Tubes, Discrete Solid State Devices, and Integrated Circuits 331signal is small compared to the dc bias current and
voltage. Transistors are normally biased at current
values between 0.1 mA and 10 mA. For large-signal
operation, the design procedures become quite involved
mathematically and require a considerable amount of
approximation and the use of nonlinear circuit analysis.
It is important to provide an impedance match
between cascaded stages because of the wide difference
of impedance between the input and output circuits of
transistors. If the impedances are not matched, an appre-
ciable loss of power will take place.
The maximum power amplification is obtained with
a transistor when the source impedance matches the
internal input resistance, and the load impedance
matches the internal output resistance. The transistor is
then said to be image matched.
If the source impedance is changed, it affects the
internal output resistance of the transistor, requiring a
change in the value of the load impedance. When tran-
sistor stages are connected in tandem, except for the
grounded-emitter connection, the input impedance is
considerably lower than the preceding stage output
impedance. Therefore, an interstage transformer should
be used to supply an impedance match in both directions.
When working between a grounded base and a
grounded-emitter circuit, a step-down transformer is
used. Working into a grounded-collector stage, a
step-up transformer is used. Grounded-collector stages
can also be used as an impedance-matching device
between other transistor stages.
When adjusting the supply voltages for a transistor
amplifier employing transformers, the battery voltage
must be increased to compensate for the dc voltage drop
across the transformer windings. The data sheets of the
manufacturer should be consulted before selecting a
transformer to determine the source and load
impedances.
Transistor Noise Figure (nf ). In a low-level ampli-
fier, such as a preamplifier, noise is the most important
single factor and is stated as the SNR or nf. Most ampli-
fiers employ resistors in the input circuit which
contribute a certain amount of measurable noise
because of thermal activity. This power is generally
about –160 dB, re: 1 W, for a bandwidth of 10,000 Hz.
When the input signal is amplified, the noise is also
amplified. If the ratio of the signal power to noise power
is the same, the amplifier is noiseless and has a noise
figure of unity or more. In a practical amplifier some
noise is present, and the degree of impairment is the
noise figure (nf ) of the amplifier, expressed as the ratio
of signal power to noise power at the output:
(12-29)where,
S 1 is the signal power,
N 1 is the noise power,
So is the signal power at the output,
No is the noise at the output.(12-30)For an amplifier with various nf, the SNR would be:An amplifier with an nf below 6 dB is considered
excellent.
Low nf can be obtained by the use of an emitter
current of less than 1 mA, a collector voltage of less
than 2 V, and a signal-source resistance below 2000:.Internal Capacitance. The paths of internal capaci-
tance in a typical transistor are shown in Fig. 12-28. The
width of the pn junction in the transistor varies in accor-
dance with voltage and current, and the internal capaci-
tance also varies. Variation of collector-base
capacitance C with collector voltage and emitter current
is shown in Figs. 12-28B and C. The increase in the
width of the pn junction between the base and collector,
as the reverse bias voltage (VCB) is increased, is
reflected in lower capacitance values. This phenomenon
is equivalent to increasing the spacing between the
plates of a capacitor. An increase in the emitter current,
most of which flows through the base-collector junc-
tion, increases the collector-base capacitance (CCB). The
increased current through the pn junction may be
considered as effectively reducing the width of the pn
junction. This is equivalent to decreasing the spacing
between the plates of a capacitor, therefore increasing
the capacitance.
The average value of collector-base capacitance
(CCB) varies from 2–50 pF, depending on the type tran-
sistor and the manufacturing techniques. The
collector-emitter capacitance is caused by the pn junc-
tion. It normally is five to ten times greater than that of
the collector-base capacitance and will vary with the
emitter current and collector voltage.nf SNR
1 dB 1.26
3 dB 2
10 dB 10
20 dB 100nfS 1 uNo
SouN 1------------------=nfdB= 10 log nf of the power ratio