Electronic Circuits LaboratoryEE462GLab #8

BJT Common Emitter Amplifier

npn Bipolar Junction Transistor

BJT in a common-emitter configuration

+VBE_

+VCE_

C

B

EB

B – Base

C – Collector

E – Emitter

For most applications the BJT is operated in the active region where:

p

n

n

B

C

E

BECEBE VVV and V60.

npn BJT Operation

BJT in a common-emitter configuration in active region (VCE > VBE ~ .6V):

+VBE_

+VCE_

C

B

E

The pn junction for VBE is forward biased and current iBE flows according to the Shockley equation:

n

n

1

T

BEESE V

VIi exp

where VT .26 mV and IES ranges from 10-12 to 10-17.

Electrons from the emitter flow into the base and are pulled into the depletion region of the reversed biased collector-base junction.

iE

- - - - -

+ + + + +

(-)

npn BJT Characteristics

BJT Transfer Characteristics in active region with = IC / IB = 100:

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1x 10

-5

Volts (VBE)

Am

ps

(Ba

se C

urr

en

t)

0 1 2 3 4 5 6 70

1

2

3

4

5x 10

-4

Volts (VCE)

Am

ps

(Co

llect

or

Cu

rre

nt) IB = 4 microamps

IB = 3 microamps

IB = 2 microamps

IB = 1 microamp1

DC (Biasing) Model

Equations for the DC operating point,

assume IB <<<< I2:

21

2

RR

RVV CCBB

R1

R2

RC

RE

VCC

VBBIB

BE

C

IC

IE

I2 EECECCCC RIVRIV

BC II EBC III

Key Result (Load-line Equation):

EC

CE

EC

CCC

RR

V

RR

VI

11

How sensitive is Ic to changes in ?

R1

R2

RC

RE

VCC VCC

DC (Biasing) Equivalent Model

Apply Thévenin model to base terminal:

21

2

RR

RVV CCB

21

12

RR

RRRB

Load-Line Equation

EB

BE

EB

BB RR

V

RR

VI

)()(

11

VB

RC

RE

RB

IB

VCC

Load Line Analysis:

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1x 10

-5

Volts (VBE)

Am

ps

(Ba

se C

urr

en

t)

0 1 2 3 4 5 6 70

1

2

3

4

5x 10

-4

Volts (VCE)A

mp

s (C

olle

cto

r C

urr

en

t) IB = 4 microamps

IB = 3 microamps

IB = 2 microamps

IB = 1 microamp1

EB

B

RR

V

)( 1

BV

EC

CC

RR

V

1

CCV

Qualitatively describe what happens in both curves when increases(or decreases).

Large-Signal (DC) Model

VB

VBE VCCRB

RC

RE

IB

IB

IC

IE

BE

C

R1

R2

RC

RE

VCC

VBBIB

BE

C

IC

IE

I2

BJT Amplifier

Once the DC operating point is set, the AC input and output are coupled to the amplifier with capacitors so as not to perturb the operating point.

vs

Rs Cin

R1

R2

RC

RE

Cout

CE

RL

VCC

BJT Amp Small-Signal ModelConsider the capacitors as short circuits for the small signal AC and open circuit for DC to obtain the model below. The resistor ro accounts for the small slope of the I-

V characteristics in the forward-active region (often assumed to be infinite). The resistance r is found from linearizing the nonlinear base-emitter characteristic,

which is an exponential diode curve.

21

12

RR

RRRB

Rs

vs r

iB

BC

Ero RL

+vin

-

iB +vout

-

RCRB

BJT Amp Small-Signal Model

Determine the voltage gain. How would the emitter resistor affect the gain if it was not bypassed?

Rs

vs r

iB

BC

Ero RL

+vin

-

iB +vout

-

RCRB

RE replaces the short here!

BJT Circuit Parameters

How can be found experimentally using the curve tracer?

How can the input and output resistances be determined experimentally?

How can voltage gain be determined experimentally?

Taylor Series

Recall that a function can be expressed as a polynomial through a Taylor Series expansion:

where a is a point about which the function is expanded. Note that if a represents a quiescent point for a voltage, then the reciprocal of the coefficient first linear represents the small signal impedance.

axax dx

xfdax

dx

xdfaxafxf

2

22 )(

!2

)()(

!1

)()()(

SPICE Example

The amplifier circuit can be constructed in B2SPICE using the BJT npn (Q) part from the menu. The “edit simulation model” option can then be used to set the “ideal forward beta”

The input can be set to a sinusoid at desired frequency and amplitude for a transient analysis.

SPICE can also do a Fourier analysis to observed effects of clipping and distortion. There should be no harmonic energy for perfect amplification.

SPICE Example

Example circuit with meters to monitor input and output:

Q1beta= 100

R1

10

KR

25

k

R3

1K

V1

12

R5

50

R4

60

0

C11u

V2

0

C2

1u

C31u

R6

1KIVm1

IVm2

SPICE Example

Graphic output for .03 V sine wave input (IVM1) at 10 kHz. Output is shown for meter IVM2. What would the gain of this amplifier be at 10 kHz?

bjtexamAC-Transient-0-Graph Time (s)

0.0 200.000u 400.000u 600.000u 800.000u 1.000m

(V)

-200.000m

0.0

200.000m

max:355.280u, 0.286

min:405.280u, -0.291

max:625.280u, 0.00981

min:675.280u, -0.00980

TIME 586.280u v(IVm1) -7.532m v(IVm2) -107.794m

D(TIME) 0.0 D(v(IVm2)) 0.0

SPICE Example

Fourier analysis of output. Frequency magnitude plot for output at meter IVM2.

Where should the energy be in the frequency domain for this output? bjtexamAC-Fourier-0-Graph Frequency (Hz)

0.0 20.000k 40.000k 60.000k 80.000k

0.0

500.000m

freq -1.000 norm_mag_v8 -1.000 D(freq) -7.623

D(norm_mag_v8) -1.000

SPICE Example

Graphic output for .08 V sine wave input (IVM1) at 10 kHz. Output is show for meter IVM2. What would the gain of this amplifier be at 10 kHz?

bjtexamAC-Transient-1-Graph Time (s)

0.0 200.000u 400.000u 600.000u 800.000u 1.000m

(V)

-2.000

-1.000

0.0

1.000

max:560.280u, 1.971

min:603.280u, -2.396

max:725.280u, 0.0785

min:775.280u, -0.0782

TIME 709.280u v(IVm1) 42.303m v(IVm2) -2.217

D(TIME) 0.0 D(v(IVm2)) 0.0

SPICE Example

Fourier analysis of output. Frequency magnitude plot for output at meter IVM2.

Where should the energy be in the frequency domain for this output? bjtexamAC-Fourier-2-Graph Frequency (Hz)

0.0 20.000k 40.000k 60.000k 80.000k

0.0

500.000m

freq -1.000 norm_mag_v8 -1.000 D(freq) -5.780

D(norm_mag_v8) -2.653Meg

SPICE Example

Graphic output for 1.2 V sine wave input (IVM1) at 10 kHz. Output is show for meter IVM2. What would the gain of this amplifier be at 10 kHz?

bjtexamAC-Transient-3-Graph Time (s)

0.0 200.000u 400.000u 600.000u 800.000u 1.000m

(V)

-4.000

-2.000

0.0

2.000max:539.280u, 2.434

min:610.280u, -4.323

max:833.280u, 1.040

min:875.280u, -1.166

TIME 850.280u v(IVm1) -4.199m v(IVm2) 2.322

D(TIME) 0.0 D(v(IVm2)) 0.0

SPICE Example

Fourier analysis of output. Frequency magnitude plot for output at meter IVM2.

Where should the energy be in the frequency domain for this output? bjtexamAC-Fourier-3-Graph Frequency (Hz)

0.0 20.000k 40.000k 60.000k 80.000k

0.0

500.000m

freq -1.000 norm_mag_v8 -1.000 D(freq) -1.661

D(norm_mag_v8) -1.661