Editor's Note: The following tutorial is one of a series of six on transistor theory by Howard Skolnik, retired Burr-Brown designer. Skolnik and Bob Dobkin, CTO of Linear Technology, will be our experts on March 26, 2014, at 1:00 p.m. ET (10:00 a.m. PT) for the first session of “Ask the Experts” on Planet Analog. (Note: This is a time change from our original date.) This three-part tutorial from Skolnik is meant to mentor and provoke questions for those who need to know analog design but are not analog engineers. Please bring your questions on March 26. I will be sending instructions for signing in to this chat session shortly.
There are many applications for one or two transistors. These include voltage and current amplification. Described here, in part 1 of a series, is an easy to understand way of visualizing the operation of a bipolar transistor in its linear operating region. This technique can be applied to the design of simple as well as sophisticated circuits.
It is most important to understand that a transistor is an impedance converter . The key to grasping practical transistor circuit design is to visualize the impedances seen when “looking into” the base, emitter, and collector terminals. One useful result is that the ratio of the collector to emitter impedances is the transistor’s small signal “voltage amplification.”
- Operating Point: The terminal impedances are dependent upon the quiescent operating point of the transistor (IE ).
- Current Gain: The transistor exhibits a current gain between the base and collector terminals. This current gain (beta, β) is defined as IC /IB , where IE = IC + IB . Typically, β > 200 and we can say that IE ≈ IC .
- Emitter: Once the operating point is found (or selected), the internal emitter impedance (Re) is easily calculated. If an external resistor (RE ) is inserted at the emitter, the total becomes Re + RE . In common usage, we generally take RE to mean either the external emitter resistor or the total.
- Base: The impedance looking into the base is β * Re, or β * (Re + RE ), when an external RE is present.
- Collector: The collector impedance (Ro ) is not so intuitive. Figure 2 shows the characteristic curves for a typical NPN transistor. An “ideal” transistor would have an infinite Ro . That is, the collector current would not change as the collector-emitter voltage changes (Ro = ΔVCE /ΔIC). For a “real” transistor, Ro is the slope of the Vce verses IC curve. If external components are present at the collector, the total Ro (Rotot ) is the parallel combination of these impedances. In some cases, the external impedances (RL ) will be much lower than Ro , and Ro can be ignored.
While not necessary, it is sometimes appropriate to start by making certain assumptions. These often include a high (>200) DC current gain (β). This means for any collector (or emitter) current, the base current can be ignored. Another common assumption is that the internal collector impedance (Ro ) is very high; that is, much higher than the external impedance (Ro ). As we will see, non-ideal β and Ro are easily included to refine the accuracy of a given transistor stage design, if needed.
Without getting into semiconductor theory, please accept that the impedance looking into the emitter is a constant at a given operating current (IE ) and temperature and changes inversely with Ie in a linear fashion. If temperature is considered constant at 25°C, Re = 26mV/IE , or 26Ω @ 1mA.
There is much more to come in future parts of this series.