BK Precision 501A, Instruction Manual

Page: 29 / 40

gm=
t.Io
at a given drain voltage
t.VG
t:NG 10.2v -o.1v1
10
l
.
.
.. �
• : • """! .. "'""":"
'
ei
······-:··········-'.
:
:
: o
····· · · · · · ' · · · · ··· · · ·
2
· ···· · · · · ·'··
·
· · · ····
4
·· · · · ·· · ·· · ··· · · ·
··
s
· · · · · · · · ·· · · · ··
Vos
:, .......... :
.
......... .i
8 10
Figure 27. FET Gain (Transconductance)
Measurement
is the ratio of change in drain current to the change
in gate voltage at a given drain voltage. Transcon­
ductance is measured in mhos. As an example, if
increasing the gate voltage from 1 volt to 1.5 volt
(6 VG = .5 V) causes the drain current to decrease
from 6 mA to 4 mA (Mo = 2 mA) at a drain voltage
of 6 volts, the forward transconductance of the FET
is 4000 micromhos. Another example is shown in
Figure 27.
To calculate gm, note the diflerence in drain cur­
rent between two curves (6 Io), at the same drain
voltage from the family of curves. Note the change
in gate voltage (6 VG) from the STEP SELECTOR
switch setting. Calcuate gm by dividing 6 VG into
6 Io.
A
•••••••••• • • •• • • •• •• , ........... , ........... , ........... , ........... , ........... 0 .......... , ••••••••••• , ••••••••••• ,
/ /',,/- .. VG=
-i
v
,
0,-t/i··t;;;/'-'·· :- i=j:;i::;;:::..+.a.-r""i"""-t··i···:
, ........ , 1., ...... 1/, .. , .... ,,"' ... ,� ........... , ........... , ... VG•
.. ..f
£.
. .. ,,;,,
/tec.---.:'.
. .
1.:,-::: .. +. .. -:.t . -
V-tG-. """1.z-vr---1
'!f l
VG• -2.5v
Vos
As with NPN and PNP transistors, the gain of a
FET is not constant over its entire voltage and cur­
rent range. The gain is normally calculated in its
typical operating range. Distortion and linearity
may be determined by the same method as desribed
for transistors; if the spacing between curves is
equal, the FET is linear.
Pinch-Off
(Vp)
Voltage Measurement
An important characteristic for depletion mode
FET's is the amount of gate voltage required to turn
off drain current. This value is called the pinch-off
voltage characteristic and may be measured from
the family of curves.
An example is shown in Figure 28. Figure 28 A
shows the STEP SELECTOR set at .5 volt per step
and the entire family of curves is displ
ay
ed. Drain
current continues to flow at its highest step of -2.5
volts. Figure 28 B shows that when the STEP SELEC­
TOR is increased to 1 volt per step that the entire
family of curves is not displayed. In fact, the -3
volt, -4 volt and -5 volt curves are superimposed
upon each other at zero drain current. All gate volt­
age values greater than -3 volts prevent drain cur­
rent. We can conclude that pinch-off occurs between
-2.5 volts and -3 volts. A more precision meas­
urement could be made,
if
necessary, by connecting
an external de bias supply to the gate of the FET,
adjusting the bias supply, and observing the exact
value of pinch-off voltage on the oscilloscope.
B
...........
'o•
'
'
/
/
'
VG -lv
Io
.,:.---
//
!
!
/
..
VG•
'
J . .
,,......
,
VG=
-3v (PINCHED
Vos
Figure 28. Determining FET Pinch-Off Voltage
23