So… But, since IC ≈ IE for large β… independent of VC, as long as the transistor is not forced into saturation (VC ≥ VE + 0.2 volts) or cutoff (VC ≈ VCC). This transistor configuration works as a current source because the collector is reverse-biased and therefore its output impedance is quite large. It works much better than simply attaching a voltage source to a relatively large resistor, but it’s not ideal. Collector impedances are on the order of mega-ohms, which, however, do show a relatively small variation with voltage. There are fixes, though, which are even better, but we leave them to a later discussion…or never, depending upon time. D. Biasing the Transistor Current Source As in the example 5.1, you can use a voltage divider as long as it’s stiff enough…its impedance should be about 10 times less than the impedance seen looking into the base, namely, β RE. You could also use a zener diode (or a two-terminal IC voltage reference like the LM385), biased from VCC or even a few forward-biased diodes in series as shown in Figure 9. In cases A and B, the npn transistors actually behave as current sinks (although most people call them current sources)…they sink the current flowing through the load from the power supply. In case C, the pnp transistor sources the current into the load.
The output range over which a current source (all of these circuits) behaves well is called compliance…set by the requirement that the transistor is not forced out of the active region. Thus, in A, VC can range from almost 10 volts down to about 1.1 – 1.2 volts. In B, it can only drop down to about 5.1 volts. In C, VC can range from ground up to about 8.6 volts. The current source doesn’t have to have a fixed voltage VB. If the base is driven by a variable voltage source vin, the output current will vary proportionately and smoothly (as long as the transistor remains within compliance) as…iout = vin / RE. In fact, this property is the basis of the common emitter amplifier, which is capable of generating a large gain, as we’ll see in the next section. E. Common-Emitter Amplifier Figure 10 looks a lot like Figure 9A. Indeed, it is a current source with a resistor RC as a load. The collector voltage is VC = VCC – IC RC We could vary VC by connecting a signal source to the base, but most likely we’d need to insert a coupling capacitor between the two in order to isolate their dc levels. We show such an example in Figure 11. Suppose we wish to pass input signals whose frequencies are greater than 200 Hz. Then Note, we’ve neglected the impedance seen looking into the base for the simple reason that it is on the order of β RE ≈ 100 K and
There is another effect not caused directly by temperature that can also create instability problems in a poorly designed circuit. It’s called the Early effect. VBE (at constant IC) varies slightly with VCE. This effect is caused by a change in the effective width of the transistor base region as VCE changes. It is given approximately by… ΔVBE = - η ΔVCE where η ~ 10-4 – 10-5 (5.9) This is often expressed as a increase of collector current with increasing collector voltage when VBE is held constant. It is expressed as… (5.10) where VA is known as the Early voltage and is typically 50 – 500 V. It is shown graphically in Figure 24. A low Early voltage leads to steeper curves as shown in the graph and this translates to lower collector resistance. Thus, the collector of the transistor doesn’t act as a good current source. This can have a deleterious effect on transistors driving moderately high impedances or on current mirror circuits, which we’ll discuss later on. Figure 24 5.9 Temperature Stability and High Gain Signal distortion and variation of gain can be greatly minimized by applying negative feedback. We’ve already demonstrated this with the addition of RE in the circuit of Figure 23. Also, negative feedback almost completely renders the effects of temperature variation inconsequential. However, this benefit came at the expense of high gain. But could we circumvent this cost so that we could have our cake and eat it, too? Unlike with cake, it’s possible with electronic circuits. The trick is to include