0195136047.pdf

414 TRANSISTOR AMPLIFIERS

+

−

vin vd

vout

A

B

Transistor

amplifying unit with

gain A

Feedback

with feedback factor B

vf

= vin − vf

= Bvout

= (AB)vd

vout = Avd = A(vin−vf)

= A(vin−Bvout)

Figure 8.5.3Voltage amplifier with negative feedback.

vd

vin

=

1

1 +AB

(8.5.20)

The productABis known as the loop gain becausevf=B(Avd)=(AB)vd. In order to obtain

negative feedback, the loop gain must be positive. WithAB >0 in Equation (8.5.19) it follows

that|Af|<|A|. Thus, a negative feedback amplifier has always less gain than the amplifying unit

itself. WithAB >>1,Afremains essentially constant, thereby reducing the effect of transistor

parameter variations in the amplifying unit. If the feedback unit has|B|<1, one can still obtain

useful amplification since|Af|∼= 1 /|B|>1.

The amplifiers discussed in Sections 8.3 and 8.4 as well as in this section are small-signal

linear stages that use capacitance coupling. These are also known asRC amplifierssince their

circuits need only resistors and capacitors. There are of course other transistor amplifiers that do

not have the small-signal limitation and are also not limited to the use of resistors and capacitors

in their circuits. For larger input signals the linear dependence of the current on the signal level

may not be maintained.

8.6 Learning Objectives

Thelearning objectivesof this chapter are summarized here so that the student can check whether

he or she has accomplished each of the following.

• Biasing the BJTs and FETs.


• BJT and FET amplifier configurations, and their small-signal equivalent circuits, based on
  which voltage and current gains are computed.


• Frequency response of amplifiers, and low- and high-frequency designs.


• Advantages of negative feedback in amplifier circuits.

8.7 PRACTICAL APPLICATION: A CASE STUDY

Mechatronics: Electronics Integrated with Mechanical Systems

Electronic circuits have become so intimately integrated with mechanical systems that a new term,

mechatronics, has been coined for the combination. Various subjects related to mechatronics are

being developed at a number of universities in the United States, supported by major industrial

sponsors. Too many engineers, at present, are not well equipped and trained to design mechatronic