Our first article gave an introductory outline of bipolar transistor principles, characteristics, and basic circuit configurations. This time we'll concentrate on practical ways of using bipolar transistors in useful common-collector (voltage follower) circuit applications.
COMMON-COLLECTOR AMPLIFIER CIRCUIT
The common-collector amplifier (also known as the grounded-collector amplifier, emitter follower, or voltage follower) can be used in a wide variety of digital and analog amplifier and constant-current generator applications. This month we start off by looking at practical “digital” amplifier circuits.
DIGITAL AMPLIFIERS
Figure 1 shows a simple npn common-collector digital amplifier in which the input is either low (at zero volts) or high (at a Vpeak value not greater than the supply rail value). When the input is low, Q1 is cut off and the output is at zero volts. When the input is high, Q1 is driven on and current IL flows in RL, thus generating an output voltage across RL — intrinsic negative feedback makes this output voltage take up a value one base-emitter junction volt-drop (about 600mV) below the input Vpeak value. Thus, the output voltage “follows” (but is 600mV less than) the input voltage.
FIGURE 1. Common-collector digital amplifier basic details.
This circuit’s input (base) current equals the IL value divided by Q1’s hfe value (nominally 200 in the 2N3904), and its input impedance equals hfe x RL, i.e., nominally 660k in
