• If base voltage is nearly constant, an increase in current due to β-variation will tend to decrease VBE because of RE

Lecture 14-

Universal Amplifier Configuration

• Dual supplies for generality, makes base biasing simpler

VCC

RB

RC

RE

• Use decoupling for input and output signals

• But what does the capacitor at the emitter do?

-VEE

Lecture 14-

Decoupling Capacitors

• Decoupling C’s are generally used only for discrete design since it is difficult to build a large capacitor on a chip

• Direct coupled amplifiers are generally used on ICs, but they are similar in form to the amplifiers that we will study here

VCC

RB

RC

RE

-VEE

Lecture 14-

Common Emitter Amplifier

• Popular discrete-component amplifier configuration

VCC

RB

RC

RE

-VEE

C1

C2

C3

RL

vs

BypassCapacitor

RS

Lecture 14-

Common Emitter Design Example

VCC

RC

RE

-VEE

C2

C3

RL

vs

• If there is no dc component in vin, then we don’t need C1or RB with -VEE.

RS

Lecture 14-

Common Emitter Design ExampleVCC=15v

RC

RE

-VEE=-15v

vs

RS=2.5k

Lecture 14-

Common Emitter Design Example

Lecture 14-

Common Emitter Design Example

Lecture 14-

SPICE dc Analysis

Q1Custom

C11E-6F+ -

RC10E3Ω

C21E-6F+ -

+ 15VVCC

+

-

VOUTAC

0.000 pV

+ -15VVEE

RB2500Ω

IC

983.289 µA

+ SINVSSIN

+

-

VIN

-24.582 mV

RE14300Ω

RL5E3Ω

+

-

VOUT

5.167 V

+

-

VINAC

0.000 pV

Lecture 14-

SPICE ac Analysis

frequencye2 e3 e4 e5 e6 e7 e8 e9

0

20

40

DB(VOUTAC)