In previous parts of this series we reviewed the basic transistor parameters that determine ac voltage gain and practical considerations limiting high gain. The role of a coupling capacitor was introduced. In this section we see how to minimize output loading to maintain high gain along with more practical DC biasing.
Minimizing the Output Load Effect
It is important to note that the voltage gain at T1 could be heavily influenced by the output load. In Figure 9, yet another transistor (T4) is added to act as an output buffer to maintain higher impedance at the collector of the gain stage (T1). While this “emitter follower” has a voltage gain of one, it has significant current gain (β).
In Figures 10 and 11 a practical way to bias T1 is suggested. R1 replaces the previous voltage divider. Note that R1 is connected to the collector of T1 (as highlighted in red) and not the power supply. This introduces additional negative feedback to further stabilize the operating point. This technique tends to “automatically” adjust T1s operating point to T3s output current. However, as a result of the feedback, gain is reduced. Still, much higher gains are possible than with a passive load.
Tailoring the feedback
The proper use of feedback, involving passive R and C components is very powerful, not only for circuit stabilization, but for creating desired frequency characteristics.
The example in Figure 11 involves placing a capacitor in parallel with RE (in this case, R2+R3). This allows stabilizing the DC operating point without limiting the mid-band gain. However, a disadvantage of this technique is that it can dramatically decrease the input impedance (β*RE ) as frequency increases.
The red dotted lines suggest another variation. By bypassing only a portion of RE , you can optimize the ac and DC characteristics to suit a particular application. Figure 12 suggests a possible result.
Demonstrated in this series has been an easy way of visualizing the terminal impedances of a bipolar transistor. With this understanding, one can design simple, yet reliable, voltage or current gain stages to satisfy a variety of interface needs. Also suggested were “advanced” techniques to optimize performance for a particular application.
In the next and last part of this series we will summarize the most important points.