Handbook for Sound Engineers

330 Chapter 12

emitter-to-base circuit is open. Emitter-cutoff current
flows when the emitter to base is reverse biased and the
collector-to-base circuit is open.

Small- and Large-Signal Characteristics. The tran-

sistor, like the vacuum tube, is nonlinear and can be

classified as a nonlinear active device. Although the

transistor is only slightly nonlinear, these nonlinearities

become quite pronounced at very low and very high

current and voltage levels. If an ac signal is applied to

the base of a transistor without a bias voltage, conduc-

tion will take place on only one-half cycle of the applied

signal voltage, resulting in a highly distorted output

signal. To avoid high distortion, a dc-biased voltage is

applied to the transistor, and the operating point is

shifted to the linear portion of the characteristic curve.

This improves the linearity and reduces the distortion to

a value suitable for small-signal operation. Even though

the transistor is biased to the most linear part of the

characteristic curve, it can still add considerable distor-

tion to the signal if driven into the nonlinear portion of

the characteristic.

Small-signal swings generally run from less than

1 μV to about 10 mV so it is important that the

dc-biased voltage be large enough that the applied ac

Figure 12-26. Basic design circuit for transistor bias circuits.

Length

Width

n-Type

Gate

S

n-Type Field

D

P-Type

VD ID

Thickness

Source Gate-1 Drain

p-Type

n-Type Channel

Gate-2

A. Plain semiconductor bar. B. Bar with gate added and drain

voltage applied.

C. Cross-sectional view of the

construction for a single- or double-

gate field-effect transistor.

Source

P Silicon

Source

Metallic

Film Drain

P Silicon

Gate insulator

n Silicon silicon dioxide

D. Internal construction of an

insulated-gate transistor (IGT).

n Input

Gate

22 M 7

20 V

2 k 7

Drain

Substrate

Source

E. Typical circuit for an IGT transistor.

+ G D+

RLD

RG RS

F. n-channel field-effect transistor

circuit.

G. p-channel field-effect transistor circuit. H. n-channel double-gate field-effect transistor circuit.

0 0

V

(^) +
S

• G D+
  RLD
  RG RS
  S


• G D+
  RLD
  RG RS
  S
  Figure 12-27. Basic bias circuits for transistors
  R 1
  R 2
  R 4
  R 3
  R 5
  R 6
  VCC
  B
  C
  E
  In
  Out