ECEN Lab 7: Characterization and DC Biasing of the BJT Solution

Objectives

The purpose of this lab is to characterize NPN and PNP bipolar junction transistors (BJT), and to analyze and design DC biasing circuits to set the DC operating point of BJTs.

Introduction

Figure 1 shows typical symbols for the NPN and PNP BJTs. Depending on the applied DC bias, BJT has three regions of operation:

Cutoff Region: If both base-emitter and base-collector junctions are reverse biased, the BJT enters the cutoff region. All terminal currents are extremely small, and the transistor is off.

Active Region: The base-emitter junction is forward biased, and the base-collector junction is reverse biased to make a BJT operate in the active region. The active region is used to design a linear amplifier.

Saturation Region: When both the base-emitter and base-collector junctions are forward biased, the BJT enters the saturation region.



IB


B 

VBE

C

IC
VEB


VCE
B

IB
IE

E

E

IE


VEC
IC


C

(a)    (b)

Figure 1: Bipolar junction transistor (BJT) (a) NPN (b) PNP

In the active region, the collector current (IC ) of NPN and PNP devices are exponential functions of base-emitter voltage (VBE ) and emitter-base voltage (VEB ), respectively, given by

IC ,npn = IS eVBE =VT    IC ,pnp = IS eVEB =VT    (1)

where IS is the saturation current and VT is the thermal voltage, which is approximately 25mV at room temperature. For both NPN and PNP, the base current IB is a small fraction of IC , given by

IB
=
IC




(2)















and the emitter current IE is the sum of the base and collector currents, given by

IE =IC +IB =(  +1)IB =
IC
(3)













where






=





(4)








+ 1




is known as the current gain of the transistor, which varies significantly with temperature, and it can be different between two transistors of the same type. Typical value of is around 100, resulting in = 0.99.




c    Department of Electrical and Computer Engineering, Texas A&M University

1
BJT Characterization

Figure 2 shows a characterization circuit for an NPN BJT. To obtain IC is kept constant, resulting in the exponential function in Fig. 3(a). If V1 obtained as a function of VCE as shown in Fig. 3(b).

as a function of VBE , V1 is swept while V2 is kept constant and V2 is swept, IC can be













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Figure 3: Collector current (IC ) of an NPN BJT as a function of (a) VBE (b) VCE

Characterization circuit for a PNP BJT is shown in Fig. 4. Keeping V2 constant and sweeping V1 provides IC as an exponential function of VEB as shown in Fig. 5(a). Sweeping V2 while V1 is kept constant provides the IC vs. VEC characteristics as shown in 5(b).



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Figure 5: Collector current (IC ) of a PNP BJT as a function of (a) VEB (b) VEC


2
BJT DC Biasing - Resistive

Figures 6(a) and (b) show typical resistive biasing circuits for NPN and PNP transistors, respectively.


VCC

RB1
RC
RB2

VC


IB


VCE


0.7

RB2
V2      VRE        RE
RB1

−VEE

VCC


V2    VRE    RE

0.7
VEC
IB
VC
RC

−VEE

(a)    (b)

Figure 6: Resistive DC biasing circuit for (a) NPN (b) PNP

For each circuit in Figs. 6(a) and (b), assume that the transistor is active, and IB is negligible, which means RB1 and RB2 form a voltage divider to set the V2 voltage. Therefore, IE and IC can be found as
V2
RB2
(VCC + VEE) ) IE =
V2
0.7
IC
(5)

RB1 + RB2


RE




All assumptions must be verified to complete the DC analysis. For the circuits in Figs. 6(a) and (b), IB is negligible only if IB IRB1, which requires

IC
VCC + VEE
(6)
IB =

IRB1






RB1 + RB2


To verify that the NPN transistor is active, VCE    VCE ,sat should be satisfied as follows

VCE = VCC + VEE
IC (RC + RE )   VCE ,sat
(7)
For the PNP transistor, active operation requires VEC    VEC ,sat as follows

VEC = VCC + VEE
IC RC + RE    VEC ,sat
(8)
where VCE ,sat    VEC ,sat    0.2V





BJT DC Biasing - Current Source

An alternative method for BJT DC biasing is to use a current source connected to the emitter terminal, which directly sets the IE current, and hence the IC current. Figure 7(a) shows the DC biasing of an NPN BJT using a current source, which can be realized using the circuits in Fig. 7(b) or (c). Figure 8 shows the DC biasing circuit of a PNP BJT using a current source, as well as current source and current mirror realizations.



VCC




VCC
RB1
RC

VCC


R4















VC

Ix
R1

Ix

IB






VCE






0.7





RB2
V2VxIx
R3
Vy
R2
R6
R5

−VEE

−VEE


−VEE

(a)

(b)


(c)


Figure 7: (a) DC biasing circuit for an NPN BJT using a current source (b) Current source (c) Current mirror



3


VCC


VCC




VCC
RB1
V2VxIx
R3
Vy
R2

R6
R5


0.7








IB
VEC


R






VC

Ix

1


Ix




















RB2

RC

−VEE




R4











−VEE







−VEE











(a)


(b)




(c)
Figure 8: (a) DC biasing circuit for a PNP BJT using a current source (b) Current source (c) Current mirror
For the current sources in Figs. 7(b) and 8(b), Ix can be calculated as





R2
(VCC + VEE ) ) Ix
Vy
0.7
(9)


Vy    R1 + R2

R3



For the current mirrors in Figs. 7(c) and 8(c), assuming matching transistors and R5 = R6, Ix can be calculated as

VCC + VEE
0.7
Ix


(10)

R4+R5


All transistors in Figs. 7 and 8 are assumed to be active, and all IB currents are assumed to be negligible. These assumptions need to be verified after finding the DC solution.

Calculations

    1. Design the circuits in Figs. 6(a) and 6(b) with the following specifications:

NPN
IC
1mA
VC
3.5V
VCE
1V
VRE
1V
VCC
5V
VEE
0

100


VT
25mV
Isupply
2mA

PNP
IC
1mA
VC
1.5V
VEC
1V
VRE
1V
VCC
5V
VEE
0

100
VT
25mV
Isupply
2mA

For both circuits, DC biasing should be insensitive to variations in and jVBE j, and IB currents should be designed to be negligible.

    2. Design the circuits in Figs. 7(a) and 8(a) using the current sources in Figs. 7(b) and 8(b), respectively, with the following specifications:

NPN
IC
2mA
VC
3.5V
VCE
1V
Vx
1.5V
VCC
5V
VEE
0

100
VT
25mV
Isupply
5mA

PNP
IC
2mA
VC
1.5V
VEC
1V
Vx
1.5V
VCC
5V
VEE
0

100
VT
25mV
Isupply
5mA

For both circuits, DC biasing should be insensitive to variations in and jVBE j, and IB currents should be designed to be negligible.

4
Simulations

For all simulations, provide screenshots showing the schematics and the plots with the simulated values prop-erly labeled.

    1. Draw the schematics for the NPN characterization circuit in Fig. 2 using the 2N3904 transistor

Perform a DC sweep of V1 from 0 to 5V, while V2 = 5V . Export the simulation data to Excel, and plot IC as a function of VBE .

Perform a DC sweep of V2 from 0 to 5V, while V1 = 2V . Export the simulation data to Excel, and plot IC as a function of VCE .

    2. Draw the schematics for the PNP characterization circuit in Fig. 4 using the 2N3906 transistor

Perform a DC sweep of V1 from -5V to 0, while V2 =    5V . Export the simulation data to Excel, and plot
IC as a function of VEB .

Perform a DC sweep of V2 from -5V to 0, while V1 =    2V . Export the simulation data to Excel, and plot
IC as a function of VEC .

    3. Draw the schematics in Figs. 6(a) and 6(b) using the calculated component values and 2N3904 and 2N3906 transistors. For both circuits, perform DC operating point or interactive simulation to obtain the DC solution for IC , VC , VRE and V2.

    4. Draw the schematics in Figs. 7(a) and 8(a) using the current sources in Figs. 7(b) and 8(b), respectively, with the calculated component values and 2N3904 and 2N3906 transistors. For both circuits, perform DC operating point or interactive simulation to obtain the DC solution for IC , VC , V2, Vx and Vy .

Measurements

For all measurements, provide screenshots showing the plots with the measured values properly labeled.

    1. Build the NPN characterization circuit in Fig. 2 using the 2N3904 transistor

Apply a ramp signal from 0 to 5V at 1Hz for V1 while V2 = 5V . Export the voltage measurements from the scope to Excel, and plot IC as a function of VBE .

Apply a ramp signal from 0 to 5V at 1Hz for V2 while V1 = 2V . Export the voltage measurements from the scope to Excel, and plot IC as a function of VCE .

    2. Build the PNP characterization circuit in Fig. 4 using the 2N3906 transistor

Apply a ramp signal from -5V to 0 at 1Hz for V1 while V2  =    5V . Export the voltage measurements
from the scope to Excel, and plot IC as a function of VEB .

Apply a ramp signal from -5V to 0 at 1Hz for V2 while V1  =    2V . Export the voltage measurements
from the scope to Excel, and plot IC as a function of VEC .

    3. Build the circuits in Figs. 6(a) and 6(b) using the calculated component values and 2N3904 and 2N3906 tran-sistors. For both circuits, measure the DC values for IC , VC , VRE and V2 using the voltmeter or scope.

    4. Build the circuits in Figs. 7(a) and 8(a) using the current sources in Figs. 7(b) and 8(b), respectively, with the calculated component values and 2N3904 and 2N3906 transistors. For both circuits, measure the DC values for IC , VC , V2, Vx and Vy using the voltmeter or scope.

Report

    1. Include calculations, schematics, simulation plots, and measurement plots.

    2. Prepare a table showing calculated, simulated and measured results.

    3. Compare the results and comment on the differences.


5
Demonstration

    1. Build the circuits in Figs. 2, 4, 6(a), 6(b), 7(a)&(b) and 8(a)&(b) on your breadboard and bring it to your lab session.

    2. Your name and UIN must be written on the side of your breadboard.

    3. Submit your report to your TA at the beginning of your lab session.

    4. For the NPN characterization circuit in Fig. 2:

Apply a ramp 0 to 5V at 1Hz for V1 while V2 = 5V , and export the measurements from scope to Excel. Plot IC as a function of VBE in Excel.

    5. For the PNP characterization circuit in Fig. 4:

Apply a ramp -5V to 0 at 1Hz for V2 while V1 = 2V , and export the measurements from scope to Excel. Plot IC as a function of VEC in Excel.

    6. For the resistive NPN and PNP biasing circuits in Figs. 6(a) and 6(b):

Measure the DC voltages VC , VB , VE .

Calculate IC from the voltage measurements.

    7. For the current-source NPN and PNP biasing circuits in Figs. 7(a)&(b) and 8(a)&(b):

Measure the DC voltages VC , VB , VE for both transistors. Calculate IC from the voltage measurements.







































6