Amplification and Feedback

First created: Mon Feb 21 2000
Last updated: Mon Feb 21 2000

Amplification and feedback circuit

The output of the front end, centered around 6 V, is plugged into the following circuit where it is compared to a 6 V reference and the difference amplified.

amplification and feedback circuit
ampl_feedback.sch Cadsoft EAGLE schema file.


This set of pictures show the clear effect of amplification.


The input voltage V1 is the output of the phototransistor front-end. It swings between 1V and 11V, with a middle value of 6V. This input is immediately buffered by a voltage follower op-amp that isolates the sensing front-end circuit from the amplifying circuit.

The middle of the diagram shows a similarly buffered virtual 6 volt reference, for comparison with the input signal.

The IC1-C op-amp amplifies the difference between V1 and V2. If r = R4/R3, the amplified difference V3 is V3 = V2 + (V2 - V1)*r. For this circuit r = 10. The relation is obtained from expressing the op-amp draws no current: (V2 - V3)/R4 = (V1 - V3)/(R3 + R4).
(R2 has no real role since the op-amp, to first approximation, draws no current.)

The amplified signal (difference) drives an LED.

The top of the diagram is the feedback circuit. (The feedback does not use the amplified signal.) The L1 coil is positioned inside the probe, between the lower magnet stack and the magnet float. Its role is to counteract the movement of the float to the extent that it can. Because the float signal is amplified, the float swing must be kept small or else it will bring the amplifier to saturation. The main benefit of keeping the probe movement small is that this will keep it in the light sensitive region, where slight movements either way still affect the amount of light transmitted. The current through the coil now becomes the best measure of the forces trying to move the float.