Output limits, operational amplifiers

The voltage limits of the amplifier are controlled by the power supplies. They usually are quite close to the supply values. For most devices, the limits are 13-14 V. Currents will be supplied freely to a load until the current limits are reached, typically at 5-100 mA. Special devices with larger current or voltage output limits are available, but high output power in operational amplifier circuits is usually obtained by booster stages, as described below. 

FIOURE 3-4 (a) Comparator mode. Note that the operational amplifier has no feedback and is thus an open-loop amplifier, (b) Output voltage v. of operational amplifier as a function of input difference voltage i. Nolo that only a very small voltage difference at the two inputs causes the amplifier output to go to one limit or the other. 

Since most electrochemical measurements relating to corrosion of metals are satisfied with a sensitivity of 1 jiV or 1 xA, modern instrumentation usually employs electronic operational amplifiers where the noise limits control the range of measurements. The function of the operational amplifier is to amplify the potential (Vg) applied at the input so that it can be displayed on a low impedance analogue or digital meter (V ) as shown in Fig. 1.1. The output potential of the operational amplifier is proportional to the source potential and is required to have sufficient input impedance to avoid polarisation of the potential source. 

The development of electronic instrumentation has effectively displaced mechanical devices as the prime element of coulometers. Those in use today almost universally utilize a capacitor charged by the current being measured. The capacitor is placed in the feedback loop of an operational amplifier (op amp), the output of which registers the time-integral of the current, a principle previously used in analog computation circuitry (20). One of the first to use this method for chemical coulometry was Booman in 1957 (21). The difficulty with this approach is that a capacitor of reasonable size (Booman used a 30-yF non-electrolytic capacitor, a giant of its kind) cannot accomodate sufficient charge at the potentials suitable for chemical use. This limitation has been overcome in several ways. 

In the comparator mode, the operational amplifier is used open loop, without any feedback as show n in Figure 3-4a. In this mode, the amplifier is almost always at one of the limits imposed by the + and - powersupplies (often 15- supplies). Usually, the voltages to be compared are connected directly to the two op amp inputs. The amplifier output is given by v = =... 

A major source of deviation from linearity is saturation of the detector or electronic circuit. By this we mean that, as the irradiance and signal increase, some physical constraint is reached, and the signal cannot continue to increase. This could be due to the electrical breakdown of the detector itself or of some electronic components. More generally, there is an amplifier in the circuit that will put out only a limited voltage. For example, many operational amplifiers are powered by 6-V supplies and cannot support more than a 6-V output signal. 

We have already noted that a negligibly small voltage differential at the inputs will drive a practical amplifier to its limit thus we almost never use the amplifier to deal with an input signal without elaboration of the circuit. Normally the amplifier is stabilized by feeding back part of its output to the inverting input. The manner in which the feedback is accomplished determines the operational properties of the whole circuit. Here our concern is with circuits involving the routing of a current from the output to the input (1-7). 

These amplifiers are active devices in which the inputs need a certain bias current and must operate within the limits of the power supply voltage. The power supply wires and the reference wire (0 V), as illustrated in Figure 8.12(a), are frequently omitted on most circuit diagrams. Notice that the inputs are galvanically separated neither from the output, nor from the power supply. Transcouplers or optocouplers are needed if galvanic separation is necessary (Min et ah, 2006). 

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