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Examining system behavior at different gain values
Resistance = 10 MΩ
Right now, the OpAmp's output and minus input are connected with a 10 MΩ resistor.
- What happens to the LED when you power up the circuit?
- What happens to the LED when you shine the flashlight on the photodiode?
- Can you get the LED to hold steady at 1/2 its maximal brightness, by moving the flashlight farther away, shading it, etc?
A circuit with very tight fully-on-or-fully-off behavior is more "digital", or switch-like, while a circuit where the LED can have a wide middle range of brightness is more "analog", or dial-like. The range of flashlight intensities that can hold the LED half-lit is a measure of the gain or strength of the amplifier. More precisely, the gain is the slope of the LED-output-vs.-photodiode-input line. Because the maximum brightness is the same for every circuit, a high-gain amplifier will cause the LED's brightness to max out even at a low level of input to the photodiode, whereas a low-gain amplifier will cause the LED's brightness to increase slowly before maxing out, as the photodiode input increases. We can tune this gain by changing the value of the gain resistor.
4. Sketch a graph with flashlight intensity on the x-axis and LED light intensity on the y-axis. At infinite resistance in place, is the circuit's behavior better described as a switch or a dial?
Resistance = 0Ω
Replace the 10MΩ resistor with a wire.
- What happens to the LED when you power up the circuit?
- What happens to the LED when you shine the flashlight on the photodiode?
- Can you get the LED to hold steady at 1/2 its maximal brightness?
- Add a line for this circuit to your graph. Is this circuit's behavior better described as a switch or a dial?
Resistance = infinite Ω
Remove the wire connecting the OpAmp's output to its negative input.
- What happens to the LED when you power up the circuit?
- What happens to the LED when you shine the flashlight on the photodiode?
- Can you get the LED to hold steady at 1/2 its maximal brightness?
- Add one last line to your graph. Is this circuit's behavior better described as a switch or a dial?
Discussion
- How does this relate to the end of the computer simulation exercise, when you tuned the system with sliders?
- When might you want switch-like behavior in a biological system, and when might you want dial-like behavior?
- What are some everyday examples of switch-like and dial-like systems? What element or aspect plays the role of "gain" in each one?
 An air conditioner is an example of a switch-like system. The cold air is either all the way on or all the way off, depending on the current temperature. The gain is how powerful the air stream is and how cold it is -- these aspects control how fast the A/C can change the temperature of the room. A cruise control is an example of a dial-like system. The amount by which it depresses or lets up on the gas pedal is directly proportional to the difference between the current speed and the desired speed, not just on-off. The gain is the relationship between the gas pedal angle and the acceleration; it will be affected by factors like the construction of the gas pedal, the engine power, the weight of the car, and air resistance.
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