1

Opamp open-loop operation

The MCP6002 8-pin chip IC1 is revealed to be a dual op-amp. You can now add the two op-amps to your schematic diagram.

Previously, you observed how the IC1 pin.1 and pin.7 voltages were related to the capacitor voltage. With the op-amps performing the role of voltage comparators, explain in detail how the voltage levels at the V+ and V- op-amp inputs result in the observed outputs at the corresponding Vout pins.

Setup your working mystery circuit, with C=0.01µF.

  • Which components are connected to the inputs A+ and A-?
  • Which components are connected to the inputs B+ and B-?
  • Which inputs are the comparator reference voltages?
  • What are the measured reference voltage values?
  • What determines when the op-amps change output state?
  • Describe the waveform you observe at the output pins.
  • Explain this variation in Vout relative to changes in the voltages at the corresponding in V+ and V- pins.
2

Op-amp closed-loop operation

Application of feedback from Vout to V- causes V+ to track V-. The simplest arrangement, a direct connection from Vo to V-, is the voltage follower or unity gain amplifier.
  • What is being amplified? Derive the gain equation.
Shown is an analog memory cell using two voltage followers.
  • Use an MCP6002 to assemble the memory cell, with C=10μF. Label your circuit schematic with the correct pin numbers.
  • Verify the MCP6002 power connections: Vdd=+5V and Vss=0V.
  • For Vin, use a 1Hz, 5Vpp sine wave with a 2.5V offset so that it fits between +0V and +5V.
  • Record Vin and Vout as the switch is toggled. What voltage does Vout represent, with the switch open/closed?
  • Open the switch and note how quickly Vout=VC decreases.
  • Which op-amp characteristics are desirable in this circuit? Is the MCP6002 a good op-amp?
3

Op-amp arithmetic

An op-amp can be used to apply a linear scaling of an input voltage to obtain a desired output value. The figure shows a two op-amp circuit that can be used to evaluate the equation

Y = M*X + B = M*(X + B / M)

The first op-amp adds an offset V2 = B / M to an input voltage V1. The second op-amp sets the gain, or slope M = -RF / R.

  • Derive the transfer function for each of the op-amps.
  • Combine the results to yield the scaling equation.
This circuit requires a bipolar power supply to power the op-amps (V+ and V- relative to GND) since it uses inverting amplifier stages that produce voltage swings below GND. It cannot be used since the breadboard has a single +5V power source.

However, there is an equivalent circuit that avoids the voltage inversion and operates from a single +5V supply.

3

The single op-amp circuit shown has the linear tranfer function:

Vout = ( R + Rf )*( V1 + V2 ) / 2R

Suppose that a sensor outputs a voltage V1 representing some quantity that needs to be linearly scaled to another voltage, such that when V1=0.0V, Vout = 1.374V and when V1 = 5.0V, Vout = 4.126V.

  • Derive the circuit transfer function, assuming an ideal op-amp. What is the slope M? and the y-intercept B?
  • Manually determine values for V2 and Rf, with R = 100kΩ.
  • Construct this circuit using the MCP6002 op-amp, with power connections Vdd=+5V and Vss=0V.
  • How did you set up the voltage V2? Explain.
  • Set V1 to GND and measure Vout to verify that the output is as expected. Repeat the test with V1=5V.
  • Set up several other voltages for V1 and record Vout.
  • Tabulate and plot the data, then fit it to a straight line to get the slope and y-intercept. Compare these with M and B.