User guide

Atlas DCA Pro User Guide December 2012 – Rev 1
Page 26
I
D
=6.67mA
at V
GS
=0.00V
and V
DS
=4.78V
V
GS
(on)=-1.10V
at I
D
=5.00mA
g
fs
=13.6mA/V
at I
D
=3mA to 5mA
“On” Characteristics
The DCA Pro measures the gate-source
voltage required to reach the onset of
good conduction through the JFET’s
drain-source. Good conduction is
determined when the drain-source
current reaches 5mA. For JFETs that have a lower saturation current than 5mA,
the DCA Pro will try to use a lower “on” current.
Transconductance
While the JFET is conducting, the JFET’s gain (transconductance) is measured.
Transconductance is often measured in mA/V (sometimes known as mmhos or
mSiemens). This refers to the change in drain current that results from a change
in gate-source voltage:
g
fs
= ∆I
DS
/ ∆V
GS
The DCA Pro measures transconductance by determining the gate voltage
change necessary to obtain a drain
current change from 3.0mA to 5.0mA.
Zero Gate-Source Voltage
Finally, the drain current for a zero
gate-source voltage is measured.
Transconductance is measured by the DCA Pro over a small range of
drain current (typically a span of 3mA-5mA). Values of
transconductance higher than 20mA/V can yield a coarse
measurement resolution because the required change in gate voltage is
so tiny. Additionally, values above 99mA/V are displayed as
“>99mA/V”.