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AN211A View Datasheet(PDF) - Motorola => Freescale

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AN211A Datasheet PDF : 12 Pages
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Freescale Semiconductor, Inc.
AN211A
VGS
µA
Figure 12a. V(BR)GSS Test Circuit
VDS
µA
VDS
µA
VGS
TYPE C
TYPE B
Figure 12b. V(BR)DSS and V(BR)DSX Test Circuit
(Usually Used for MOSFETs Only).
Audio
yfs (1 kHz)
Ciss
Crss
yos (1 kHz)
NF
RF-IF
yfs (1 kHz)
Ciss
Crss
GP
Re(yis) (HF)
Re(yos) (HF)
NF
Switching
Ciss
Crss
Cd(sub)
rds(on)
td1, td2
tr, tf
Chopper
Ciss
Crss
Cd(sub)
rds(on)
yfs The forward transadmittance is a key dynamic
characteristic for field-effect transistors. It serves as a basic
design parameter in audio and rf circuits and is a widely
accepted figure of merit for devices.
Because field-effect transistors have many characteristics
similar to those of vacuum tubes, and because many
engineers still are more comfortable with tube parameters,
the symbol gm used for tube transconductance is often
specified instead of yfs. To further confuse things, the “g”
school also uses a variety of subscripts. In addition to gm,
some data sheets show gfs while others even show g21.
Regardless of the symbol used, yfs defines the relation
between an input signal voltage and an output signal current:
It is interesting to note that yfs varies considerably with
ID due to nonlinearity in the ID – VGS characteristics. This
variation, for a typical n-channel, JFET is illustrated in Figure
14. Obviously, the operating point must be carefully selected
to provide the desired yfs and signal swing.
For tetrode-connected FETs, three yfs measurements are
usually specified on data-sheet tables. One of these, with
the two gates tied together, provides a yfs value for the
condition where a signal is applied to both gates
simultaneously; the others provide the yfs for the two gates
individually. Generally, with the two gates tied together, yfs
is higher and more gain may be realized in a given circuit.
Because of the increased capacitance, however,
gain-bandwidth product is much lower.
For rf field-effect transistors, an additional value of yfs is
sometimes specified at or near the highest frequency of
operation. This value should also be measured at the same
voltage conditions as those used for ID(on) or IDSS. Because
of the importance of the imaginary component at radio
frequencies, the high frequency yfs specification should be
a complex representation, and should be given either in the
specifications table or by means of curves showing typical
variations, as in Figure 15 for the MPF102 JFET.
The real portion of this high-frequency yfs, Re (yfs) or G21,
is usually considered a significant figure of merit.
yos Another FET parameter that offers a direct vacuum
tube analogy is yos, the output admittance:
yos = ID/VDS
VGS = K
In this case, the analogous tube parameter is rp — i.e.,
yos = 1/rp. For depletion mode devices, yos is measured with
gate and source grounded (see Figure 16). For enhancement
mode units, it is measured at some specified VGS that
permits substantial drain-current flow.
As with yfs, many expressions are used for yos. In addition
to the obvious parallels such as y22, gos, and g22, it is also
sometimes specified as rd, where rd = 1/yos.
VDD
BYPASS
Vin
GI
RL
Vout
D
yfs = ID/VGS
VDS = K
The unit is the mho — current divided by voltage. Figure
13 is a typical yfs test circuit for a junction FET.
As a characteristic of all field-effect devices, yfs is
specified at 1 kHz with a VDS the same as that for which
ID(on) or IDSS is characterized. Since yfs has both real and
imaginary components, but is dominated by the real
component at low frequency, the 1 kHz characteristic is given
as an absolute magnitude and indicated as yfs.
AC
VTVM
AC
Rg
VTVM
S
Yfsl =
Vout
Vin RL
Rg TYPICALLY 1 M
RL OF SUCH VALUE AS TO CAUSE
NEGLIGIBLE DC DROP AT IDSS
Figure 13. Typical yfs Test Circuit
MOTOROLA SEMICONDUCTOR AFPPoLrICMAoTrIOeNInINfFoOrmRMaAtTioIOnNOn This Product,
7
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