Tubes, Discrete Solid State Devices, and Integrated Circuits 329(12-28)where,
Zo is the output impedance in ohms,
Zin is the input impedance in ohms.Forward-Current-Transfer Ratio. An important char-
acteristic of a transistor is its forward-current-transfer
ratio, or the ratio of the current in the output to the
current in the input element. Because of the many
different configurations for connecting transistors, the
forward transfer ratio is specified for a particular circuit
configuration. The forward-current-transfer ratio for the
common-base configuration is often referred to as alpha
(D) and the common-emitter forward-current-transfer
ratio as beta (E). In common-base circuitry, the emitter is
the input element, and the collector is the output
element. Therefore, Ddc is the ratio of the dc collector
current IC to the dc emitter current IE. For the common
emitter, the Edc is then the ratio of the dc collector
current IC to the base current IB. The ratios are also given
in terms of the ratio of signal current, relative to the
input and output, or in terms of ratio of change in the
output current to the input current, which causes the
change.
The terms Dand E are also used to denote the
frequency cutoff of a transistor and is defined as the
frequency at which the value of D for a common-base
configuration, or E for a common-emitter circuit, falls to
0.707 times its value at a frequency of 1000 Hz.
Gain-bandwidth product is the frequency at which
the common-emitter forward-current-transfer ratio E is
equal to unity. It indicates the useful frequency range of
the device and assists in the determination of the most
suitable configuration for a given application.Bias Circuits. Several different methods of applying
bias voltage to transistors are shown in Fig. 12-26, with
a master circuit for aiding in the selection of the proper
circuit shown in Fig. 12-27. Comparing the circuits
shown in Fig. 12-26, their equivalents may be found by
making the resistors in Fig. 12-27 equal to zero or
infinity for analysis and study. As an example, the
circuit of Fig. 12-26D may be duplicated in Fig. 12-27
by shorting out resistors R4 and R5 in Fig. 12-27.
The circuit Fig. 12-26G employs a split voltage
divider for R 2. A capacitor connected at the junction of
the two resistors shunts any ac feedback current to
ground. The stability of circuits A, D, and G in Fig.
12-26 may be poor unless the voltage drop across the
load resistor is at least one-third the value of the power
supply voltage Vcc. The final determining factors will be
gain and stability.
Stability may be enhanced by the use of a thermistor
to compensate for increases in collector current with
increasing temperature. The resistance of the thermistor
decreases as the temperature increases, decreasing the
bias voltage so the collector voltage tends to remain
constant. Diode biasing may also be used for both
temperature and voltage variations. The diode is used to
establish the bias voltage, which sets the transistor
idling current or the current flow in the quiescent state.
When a transistor is biased to a nonconducting state,
small reverse dc currents flow, consisting of leakage
currents that are related to the surface characteristics of
the semiconductor material and saturation currents.
Saturation current increases with temperature and is
related to the impurity concentration in the material.
Collector-cutoff current is a dc current caused when the
collector-to-base circuit is reverse biased and theFigure 12-24. Signal-voltage polarities in a p-channel
field-effect transistor (FET).Figure 12-25. Typical voltage, power, and current gains for
a conventional transistor using a common-emitter
configuration.0 o180 o+VDSVDSRSRDRGG
SD
Outputs(^)
Input
AV = Voltage Amplification
AI = Current Amplification
Load Resistance (RL)—Ohms
Power Gain—dB
50
40
30
20
1k 2 5 10k 20 50 100k 200k
1500
1000
500
0
75
50
25
0
Volt
Gain
Current
Gain
Power Gain
Current Gain
Voltage Gain
dB 10
Zo
Zin
= log------ -