Tell us about µA741, most popular op-amp of all time (7)

We continue previous article and continue analysis of output stage of operational amplifier. Limited by size of op-amp package, op-amp pull and pull current will not be too large. Then, in order to limit current beyond a certain value, uA741 also specially designed an internal current limiting circuit, including a push current limit circuit and a pull current limit circuit.

First, let's look at pulse current limiting circuit. The implementation of this function mainly depends on T13 lamp. When current flowing from power supply through T12, R9, and then to load (Vo) reaches a certain level, voltage across R9 will reach 0.7V. Let's calculate this current value. I=0.7V/R9=0.7V/27R=25mA. That is, when value of current flowing through R9 reaches approximately 25 mA, voltage across R9 reaches 0.7 V. Let's see if voltage across resistor R9 exactly matches voltage at base and emitter terminals of T13, then this means that T13 is this time is saturated and turned on. After T13 saturates and turns on, will T12's Vbe be less than 0.7V and then T12 turns off? Then current flowing naturally through R9 becomes smaller. The two devices R9 and T13 play role of negative feedback here, which is essentially implemented by using characteristics of opposite phase of base potential and collector potential of N-tube.

Let's see how source current is limited. This function is mainly implemented by three devices Q8, T15 and T14.

When output of op-amp is low, for example, emitter voltage of Q6 is lower than output voltage Vo, then for op-amp, current flows from load to Q7. When current flowing through R10 reaches a certain level, lamp Q8 turns on. After tube Q8 is open, current will flow through tube T15. Tube T14 is a mirror image of T15, here is a mirror current source, that is, collector potentials of T15 and T14 are same. As current flowing through T15 gets bigger and bigger, collector voltage at T15 gets lower and lower. Similarly, voltage across collector of T14 gets lower and lower. When voltage across collector of T14 drops to a certain level, two tubes T8 and T9 will close. At this time, collector voltage of T9 will increase, then voltage at emitter of Q6 will also increase, and then Ib and Ic flowing through Q7 will become smaller. It also plays a protective role when drawing current.

In addition, inside 741, R&D staff at that time also made a relatively amazing invention, that is, a capacitor was added between amplifier stage and output stage, and this capacitor was also called Miller capacitor.

Because T8 and T9 form a Darlington tube, magnification will be very large. If magnification of each tube is 100 times, then two tubes add up to a magnification of 10,000 times. In other words, a small change in voltage at point F will result in a very large change in collector voltage at T9. This serious dv/dt is actually detrimental to stability of op amp. To solve this stability problem, scientists have tried many methods and finally found a relatively ideal solution inside uA741 op-amp, which is to add a small capacitor. Before that, there were no capacitors inside op-amp. Let's see what effect this capacitor can have. We are assuming that voltage at point F decreases downward, so voltage at pin C of T9 rises sharply? This sharply rising voltage will charge point F through Miller capacitor, right? Then potential at point F will not fall so fast. Thus, voltage at point C of T9 will not rise as fast, i.e. dv/dt at that point will not be as strong, and stability of entire op-amp system will improve. But this improvement must come at a price. For amplifying AC signals, it is easy to see that overall gain of op-amp decreases, higher frequency of signal, lower overall gain of the op-amp. In a sense, this capacitor acts as a low-pass filter here.