Lab 3 Slide 1 PYKC 19 May 2016 EA1.3 - Electronics Laboratory Experiment 3 Operational Amplifiers Peter Cheung Department of Electrical & Electronic Engineering Imperial College London URL: Design Engineering 1.3 – Electronics

Lab 3 Slide 2 PYKC 19 May 2016 EA1.3 - Electronics Preparation u You will be using the breadboard on your BB to build circuits. u You will also be using signals produced by your BB as inputs to your circuits. u You will need the following special components which you will find either already on the breadboard or in the component box for your group. It includes: LM324 Quad Op Amp in 14-pin DIP – installed (dual in-line package) LM386 Audio Amplifier in 8-pin DIP – installed AP430i voltage regulator in a TO-92 package (not installed) 4700F electrolytic capacitor 32  miniature speaker u You will also need various capacitors, resistors and wires to construct your circuit. These can be found in the Lab. u As for tools, you will need a wire cutter, a pair of pliers and a wire stripper. These will be provided on your bench.

Lab 3 Slide 3 PYKC 19 May 2016 EA1.3 - Electronics Further preparation u Shown below is how you would put the LM324 in the breadboard. u The breadboard is powered directly by the Power Pack with a separate red-and-black wire as shown. This also powers the BB through the ribbon cable. LM324 u The pin connection of the LM324 is shown here.  This is already connect to +5V and GND terminal, which is decoupled with a 0.1F capacitor. LM324 LM386 +5V 0.1  F GND Pin 1

Lab 3 Slide 4 PYKC 19 May 2016 EA1.3 - Electronics Task 1: Unity Gain Buffer Amplifier  Write down the equation for the signal V S.  Measure and explain the waveform at V O.  Why is such a unity gain amplifier useful in a practical engineering situation? Vcc = +5v + R2 BB SW=9 Vo Vs 200k 0.1 ¼ LM u Build this circuit on the breadboard. u Test the circuit with the Black Box producing a source signal under the switch setting of 9. u The source signal is a 0.7V, 1kHz sine wave sitting on a 1.6V DC. C1

Lab 3 Slide 5 PYKC 19 May 2016 EA1.3 - Electronics Task 2: Failed x2 Amplifier  With a gain of 2, the signal is now a 1.4V sine wave on a 3.2 DC offset. This op-amp can only drive an output range of around 0V to 4V (supply voltage – 1.0V).  Explain what you see at V O. V + = +5v + R2 BB SW=9 Vo R1 200k 0.1 200k ¼ LM u Now add R1=200k to your circuit and derive the amplifier gain equation to confirm that this amplifier has a gain of 2. u Test this circuit with the Black Box switch setting on 9 again. Vs

Lab 3 Slide 6 PYKC 19 May 2016 EA1.3 - Electronics Task 3: Build a 2.5V voltage reference  This circuit provides a near perfect dc voltage source (V TH ) at 2.5V with very low equivalent (R TH ) resistance of 0.5.  Check that the output voltage at the cathode terminal (3) is indeed 2.5V. You may put a load resistance of, say 1k (i.e with a load current of 2.5mA). The output voltage should still be 2.5V. u Before we deal with the problem caused by single power supply opamp, we need to have a good voltage reference at 2.5V. u Build the circuit shown here using a voltage regulator circuit AP431i. u We will be using this reference voltage later. R V2.5V reference cathode anode ref AP431i 3 1 2

Lab 3 Slide 7 PYKC 19 May 2016 EA1.3 - Electronics Task 4: AC coupled Amplifier  Replace the 200k R1 with a 20k resistor. Calculate the gain of the amplifier.  Change the BB setting to 10, which produces a 100mV sine wave with a 1.6V DC offset at 440Hz (musical note A4). Unplug and plug the power to change the waveform output.  Check that V O is as you expected. u Read the description of the circuit in the notes below. u Build this circuit on the breadboard. u Test the circuit with the Black Box in switch setting of 9 again. u Unlike Task 2, you now have a x2 amplification of the 0.7V sine wave without distortion. 3 +5v + R2 BB SW=9 Vo Vs R1 200k 0.1 200k ¼ LM  R4 2.5v 5v R3 10 AP431i 200k 510 C1C2 C3

Lab 3 Slide 8 PYKC 19 May 2016 EA1.3 - Electronics  You will see that the LM324 OpAmp does not have the output current capability to drive a 32 ohm load.  We need a different solution. Task 5: Failure to drive a speaker  3 +5v + R2 BB SW=10 Vo Vs R5 200k 0.1 20k ¼ LM  R4 2.5v 5v R3 10 AP431i 200k 510 C1C2 C3 u We want to hear the amplified signal on a speaker.  Connect the output of OpAmp to 4700F capacitor in series with a 32 speaker as shown. u Measure the waveform at output V O again.

Lab 3 Slide 9 PYKC 19 May 2016 EA1.3 - Electronics  Measure VO to confirm that indeed you get a nice 440Hz sine wave and the sound from the speaker is now NOT distorted. Task 6: Using an Audio Amplifier  3 +5v + BB SW=10 Vo Vs R6 0.1 LM  C1C2 u In order to drive a 32W speaker, we need to use an amplifier designed to drive a low impedance. u LM386, which is already on the breadboard, is a specially designed audio frequency amplifier with a gain of 20dB. u Build the circuit as shown here. 0.05 10 C3

Lab 3 Slide 10 PYKC 19 May 2016 EA1.3 - Electronics Task 7: Hear the Heart Beat  3 +5v + BB SW=11 Vo Vs R6 0.1 LM  C1C2 u Use the BB with the switch setting on 11, and obtain the cardiac signal output. u Measure the signal at V O, and hear the signal on the speaker. 0.05 10