So… But, since IC IE for large ≈ β… independent of as long as the transistor is not forced into VC, (VC + 0.2 volts) or (VC VCC). saturation VE cutoff ≥ ≈ 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 stiff impedance should be about 10 times less it’s enough…its than the impedance seen looking into the base, namely, RE. β 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.
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.