
Current can be determined out by measuring the voltages at
both ends of the resistance based on the above relationship.
Fig 4
Note: The “INPUT” terminal of this instrument is grounded
at one end.
8. Functional description
LMV-181A and LMV-181B are similar as far as functions are
concerned. They consist of a60 dB attenuator, input protective
circuit, impedance conversion circuit, 10 dB step attenuator,
meter pre-amplifier, meter amplifier, meter circuit, monitor
amplifier and stabilization power circuit.
1. 60 dB attenuator
Controls input voltage so that the impedance conversion
circuit will work normally. The attenuation quantity is 0dB in
the 1mV —300 mV [1.5 mV —500 mVJ range, and 60 dB in
higher ranges.
2. Input protective circuit
It consists of Qi and Q2, and protects FET Q3 from
excessively large input voltages.
3. Impedance conversion circuit +10 dB amplifier
It consists of Q3 (FET) and Q4 converts high input impedance
into low impedance, and has an amplification factor of +10
dB.
4. 10 dB step attenuator
In conjunction with the 60 dB attenuator, it selects proper
values to be measured.
5. Meter pre-amplifier
It consists of Qs and Q7 ,and amplifies small signals into large
signals.
6. Meter amplifier and meter circuit
Consists of Q9, Qio, Qii, D2, D3 and meter. The rectifier
circuits of D2 and D3 and the meter are in the feedback circuit
of the amplifier, so, the indications of the meter are in
proportion to the input voltage.
7. Monitor amplifier
1Vrms output is obtained at the “OUTPUT” terminal when
the meter indicates full scale after asignal is taken out of a
part of the meter preamplifier and amplified.
8.Stabilization power circuit
This circuit consists of D4 ,D5 ,and D7 rectifier circuits
and Dg, Qi 2and Q13 stabilization circuits. With Dg Zener
diode as the reference voltage, it supplies +18V stabilized
voltage to the impedance conversion circuit, meter pre-
amplifier, meter amplifier and monitor amplifier.
9. Maintenance
9-1 Changing fuses
Afuse can be removed by turning the fuse holder on the rear
side in the direction of the arrow. Use 0.1 Afuses. After fuses
are changed, be sure to investigate the cause. Take appropriate
steps before power is turned back on.
11 12