Physics 3108 OpAmp Lab Purpose To introduce the student to the basic operation of an OpAmp, and to the electronic breadboard that is used for prototype testing of the electronic circuits. Electronic Breadboard An electronic breadboard is used to quickly put together electronic circuits without the need to use solder. This enables rapid prototyping of circuits. For more information look at: http://en.wikipedia.org/wiki/Breadboard Figure 1 – schematic of breadboard The breadboard is internally wired according to the diagramme in Figure 2. The two top and two bottom rows are referred to as bus strips. They are used to provide power and ground to the circuits. The columns (labelled A-J in Figure 1) are used to connect electronic devices. Figure 2 – electrical schematic of breadboard. The orientation can be either way. The electronic breadboard is design to be used with simple devices (resistors, capacitors, inductors) as well as dual in-line packaged devices (as shown in Figure 3) using pins spaced by 0.1” (2.54mm) . Figure 3 DIP (dual in-line package) OpAmp Background Most operational amplifiers have two input terminals, an inverting (-) and a non-inverting (+) input, as shown in Figure 4. V2 Vout V1 Figure 4 Schematic of basic OpAmp This arrangement is know as a differential (or difference) amplifier because the output voltage is proportional to the difference between the two input voltages: Vout = A0 (V1 − V2 ) where A0 is the open-loop (i.e. without feedback) gain of the opamp (typically 105-106). Opamps are designed for use in negative-feedback circuits; they have high input impedances and low output impedances and in many applications they can be treated as ideal amplifiers. With suitable feedback they can be used to carry out various analogue operations (integration, differentiation, summation, …) whence the name. The device requires positive and negative power supplies (in this experiment +12V and -12V). Figure 2 An inverting amplifier Consider the circuit in Figure 2. a simple way of getting the voltage gain of this circuit, and one which makes it easy to analyse similar circuits, is to use the idea of a ‘virtual earth’. If the open-loop gain, A0 is large and the device is not to be overloaded, the input voltage at point v- must be small (a few microvolts); point v- is a ‘virtual earth’. As a result, the input current iS = VS RS and the same current flows in RS and RF since none goes into the opamp. Therefore: V V V R iS = S = −iF = − out , and the voltage gain is: out = − F . RS RF VS RS In this circuit, the ‘operation’ performed is that of multiplication by a constant. A Typical Opamp. Figure 5 – A typical opamp including pin configuration. Procedure: Part A – Inverting Amplifier – double sided 1. Make sure that the prototyping board is switched off. 2. Build an inverting amplifier as shown in Figure 2 powering the OpAmp with ±12V, and using RS=1kΩ, and RF=10kΩ 3. Switch on the power and apply a 1kHz sine wave as an input signal. Measure the voltage gain of the amplifier. 4. Measure the voltage gain for RF=100kΩ and RF=1MΩ. 5. Examine the saturation of the output. How high a voltage can the output of the OpAmp reach? How does this compare to the voltage that supplies the OpAmp? 6. Confirm that you are able to theoretically analyse the circuit yourself. Part B– Inverting Amplifier – single sided 1. Make sure that the prototyping board is switched off. 2. Change the –V input to the OpAmp to ground, so that your circuit is powered by +12V and 0V. 3. Now examine the output of the circuit. 4. Examine the low voltage output of the OpAmp. For example: is the OpAmp able to source 0V? This lab will be demonstrated for those who are interested. It will therefore be assumed that the students understand the operation of the OpAmp. There is nothing to hand in.