HW8_Soln - Escuti/Brickley 2008 ECE200 Fall 2008 Homework 8...

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Escuti/Brickley 2008 ECE200 – Fall 2008 – Homework 8 Solutions Amplification and Operational-Amplifiers Page 1 of 13 Problem 1 – [12 pts] Find an equation for the output voltage in terms of the resistors and the given voltages. Note that a photocell is included as a light-sensitive resistive element. (a) (b) (c) If the 100 Ω resistors were light bulbs, these circuits could be used to create a night-light (a lamp that turns on when the ambient light gets dark). Knowing that the photocell can generally have a range of ~100 Ω (when light) to 500 k Ω (when dark), which one of these circuits would be best in this application? Why? (note: you may assume that R f = R n = 500 k Ω ) Solution: a) This is simply a non-inverting amplifier with output-input relationship: b) Similar to the circuit in part (a), but with the inverting amplifier instead:
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Escuti/Brickley 2008 ECE200 – Fall 2008 – Homework 8 Solutions Amplification and Operational-Amplifiers Page 2 of 13 c) The following matrix applies to part (a) and part (b): Bright Light ( R photo = 100 Ω ) Low Light ( R photo = 500k Ω ) Output of Circuit (a) which will of course be clipped by the power supply voltage Output of Circuit (b) It should be obvious that Circuit (b) is best for the night light application since it provides a moderate voltage across the lamp resistor (–5 V) when it is dark and a tiny voltage (–1 mV) when it is bright. Circuit (a) will not work at all for this.
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Escuti/Brickley 2008 ECE200 – Fall 2008 – Homework 8 Solutions Amplification and Operational-Amplifiers Page 3 of 13 Problem 2: [12 pts] Consider the op-amp circuit below. (a) Find an equation for the output voltage of the circuit below in terms of the resistors and the given voltages. Note that R var is a potentiometer. (b) The circuit at left is essentially a
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HW8_Soln - Escuti/Brickley 2008 ECE200 Fall 2008 Homework 8...

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