AN211A View Datasheet(PDF) - Motorola => Freescale
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AN211A Datasheet PDF : 12 Pages
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AN211A
Freescale Semiconductor, Inc.
For high-frequency circuits, the input capacitance (Ciss)
and the Miller-effect capacitance (Crss) become important,
so yfs/(Ciss + Crss) indicates a relative measure of device
performance. For switching and chopper circuits, a figure of
merit is not often useful. Here the magnitudes of Ciss, Crrs,
Cd(sub) and rds are of primary interest.
Circuits
With its high input impedance, the field-effect transistor
will play an important role in input circuitry for instrumentation
and audio applications where low-impedance junction
transistors have generally been least successful.
+ 12 V
The types of circuits that can utilize FETs are practically
unlimited. In fact, many circuits designed to utilize
small–signal pentode tubes can utilize FETs with only minor
modifications. For example, the circuit in Figure 21 shows
a typical rf stage for a broadcast-band auto radio. In this
circuit, a MPF102 n-channel JFET has replaced the 12BL6
pentode normally employed. The specifications for the two
devices, including the AGC characteristics, are similar
enough to perform adequately in the circuit of Figure 21.
In an audio application, a field-effect transistor such as
the 2N5460 can be combined with a high voltage bipolar
transistor to make a simple line-operated phonograph
amplifier such as that shown in Figure 22. The ceramic
pickup is connected through a potentiometer volume control
to the field-effect transistor. Collector current of the transistor,
in turn, is set by the potentiometer in the source of the FET.
With the proper bipolar output transistor, the circuit can be
driven directly from the rectified line voltage, while the low
voltage for the FET can be derived from a voltage divider
in the power supply line.
Figure 23 shows three basic chopper circuits. The
advantage of the more complex series-shunt circuit (24c) is
that it balances out the leakage currents of the FETs in order
to reduce voltage error and is used to attain high chopping
frequencies. From an applications standpoint, the FET circuit
is superior to a junction transistor circuit in that there is no
offset voltage with the FET turned on. On the minus side,
however, the field-effect-transistor chopper generally has a
higher series resistance (rds(on)) than the junction transistor.
As newer and better FETs are introduced and as a larger
number of designers learn to use them, the range of
applications of FETs should broaden considerably.
TUNING
MPF102 D
S
GI
TO MIXER
AGC
Figure 21. RF Stage of Broadcast Auto Radio
MFE4007
D
G
S
VOLUME
BIAS
ADJUST
TONE
- 30 V - 120 V
Figure 22. Line Operated Phono Amplifiers
10
For More InformMaOtiToOnROOLnATShEiMsICPOrNoDdUuCcTtO, R APPLICATION INFORMATION
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