Figure 1. Basic op-amp current source (floating load). Vin might come from a voltage divider, or it could be a signal that varies with time.
In all op-amp circuits, golden rules I and II are obeyed only if the op-amp is in the active region, i.e., inputs and outputs are not saturated at one of the supply voltages. For instance, overdriving one of the amplifier configurations will cause output clipping at output swings near VCC or VEE. During clipping, the inputs will no longer be maintained at the same voltage. The op-amp output cannot swing beyond the supply voltages (typically it can swing only to within 2V of the supplies, though certain op-amps are designed to swing all the way to one supply or the other, or to both; the latter are known as “rail-to-rail output” op-amps). Likewise, the output compliance of an op-amp current source is set by the same limitation. The current source with floating load (Figure 1), for instance, can put a maximum of VCC−Vin across the load in the “normal” direction (current in the same direction as applied voltage) and Vin−VEE in the reverse direction.
The feedback must be arranged so that it is negative. This means (among other things) that you must not mix up the inverting and non-inverting inputs. We’ll learn later that you can get yourself into similar problems if you rig up a feedback network that has lots of phase shift at some frequency.
There must always be feedback at dc in an op-amp circuit. Otherwise the op-amp is guaranteed to go into saturation. For instance, we were able to put a capacitor from the feedback network to ground in the non-inverting amplifier (to reduce gain to 1 at dc, Figure 2), but we could not similarly put a capacitor in series between the output and the inverting input. Likewise, an integrator will ultimately saturate without some additional circuitry such as a reset switch.
Some op-amps have a relatively small maximum differential-input voltage limit. The maximum voltage difference between the inverting and non-inverting inputs may be limited to as little as 5 volts in either polarity. Breaking this rule will cause large input currents to flow, with degradation or destruction of the op-amp.
Op-amps are high-gain devices, often having plenty of gain even at radio frequencies, where the inductances in the power-rail wiring can lead to instabilities in the amplifiers. We solve this issue with mandatory (we mean it!) bypass capacitors on the op-amp supply rails.
Figure 2. Amplifiers for ac signals: A. ac-coupled non-inverting amplifier, B. blocking capacitor rolls off the gain to unity at dc.