Almughtaribeen University

ALMUGHTARIBEEN UNIVERSITY

College of Engineering
Department of Electrical Engineering

Analog Electronic Circuits I

Lecture 4

BJT I-V Characteristics

Almughtaribeen University – [ Abusabah I. A. Ahmed]

 Lecture Outline
❑ Introduction
ALMUGHTARIBEEN UNIVERSITY

❑ Operation Modes and Currents

❑ I-V Characteristics
❑ Operation Point
❑ BJT Analysis
❑ Practice Problem

Almughtaribeen University – [ Abusabah I. A. Ahmed] 2

 Introduction
❑ Two main categories of transistors:
✓ Bipolar Junction Transistors (BJTs).
ALMUGHTARIBEEN UNIVERSITY

✓ Field Effect Transistors (FETs).

❑ Transistors have 3 terminals where the application of current
(BJT) or voltage (FET) to the input terminal increases the
amount of charge in the active region.
❑ The physics of "transistor action" is quite different for the
BJT and FET.
❑ In analog circuits, transistors are used in amplifiers and
linear regulated power supplies.
❑ In digital circuits they function as electrical switches,
including logic gates, Random Access Memory (RAM), and
microprocessors.

Almughtaribeen University – [ Abusabah I. A. Ahmed] 3

ALMUGHTARIBEEN UNIVERSITY
Operations Modes and Currents

❑ Active:
✓ Most importance mode, e.g. for amplifier operation.
✓ The region where current curves are practically flat.
❑ Saturation:
✓ Barrier potential of the junctions cancel each other out causing a
virtual short.
✓ Ideal transistor behaves like a closed switch.
❑ Cutoff:
✓ Current reduced to zero
✓ Ideal transistor behaves like an open switch.
Almughtaribeen University – [ Abusabah I. A. Ahmed] 4
ALMUGHTARIBEEN UNIVERSITY
Operations Modes and Currents

❑Operation
✓ Forward bias of EBJ injects electrons from emitter into base
(small number of holes injected from base into emitter)
✓ Most electrons shoot through the base into the collector
across the reverse bias junction (think about band diagram)
✓ Some electrons recombine with majority carrier in (P-type)
base region
Almughtaribeen University – [ Abusabah I. A. Ahmed] 5
ALMUGHTARIBEEN UNIVERSITY
Operations Modes and Currents

Almughtaribeen University – [ Abusabah I. A. Ahmed] 6

 Operations Modes and Currents
Beta
ALMUGHTARIBEEN UNIVERSITY

❑ Can relate iB and iC by the following equation

Beta is constant for a particular transistor

On the order of 100-200 in modern devices (but can be
higher) Called the common-emitter current gain

Almughtaribeen University – [ Abusabah I. A. Ahmed] 7

 Operations Modes and Currents
Emitter Current
ALMUGHTARIBEEN UNIVERSITY

❑ Emitter current is the sum of iC and iB

❑∝ is called the common-base current gain

Almughtaribeen University – [ Abusabah I. A. Ahmed] 8

ALMUGHTARIBEEN UNIVERSITY
I-V Characteristics
IC
IC VBE3

VCE VBE2
VBE
VBE1
VBE3 > VBE2 > VBE1

VCE

❑ Collector current vs. vCB shows the BJT looks like a current
source (ideally)
❑ Plot only shows values where BCJ is reverse biased and so BJT in
active region
❑ However, real BJTs have non-ideal effects
Almughtaribeen University – [ Abusabah I. A. Ahmed] 9
ALMUGHTARIBEEN UNIVERSITY
I-V Characteristics

Base-emitter junction looks Collector-emitter is a family of

like a forward biased diode curves which are a function of
base current.

Almughtaribeen University – [ Abusabah I. A. Ahmed] 10

ALMUGHTARIBEEN UNIVERSITY
I-V Characteristics

Almughtaribeen University – [ Abusabah I. A. Ahmed] 11

 I-V Characteristics
Example 4-1
ALMUGHTARIBEEN UNIVERSITY

Calculate the values of

β and α from the
transistor shown in the
graphs.
Solution

Almughtaribeen University – [ Abusabah I. A. Ahmed] 12

 I-V Characteristics
Emitter Resistance, rE
ALMUGHTARIBEEN UNIVERSITY

Almughtaribeen University – [ Abusabah I. A. Ahmed] 13

 I-V Characteristics
Collector Resistance, rC
ALMUGHTARIBEEN UNIVERSITY

Base Resistance, rB
❑ Mainly effects small-signal and transient responses.
❑ Difficult to measure since it depends on bias condition and is influenced
by rE.
❑ In the Ebers-Moll model (SPICE’s default model for BJTs), rB is assumed
to be constant.

Almughtaribeen University – [ Abusabah I. A. Ahmed] 14

 I-V Characteristics
Input Equation
ALMUGHTARIBEEN UNIVERSITY

❑To start, let’s write Kirchoff’s voltage law (KVL)

around the base circuit.

Almughtaribeen University – [ Abusabah I. A. Ahmed] 15

 I-V Characteristics
Output Equation
ALMUGHTARIBEEN UNIVERSITY

❑ Likewise, we can write KVL around the collector

circuit.

Almughtaribeen University – [ Abusabah I. A. Ahmed] 16

 Operation Point
Base-Emitter Circuit Q point
ALMUGHTARIBEEN UNIVERSITY

The Load Line intersects

the Base-emitter
characteristics at VBEQ =
0.6 V and IBQ = 20 µA

Almughtaribeen University – [ Abusabah I. A. Ahmed] 17

 Operation Point
Collector-Emitter Circuit Q point
ALMUGHTARIBEEN UNIVERSITY

Now that we have

the Q-point for the
base circuit, let’s
proceed to the
collector circuit.

The Load Line intersects the Collector-emitter characteristic, iB = 20 µA at

VCEQ = 5.9 V , ICQ = 2.5mA, then β = 2.5m/20 µ = 125
Almughtaribeen University – [ Abusabah I. A. Ahmed] 18
 BJT Analysis
BJT - AC Analysis
ALMUGHTARIBEEN UNIVERSITY

❑Let’s assume that Vin(t) = 0.2 sin(ωt).

❑Then the voltage sources at the base vary from a
maximum of 1.6 + 0.2 = 1.8 V to a minimum of 1.6 -
0.2 = 1.4 V
❑We can then draw two “load lines” corresponding the
maximum and minimum values of the input sources
❑The current intercepts then become for the:
❑Maximum value: 1.8 / 50k = 36 µA
❑Minimum value: 1.4 / 50k = 28 µA

Almughtaribeen University – [ Abusabah I. A. Ahmed] 19

 BJT Analysis
AC Analysis Base-Emitter Circuit
ALMUGHTARIBEEN UNIVERSITY

From this graph, we find: Note the asymmetry around the Q-

point of the Max and Min Values for
At Maximum Input Voltage: the base current and voltage which is
VBE = 0.63 V, iB = 24 µA
due to the non-linearity of the base-
At Minimum Input Voltage:
VBE = 0.59 V, iB = 15 µA emitter characteristics
Recall: At Q-point: ∆iΒmax = 24-20 = 4 µA;
VBE = 0.6 V, iB = 20 µA ∆i = 20-15 = 5 µA
Bmin
Almughtaribeen University – [ Abusabah I. A. Ahmed] 20
 BJT Analysis
AC Analysis Base-Emitter Circuit
ALMUGHTARIBEEN UNIVERSITY

Almughtaribeen University – [ Abusabah I. A. Ahmed] 21

 BJT Analysis
AC Characteristics-Collector Circuit
ALMUGHTARIBEEN UNIVERSITY

Using these max and min values for the base current on the
collect circuit load line, we find:
At Max Input Voltage: VCE = 5 V, iC = 2.7mA
At Min Input Voltage: VCE = 7 V, iC = 1.9mA
Recall: At Q-point: VCE = 5.9 V, iB = 2.5mA
Almughtaribeen University – [ Abusabah I. A. Ahmed] 22
 BJT Analysis
BJT AC Analysis - Amplifier Gains
ALMUGHTARIBEEN UNIVERSITY

❑ From the values calculated from the base and collector

circuits we can calculate the amplifier gains:

Almughtaribeen University – [ Abusabah I. A. Ahmed] 23

 BJT Analysis
Example 4-2
ALMUGHTARIBEEN UNIVERSITY

Estimate terminal currents and base-emitter voltage

Given data: IS =10-16 A, aF = 0.95, VBC = VB - VC = -5 V, IE = 100 𝜇A
Assumptions: Simplified transport model assumptions, room temperature
operation, VT = 25.0 mV

Almughtaribeen University – [ Abusabah I. A. Ahmed] 24

 BJT Analysis
Solution
Analysis: Current source forward-biases base-emitter diode, VBE > 0,
ALMUGHTARIBEEN UNIVERSITY

VBC < 0, we know that transistor is in forward-active operation region.

IC = F I E = 0.95100A = 95A
F 0.95
F = = =19
1−  F 1− 0.95
IE 100A
IB = = = 5A
 F +1 20
 
 F I E 
VBE =VT ln   = 0.69V
 IS 
 

Almughtaribeen University – [ Abusabah I. A. Ahmed] 25


 Practice Problem
Estimate terminal currents, base-emitter and base-collector
voltages. Given data: IS = 10-16 A, aF = 0.95, VC = +5 V,
ALMUGHTARIBEEN UNIVERSITY

IB = 100 𝜇A .
Assumptions: Simplified transport model assumptions, room
temperature operation, VT = 25.0 mV

Almughtaribeen University – [ Abusabah I. A. Ahmed] 26

ALMUGHTARIBEEN UNIVERSITY

Thank You!
Almughtaribeen University – [ Abusabah I. A. Ahmed] 27