ADC Sound Shaper Three - LED Meter Comparator Circuit Description; Operational Amplifier Feedback Circuit

LED METER COMPARATOR CIRCUIT DESCRIPTION
Power Supply and bias configuration (Refer to LED METER circuits)
The power supply for the LED Meter Comparator circuit consists of full-wave rectifiers, which
provide plus (+) and minus (-) +15.4/-15 VDC to each of IC802 through IC814.
LED indication level is provided from the Zener diode D71 0 (-5.6 VDC).
The +15.4/-15 VDC source is applied to LED comparator ICs IC802 through IC814.
The -5.6 VDC source provides a stable voltage to VR801 (L) and VR802 (R) for LED Meter
indicator level.
Signal to the (-) input causes a change in output that is inverted in phase relative to the input.
Signal to the (+) input causes a change in output that is in phase with the input.
With no signal to Rin, the (+) input sees 0 Volts through RB, causing the output to be positive-
going. (-) input voltage is equal to (+) input voltage
(= 0 V) - known as Imaginal Short.
When the output reaches 0 V, the (-) input also sees0 V-through resistor RF and further output
change is inhibited. The output, (+) input and (-) input are now all 0 V.
The (-) input remains at 0 V regardless of the signal into Rin.
Example: An input more positive than 0 V to Rin causes the output to be negative-going. The
output continues to drop until the feedback through RF is lowered by an amount suf-
ficient to equalize the imbalance between the (+) and (-) inputs causes by the input
signal. Because of the amplifier's high gain and speed, the imbalance between the (+) and
(-) inputs is always small.
Since the voltage at the (-) input is always at 0 Volts, the (-) input is effectively at AC ground.
Therefore, the entire input signal appears across Rin and the entire output signal appears across R
F •
Since the (-) input voltage is always 0 V the current into the amplifier's (-) input is constant. Since
this current is supplied by Rin and RF any change in current due to input signal through Rin is
offset by an opposite and equal change of current through RF.
For AC signal currents, if we ignore the negative values indicating signal inversion, the gain of the
amplifier can be calculated as follows.
I (Rin)
= I (RF) Since 1= g_ E (Rin) = E (RF) __B__E = E (RF) GAIN
R' Rin RF' Rin E (Rin)
NOTE: RC4709 of this circuit operates from split power supplies. [plus (+) supply at Pin No. 14
and minus (-) supply at Pin No.7].
OPERATIONAL AMPLIFIER FEEDBACK CIRCUIT (Refer to Figure B)
AMP OUTPUT
BASIC OP. AMP. FEEDBACK CIRCUIT
Figure B
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