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【正文】 he gain is given by the expression AU =(R1+R2)/ R1 (12A5b) This shows that if A is very large, then the gain of the circuit is independent of the exact value of A and can be controlled by the choice of R1and R2. This is one of the key features of OpAmp design the action of the circuit on signals depends only upon the external elements which can be easily varied by the designer and which do not depend upon the detailed character of the OpAmp itself. Note that if A=100 000 and (R1 +R2)/R1=10, the price we have paid for this advantage is that we have used a device with a voltage gain of 100 000 to produce an amplifier with a gain of 10. In some sense, by using an OpAmp we trade off power for control. A similar mathematical analysis can be made on any OpAmp circuit, but this is cumbersome and there are some very useful shortcuts that involve application of the two laws of OpAmps which we now present. 1) The first law states that in normal OpAmp circuits we may assume that the voltage difference between the input terminals is zero, that is, U+ =U 2) The second law states that in normal OpAmp circuits both of the input currents may be assumed to be zero: I+ =I =0 The first law is due to the large value of the intrinsic gain A. For example, if the output of an Op Amp is IV and A= 100 000, then ( U+ U )= 10SV. This is such a small number that it can often be ignored, and we set U+ = U. The second law es from the construction of the circuitry inside the OpAmp which is such that almost no current flows into either of the two inputs. B: Transistors Put very simply a semiconductor material is one which can be 39。 to produce a predominance of electrons or mobile negative charges (Ntype)。holes39。P39。N39。working39。 and the emitter through base to collector circuit the output side. Although these have a mon path through base and emitter, the two circuits are effectively separated by the fact that as far as polarity of the base circuit is concerned, the base and upper half of the transistor are connected as a reverse biased diode. Hence there is no current flow from the base circuit into the collector circuit. For the circuit to work, of course, polarities of both the base and collector circuits have to be correct (forward bias applied to the base circuit, and the collector supply connected so that the polarity of the mon element (the emitter) is the same from both voltage sources). This also means that the polarity of the voltages must be correct for the type of transistor. In the case of a PNP transistor as described, the emitter voltage must be positive. It follows that both the base and collector are negatively connected with respect to the emitter. The symbol for a PNP transistor has an arrow on the emitter indicating the direction of current flow, always towards the base. (39。 for positive, with a PNP transistor). In the case of an NPN transistor, exactly the same working principles apply but the polarities of both supplies are reversed (Fig. 12B4). That is to say, the emitter is always made negative relative to base and collector (39。 for negative in the case of an NPN transistor). This is also inferred by the reverse direction of the arrow on the emitter in the symbol for an NPN transistor, ., current flow away from the base. While transistors are made in thousands of different types, the number of shapes in which they are produced is more limited and more or less standardized in a simple code TO (Transistor Outline) followed by a number. TO1 is the original transistor shape a cylindrical 39。 with the three leads emerging in triangular pattern from the bottom. Looking at the base, the upper lead
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