The operational amplifier (or 'op amp') is a critical component in any apparatus for doing electrochemistry. There are many references to op amps out there, but here, we're going to concentrate only on those features that are important to electrochemical applications.
This is the general schematic for an op amp.
Figure 1. General symbol for an op amp.
There are two input terminals (left side) and one output terminal. By convention, the upper terminal, or (-) is called the inverting input, and the lower terminal, or (+), is the noninverting input (yes, the term is redundant, but that's the convention). The input voltages can be AC or DC, and usually positive or negative.
The output voltage is determined by the following formula:
Where V- is the voltage at the inverting input, V+ is the voltage at the noninverting input, and Vo is the output voltage. Since the 'open loop' gain factor A is very high, usually 100,000 or greater, the unadorned amplifier is not very useful. Usually a feedback network is used to rein in this awesome amplifying power to some reasonable level. The formulas that follow assume that the open loop gain is much higher than the gain of the overall circuit. The reasons for this are beyond the scope of this page, but can be found in the references below.
Op amps are active devices and must be provided with power to operate. Usually, both positive and negative voltages are needed to drive op amps involved in electrochemistry applications. Typical power voltages are +5 to +15 VDC and -5 to -15 VDC. There need not be a center, or 'ground' reference unless the application demands it.
Op amps come in several packages. A common package for electrochemical construction is the 8-pin DIP package:
The pin connections of nearly all 8 pin DIP single op amps are standard
* Pin 1 - no connection
* Pin 2 - inverting input
* Pin 3 - noninverting input
* Pin 4 - negative power supply
* Pin 5 - no connection
* Pin 6 - output
* Pin 7 - positive power supply
* Pin 8 - no connection
Op amps are also supplied in dual and quad packages, containing 2 and 4 op amps, respectively.
Classic Op Amp Circuits (that also illustrate operation)
Unity-gain (buffer) amp
Here is a simple circuit with a voltage gain of 1 :
The voltage at the output is fed back to the inverting input, so the amplification is controlled by the difference between the Vin and the Vo. To understand this better, imagine what would happen if the Vin increased. The term (V+ - V-) would become positive, which would cause the output voltage to increase until Vo = Vin. If Vin decreased, (V+ - V-) would become negative, which would cause the output to decrease. The only stable point for the circuit occurs when Vo = Vin.
Why would anyone want an amplifier with a gain of only 1 ? Ops amps typically have maximum output currents of several milliamps, whereas the input terminals draw no current (in principal, at least). A high-impedance source, such as a reference electrode, can properly be buffered by a unity-gain amplifier. (The term 'buffer' is often used for such applications.) A reference electrode cannot be depended on to provide a constant voltage if significant current is allowed to pass through it. However, if connected to a unity-gain amp, the RE voltage will be reflected at the output, and there is plenty of drive capacity available to drive subsequent circuits.
Figure: conventional electrochemical cell with unity-gain amplifier buffering the reference electrode: