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Junction and circuit capacitance Capacitance limits the speed at which the voltages within a circuit can swing “slew rate”owing to finite driving impedance or current.
For tran— sistors it is important to realize that the collector current is accurately determined by the base—emitter voltage, rather than by the ad75122 current the base current is then roughly determined by hFEand that this exponential law is accurate over an enormous range of currents, typically from nanoamps to milliamps.
CMRR is usually speci- fied in decibels. This sort of prob— lem can also come up when the load of an datashset follower contains voltage or current sources of its own.
The terms “grounded emitter” and “common emitter” are sometimes used in— terchangeably, and they can be confusing. The high ef— fective collector load impedance provided by the mirror yields voltage gains of or more, assuming no load at the ampli— fier’s output. Con- sider a common—source amplifier.
Just think of it as “transistor action. A diode prevents base—emitter reverse Voltage breakdown. How- ever, it would be a mistake to continue without a few words of explanation about the other kind of transistor, the field-effect transistor FETwhich we will take up in detail in the next chapter. Once again, we favor the symbol with offset gate, to identify the source. The solution here is to add some by- passed emitter resistance for stable biasing, with no change in gain at signal frequen— cies Fig.
The best cure feedback offers another method, although it is not entirely satisfactory is to bias the push—pull stage into slight conduction, as in Figure 2. Of course, you could also use a differen— tial amplifier with both differential inputs and differential outputs.
From the preceding rules it is easy to un- derstand. Nevertheless, the distinction is an important one when it comes to circuit design. Biasing a push-pull output stage for low crossover distorion and good thermal stability. That circumvents the Early ef— fect in Q1, whose collector is now the pro- gramming terminal, with Q2 now sourc— ing the output current.
Product/Process Change Notice – PCN 12_0199 Rev. –
The compar— ison is shown graphically in Figure 3. Let’s calculate the input and output impedances of the emitter follower. In this case it is equivalent to a resistor of about ohms to ground. The voltage gain of an emitter follower is actually slightly less than 1. With the Ebers—Moll equations and a simple 3—terminal model, you will have a good understanding of transistors; you won’t need to do a lot of calculations, and your designs will be first—rate.
When biasing an emitter follower, for in- stance, you choose the base voltage datasbeet resistors so that the divider presents a stiff voltage source to the base, i. In the “linear” region of the drain curves, FETs behave like voltage—controlled resistors; in the “saturation” region they are voltage- controlled current sources. The emitter follows with Input currents measured in Page 57 picoamperes are commonplace.
An alternative approach might be to bypass the feedback resistance in order to eliminate feedback and therefore lowered input impedance at signal frequencies Fig. Thus these variations result in a less—than—perfect cur- rent source: For example, an n-channel JFET which is automatically depletion- mode uses a positive drain supply, as do all datawheet devices. E] Dynamic gate current Gate leakage is a dc effect. Furthermore, the drain voltage at which the curves enter the “saturation region,” Vpsmtequals V05 —VT, making the saturation drain cur— rent, 1mmproportional to VGS — VT 2, the quadratic law we mentioned earlier.
If you’ve studied physics or chemistry, you probably know in your bones that the resulting current is exponen— tial: The collector can swing from 0. Better stability is possible if several stages of amplification are included within the feedback loop. With MOSFETs you must never allow the gate insulation to break down; instead, gate leakage ddatasheet specified as some maximum leakage current at a specified gate—channel voltage.
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The equation for Io is known as the Ebers—Moll equation. This simplifies biasing and eliminates coupling capacitors Fig. A circuit that depends on a particular value for fin; is a bad circuit. Typically R might be a few hundred ohms in a power transistor Darlington, or a few thousand ohms for a small-signal Darlington.
Note that the phase—splitter outputs must be loaded with equal or very high impedances at the two outputs in order to maintain dtasheet symmetry. This base current “robbing” by the input capacitance causes the transistor’s small-signal current gain hfe to drop at high frequencies, eventually reaching unity at a frequency known as fT.