BRUEL & KJAER 2209 - Meter Amplifier and Hold Circuit

4.10. METER AMPLIFIER AND HOLD CIRCUIT
The mode of operation of this section can be seen from Fig.4.21. The
output of the RMS detector circuit (E
1
in Fig.4.21) is fed to an operational
amplifier which charges the capacitor C to a voltage V
2
,
very nearly equal to
E
1
.
If E
1
is then decreased or removed the diode D will cause the voltage on
C (nearly equal to V
2
) to be stored. C will only be discharged by leakage of
the components or leakage through a resistor in parallel.
The decay time constant for impulse measurements is specified in the
I EC proposal for Impulse Sound Level meters at 3 sec ± 0.5 sec. Since the
figures in this standard are determined only by the peak value of the voltage
on the averaging condenser in the RMS circuit, the "Imp. Hold" and "Im-
pulse" positions will give rise to the same reading for the figures specified.
Measurements on single impulses will, however, be much more easily
carried out in the "Imp. Hold" position. This position will also be very
convenient if the maximum RMS value of a varying Sound Level is wanted.
Imp .
1 E1
Hold
0
+
~~---+--~-------c~--~
/69/8£
Fig.4.21. Meter "Hold" circuit
The hold action can be released by the METER RESET button which
short circuits the diode D. This diode is short circuited automatically when
the meter is set on the "Fast" or "Slow" meter dampings. The high ampli-
fication of the operational amplifier prevents the voltage drop across the
diode having any significant influence on the linearity of the circuit. The
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