# Amplifiers and PCT

Today I attended a workshop on the basic science background of brainwave
training.

There was discussion of amplifiers: single-ended and differential. I was
wondering if there was a connection to negative feedback control
systems. A diagram was put on the board.
input A
0----------:\
: \
: / A-G
0----------
ground G

The output of the amplifier is A-G and looks like the error signal. The
input A looks like it could be the perceptual signal. The input G looks
like it could be the reference signal. If the error signal is connected
back to input A, do we have a negative feedback system?

Some help from the more technologically enlighted list members would be
appreciated.

···

From: David Goldstein
Subject: Amplifiers and PCT
Date: 4/18/97

[Robert Kosara, 970419.0845 MEST]

From: David Goldstein
Subject: Amplifiers and PCT
Date: 4/18/97

[...]
input A
0----------:\
: \
: / A-G
0----------
ground G

The output of the amplifier is A-G and looks like the error signal. The
input A looks like it could be the perceptual signal. The input G looks
like it could be the reference signal. If the error signal is connected
back to input A, do we have a negative feedback system?

No, you have to get the feedback signal (the CV) from the outside world,
the error signal only controls the operation of some kind of manipulator.
If I may extend your block diagram (the output is B(A-G), by the way,
where B is the amplification factor, or, in PCT-speek, the loop gain):

···

+--------------------------+
> input A |
+-----------:\+ ^ (some kind of sensor)
: \_________\ Real
: / A-G / World
Reference -------------:/- (some kind
signal ground G of manipulator)

As a concrete example, think of a (simple one-level) system
that is built to control the state of a door. It gets its input from a
potentiometer (a resistor whose resistivity changes dependent on the
position of a lever; the volume dial on your radio is one example), and
its output controls a motor. Usually, the door should be closed, so we set
the reference signal accordingly. Every time somebody opens the door, the
error signal will make the motor run until it is closed again. In case of
a fire, the reference signal could be changed to 'open', which would make
the motor run to open the door (in case it isn't already).
In this example, the manipulator is the motor, and the sensor is the
potentiometer.

Robert

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[From Bruce Abbott (970419.1150 EST)]

From: David Goldstein
Subject: Amplifiers and PCT
Date: 4/18/97

Today I attended a workshop on the basic science background of brainwave
training.

There was discussion of amplifiers: single-ended and differential. I was
wondering if there was a connection to negative feedback control
systems. A diagram was put on the board.
input A
0----------:\
: \
: / A-G
0----------
ground G

The output of the amplifier is A-G and looks like the error signal. The
input A looks like it could be the perceptual signal. The input G looks
like it could be the reference signal. If the error signal is connected
back to input A, do we have a negative feedback system?

Yes, you _do_ have negative feedback (assuming that input A is the inverting
input of the amplifier), but you do _not_ have a control system. The
circuit element you diagram is an operational amplifier, and input A would
receive the signal to be amplified. The noninverting input, which you show
tied to ground, is not a reference signal; it is simply the ground of the
circuit. The output variation will be proportional to the input variation
of the signal (but inverted) and greater in magnitude as determined by the
gain (e.g., for a gain of 100 the output amplitude will be 100 times the
input amplitude).

Without feedback, the op amp will be at maximum gain (e.g., 100,000); what
the feedback (with series resistor) does is limit the gain; by choosing an
appropriate feedback resistance, you can make the gain what you wish, up to
the open-loop maximum.

I'm not an electronics expert; Bill P. can give you a more detailed
explanation if you need it, and show how a differential amplifier could be
employed as the heart of an electronic control system. (Control systems
have gains > 1 and therefore contain an amplifier). Connected as given
(with signal and feedback to the inverting input and the noninverting input
grounded), this circuit would act as a current-to-voltage converter.

Regards,

Bruce