Principles of Analog Electronics

371

Amplifiers: Basic BJT

Configurations

11.1 CONVENTIONAL NOTATIONS

When we indicate a voltage V we need two subscripts that suggest the two nodes

across which the voltage is applied (e.g., V

BE

). We use only one subscript (e.g., V

B

)

to refer to a voltage applied across the node indicated by that subscript and the

(implicit) ground. When only one subscript that is doubled is used (e.g., V

CC

), we

refer to a voltage owing to a DC source, applied across the node indicated by the

subscript and the ground.

Examples: V

CE

indicates the voltage across nodes “C” and “E,” V

B

indicates the

voltage across node “B” and the ground, V

BB

indicates the voltage owing to a DC

source across the node “B” and the ground.

11.2 STEP-BY-STEP FIRST DESIGN

An amplier consists of one or more transistors together with their biasing circuits

and input/output stages. Now, we will deal with single-stage single-transistor class-

A ampliers, starting here to systematically design our rst, elementary, scheme.

11.2.1 DC Bias Resistor

The heart of the amplier is the transistor, and we start by utilizing the BJT

type. Power to operate is furnished by one or more batteries, which bias the

11

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Principles of Analog Electronics

372

BJT’sjunctions. With biasing, a DC operating

point Q is xed, that is all DC voltages across

the BJT’s junctions, V

BE,Q

, V

CB,Q

, and V

CE,Q

,

and all DC currents owing into the three

BJT regions, I

E,Q

,I

B,Q

, andI

C,Q

. The operating

point Q is fundamental to determine the por-

tion of the cycle during which the source cur-

rent owsthroughthe BJT (Section 10.2.4 in

Chapter 10).

To bias the two junctions we start by utiliz-

ing two DC voltage sources (by means of two

batteries in particular, as here represented, or

two DC power supplies in general), an obvious

solution. Figure 11.1 schematizes the applied

DC voltage sources, V

CB

and V

BE

, and the direc-

tions of the currents which, conventionally, are

those of the holes.

For reasons of cost and size, to bias the two

BJT junctions it is more convenient to utilize

just one DC source, here the battery V

CE

, and

a so-called bias resistor R

1

, as in the scheme

shown in Figure 11.2.

The two previous networks are equivalent;

let us see why. We can provide V

CE

equal to

V

BE

+ V

CB

and use a resistance R

1

to obtain a

voltage drop V

BC

at its terminals. Let’s assume,

for instance, V

BE

= 2 V and V

CB

= 3 V for the

twobatteries in Figure 11.1, so that the base-

emitter junction is forward biased by 2 V and

the base-collector junction is reverse biased by

3 V. Choosing for Figure 11.2 a battery V

CE

=5 V

anda resistance R

1

of such a value to obtain 3 V

across it, then the 5 V of the battery are divided

into 3 V across the base- collector junction and

2 V across the base-emitter junction. Therefore,

the two situations are equivalent.

In any arbitrary amplier’s scheme, how-

ever, the DC source is never represented with

its complete loop but, simply, with one of its

terminals, with the knowledge that the other

is always connected to the ground; the new

scheme is shown in Figure 11.3.

According to conventional notations

(Section 11.1) the DC source is represented as

V

CC

across the BJT’s collector terminal and the

ground.

This type of biasing network can be referred

to as xed bias or base bias.

B

E

C

V

CB

V

BE

I

E

I

B

I

C

+

+

FIGURE 11.1 BJT biased by

means of two batteries.

V

CE

V

BE

I

E

I

B

I

C

R

1

+

+

+

FIGURE 11.2 BJT biased

by means of one battery

and one bias resistor.

I

E

I

C

I

B

R

1

V

CB

V

CC

V

CE

V

BE

+

+

+

FIGURE 11.3 BJT biased by V

cc

and R

1

. This network is called

xed bias or base bias.

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