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参数资料
| 型号： | AN211A |
| 厂商： | Motorola, Inc. |
| 英文描述： | FIFELD EFFECT TRANSISTORS IN THEORY AND PRACTICE |
| 中文描述： | FIFELD场效应晶体管理论与实践 |
| 文件页数： | 8/12页 |
| 文件大小： | 340K |
| 代理商： | AN211A |
8
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Figure 14. Forward Transfer Admittance versus Drain
Current for Typical JFETs
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Figure 15. Forward Transfer Admittance versus
Frequency
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Figure 16. y
os
Measurement Circuit for Depletion FETs
Voltages and frequencies for measuring y
os
should be
exactly the same as those for measuring y
fs
. Like y
fs
, it is
a complex number and should be specified as a magnitude
at 1 kHz and in complex form at high frequencies.
μ
tor,
μ
:
Closely related to y
os
and y
fs
is the amplification fac-
μ
=
V
DS
/
V
GS
I
D
= K
The amplification factor does not appear on the field-effect
transistor registration format but can be calculated as y
fs
/y
os
.
For most small-signal applications,
μ
has little circuit
significance. It does, however, serve as a general indication
of the quality of the field-effect manufacturing process.
C
iss
takes the place of y
is
in low-frequency field-effect transistors.
This is because y
is
is entirely capacitive at low frequencies.
C
iss
is conveniently measured in the circuit of Figure 17 for
the tetrode JFET. As with y
fs
, two measurements are
necessary for tetrode-connected devices.
At very high frequencies, the real component of y
is
becomes important so that rf field-effect transistors should
have y
is
specified as a complex number at the same
conditions as other high-frequency parameters. For
tetrode-connected rf FETs, reading of both Gate 2 to source
and Gate 1 tied to Gate 2 are necessary.
In switching applications C
iss
is of major importance since
a large voltage swing at the gate must appear across C
iss
.
Thus, C
iss
must be charged by the input voltage before
turn-on effectively begins.
The common-source-circuit input capacitance, C
iss
,
C
rss
FET data sheets. Instead C
rss
, the reverse transfer
capacitance, is specified at low frequency. Since y
rs
for a
field-effect transistor remains almost completely capacitive
and relatively constant over the entire usable FET frequency
spectrum, the low-frequency capacitance is an adequate
specification. C
rss
is measured by the circuit of Figure 18. For
tetrode FETs, values should be specified for Gate 1 and for
both gates tied together.
Again, for switching applications C
rss
is a critical
characteristic. Similar to the C
ob
of a junction transistor, C
rss
must be charged and discharged during the switching
interval. For a chopper application, C
rss
is the feedthrough
capacitance for the chopper drive.
Reverse transfer admittance (y
rs
) does not appear on
C
d(sub)
capacitance becomes an important characteristic affecting
the switching behavior. C
d(sub)
appears in parallel with the
load in a switching circuit and must be charged and
discharged between the two logic levels during the switching
interval.
For the MOSFET, the drain-substrate junction
Noise Figure (NF)
field-effect transistors generate a certain amount of noise.
The noise figure for field-effect transistors is normally
specified on the data sheet as “spot noise”, referring to the
noise at a particular frequency. The noise figure will vary with
frequency and also with the resistance at the input of the
device. Typical graphs of such variations are illustrated in
Figure 19 for the 2N5458. From graphs of this kind the
designer can anticipate the noise level inherent in his design.
Like all other active components,
F
Freescale Semiconductor, Inc.
n
.
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