Low-Power Op Amp

Today you’ll be using a MCP6004 Quad Op Amp. This chip contains 4 low-power, low-voltage op amps that are powered by 1.8 to 6VDC. This is good choice for battery or Arduino powered applications.

• If you apply more than 6 volts you will fry this op amp!
• The op amps will be powered with +5V from your Arduino.
• Do not use the EDU36311A DC power supply* at all today.
• Power for all 4 MCP6004 op amps are supplied on pins 4 (+ve) and 11 (-ve)

MCP6004 Quad Op Amp pin assignments from its datasheet

Voltage Controlled Current Source (Transadmittance Amplifier)

• Admittance is 1/impedance.
• Transadmittance (“transfer admittance”) is the ratio of a circuit’s output current to input voltage.

The optical power emitted by a light-emitting diode (LED) is proportional to the current through it. Most power supplies deliver known voltages, and the current through a circuit depends on its (often hard to calculate and frequency dependent) impedance. The current through a diode is a very non-linear function of applied voltage, so it would be nice to directly control the current. One way to do this is with a Voltage-Controlled Current Source (VCCS) that uses an op-amp and a MOSFET. Such current sources are useful whenever current is the primary parameter of interest, e.g. LEDs or generating magnetic fields.

A voltage-controlled current source (VCCS) using an op-amp and an nMOSFET transistor to drive a current through an LED load. The optional series resistor, \(R_{stability}\), improves feedback stability.

This exercise will only be looking at positive voltages, so you should set up positive power rails supplied by the Arduino 5V pin and ground rails connected to a Arduino ground (GND) pin.

• The op amp +VDC pin 4 (\(V_{DD} \equiv V^+_{supply}\)) should be connected to the +5V rail, and the op amp -VDC pin 11 (\(V_{SS} \equiv V^+_{supply}\)) to the ground rail.
  • Be careful not to get these connections backwards!

Because the LED current is controlled, any voltage significantly above the LED’s threshold should be fine, so we can choose the LED supply voltage \(+V_{LED}\) to be \(V_{DD}\)=+5VDC, but we also need a variable DC voltage for the op amp input that sets the Gate voltage \(V_{set}\). We could use the Arduino \(5V\) pin to supply \(+V_{LED}\) and \(V_{DD}\), and your EDU36311A DC power supply for \(V_{set}\), but we want to get used to building circuits which only have a single DC supply, which is typical.

• For example, for your project you might build a standalone Arduino controlled circuit, where the Arduino is powered by a 9V battery, and the Arduino provides +5V to the circuit.

We will power the circuit from the Arduino \(5V\) pin, and use a simple trimpot voltage divider to control \(V_{set}\). Since the op amp inputs have very high impedance, we can choose a high resistance voltage divider that draws little current.

• Confirm that the voltage from the Arduino is about \(5V\).
  • The Arduino has a \(3.3V\) pin next to the \(5V\) pin, so be careful not to connect to the wrong pin.

Simulation of voltage-controlled current source with a trimpot voltage divider supplying \(V_{set}\).

Construct the above circuit shown using op amp A on your MCP6004 chip.

• As usual, the op amp power connections and decoupling capacitors are not shown, but must exist.

• Caution: You can easily destroy a MOSFET by zapping it with even small static shocks.

• The resistance between the drain (\(D\)) and source (\(S\)) pin of a 2N7000 N channel MOSFET transistor depends on the voltage \(V_{GS}\) between the gate (\(G\)) and the source.

  • Use the 2N7000 spec sheet has a diagram that clearly identifies its pins.

• A MOSFET’s gate draws essentially no DC current, so the same current flows through the load LED and the sense resistor.

• Use a clear white LED (20mA, 3.0-3.2V, 12000-14000 mcd) and choose a value for \(R_{sense}\) such that the LED’s rated current flows when \(V_{set}=2\,V\).

Confirm that the LED’s brightness changes qualitatively as expected when you vary \(V_{set}\).

• The LED may glow dimly even with \(V_{set}=0\) because of \(~\sim\mu A\) leakage currents through the transistor.

Do not disassemble this circuit; you’ll need it later.

Transimpedance Amplifier (Current Controlled Voltage Source)

• Transimpedance (“transfer impedance”) is the ratio of a circuit’s output voltage to its input current.

A photodiode produces a small current proportional to the optical power incident on it.

• Photodiodes have very large output impedance, so their output voltage will depend on the input impedance of whatever circuit they are driving unless that input impedance is huge.
  • e.g. The photodiode output impedance is comparable to the input impedance of typical oscilloscopes and voltmeters, so the measured output voltage of an isolated photodiode will depend the scope used.

To measure the current from a TEFD4300 photodiode, convert it into a voltage using the transimpedance amplifier shown in the figure below.

A transimpedance amplifier using an op-amp, with a feedback resistor \(R\sim 1 - 10 \mathrm{M}\Omega\) and a feedback capacitor \(C \sim 10-100 \mathrm{pF}\). Use a TEFD4300 photodiode.

• The photodiode only works if inserted the right way around.
  • For diodes and capacitors with different length legs, the longer leg is positive.
  • For diodes, the arrow points in the forward direction from + to -, and the bar on the physical diode corresponds to the line at the tip of arrow in diode circuit symbol.
• Use MCP6004 op amp B, C, or D. (A is in use for the previous circuit.)

The transimpedance amplifier takes advantage of the op amp’s low output impedance to decouple the photodiode signal voltage amplitude from the value of the input impedance of any subsequent circuit.

• The output voltage of this circuit is \(V_{out}=I \left( 1/R+j\omega C\right)^{-1}\). This is (again) easily derived using the op amp golden rule that it tries to adjust its output to equalize the potential at its two inputs.

Observe this output on your oscilloscope, and by covering and uncovering the photodiode, verify that the output voltage qualitatively depends on the light level.

• You may notice electrical mains (50/60Hz) noise.

Testing linearity

Place the photodiode and LED so they are point towards each other and are \(\sim 1\,\mathrm{mm}\) apart.

Apply a 100 Hz, 1Vpp, 1ms wide Pulse bias voltage to \(V_{set}\) and observe whether the photodiode is detecting the LED light.

• A pulse signal is usually defined as a square wave that does not cross zero volts. The DSOX1204G Wave Generator is a a little idiosyncratic and requires some not obvious settings:
  • In the Wavegen window, select:
    • Select Type: Pulse
    • Confirm (or set): Frequency: 100.0Hz, High Level 1.00Vpp, Low Level 0Vpp, Offset: 0V, Width: 1ms
  • Observe the pulses on your scope and confirm the bottom of the pulses is at 0 volts and the top of pulse is at 1V.
• Adjust the trimpot to see a nice signal.
• If the photodiode output is much bigger than the WaveGen input, you may want to to reduce it by pointing the LED and photodiode away from each other. This will help with the bandwidth measurement later.
• Be sure to check that the signal goes away if a strip of black paper is inserted between the LED and the photodiode. If the signal does not go away, the signal is due to electronic cross-talk between your LED and Photodiode circuits, not light transmission between them.
  • To get the cleanest signal, it is best to block or turn off any ambient light.
• Note: If the voltage divider did not have such high resistance, we’d have to think about whether the AC signal leaking into the op amp VDC power could be a problem.

Note: Falstad cannot simulate a photodiode, but if desired the LED flasher circuit can be simulated.

Light Communication

Instead of driving the LED with with WaveGen, use the output from Arduino digital pin 8 instead. Load DigitalOutputPrimes.ino into your arduino and run. It sends sequences of pulses corresponding to the first 5 prime numbers, pauses, then repeats. It should work without modification.

• Sequences of prime numbers are favourite SETI search signals.