The output range over which a current source (all of these circuits) behaves well is called by the compliance…set 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 have to have a fixed voltage VB. If doesn’t the base is driven by a variable voltage source the output vin, current will vary proportionately and smoothly (as long as the transistor remains within = / RE. In vin compliance) as…iout fact, this property is the basis of the common emitter amplifier, which is capable of generating a large gain, as see in the we’ll next section. E. Common-Emitter Amplifier Figure 10 looks a lot like Figure 9A. Indeed, it is a current source with a resistor as a load. The collector voltage is RC VC = VCC IC RC – We could vary VC by connecting a signal source to the base, but most likely need to insert a coupling capacitor between the we’d 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, neglected the impedance seen looking into the base we’ve for the simple reason that it is on the order of 100 K and RE β ≈
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