F.E.T. CAL Check

TM 11-6625-539-15-2

Figure 5-4 Ico (leakage) check, simplified schematic diagram.

to-revere current ratio is read directly on the BETA

diode is forward-biased and conducts heavily. Resis-

scale of the meter.

tors R9, R5, meter shunt R27, and the forward resist-

ance of the test device limit the current through

5-7. F.E.T. CAL Check

meter M1. CAL potentiometer R5 is adjusted to set

a. A schematic diagram of the circuit used to

the current through M1 to 100 ma which corresponds

calibrate the test set for field effect transistor

to a full-scale meter reading. After CAL potenti-

( F. E.T.) transconductance measurement is shown in

ometer R5 has been set to provide a full-scale reading,

figure 5-6. Figure 5-6 shows an N-channel F.E.T.

the polarity switch is set to NPN REV as shown in

transistor; however, the same principle applies for a

B, figure 5-5. The circuit is essentially the same as

P-chnnnel F.E.T. transistor.

the circuit used to obtain the reference reading; the

b. The F.E.T. CAL position of the function switch

sets the signal level applied to the test device at 100

only change is the reversal of the applied voltage

millivolts (mv) as follows. The 1-kHz multivibrator

and reversal of the meter. The test device is now back-

applies a symmetrical square wave between the

biased and offers a high resistance. Circuit current is

source and the gate of the F.E.T. under test. A volt-

proportionally decreased because of the increased

age divider, consisting of resistors R18 and R17,

resistence of the test device, and consequently, meter

shunts the source gate junction of the test device; a

deflection is proportionally reduced. The forward-

portion of the 1-kHz square wave applied between

5-3