Now let’s calculate the peak-to-peak output and base currents using R out equivalent output resistance.ĪC operation is illustrated in the following graph. Let’s find collector and emitter resistors.Ĭontrary to the previous part, now we have to replace all DC voltage sources and capacitors with short-circuits (because we will choose capacitor values such that they pass the designed AC input frequency with little attenuation) to obtain the AC model of the amplifier. What I did here is to approximate the resistor values to the nearest 10% standard E12 resistance since these rounding does not change the operating conditions dramatically. (Note: for high speed applications, we need to think again) As a rule of thumb, we can choose R 1 current as 10 times the base current and R 2 current as 9 times. Now we can calculate the biasing resistors R 1 and R 2. 1, that is V BEQ base-emitter voltage at Q-point. Then we can choose the voltages of collector and emitter on quiescent.Īnother value we need, hidden in Fig. Looking at the curves, we can approximate the transistor’s gain parameter β for our desired operating-point of I CQ=5mA and V CEQ=10V. With the assumptions above, we can end up with the load-line (a.k.a. 2 arbitrarily depending on the application, just like the following. Secondly, we can choose the collector-emitter voltage and quiescent collector-current looking at Fig. We need to replace all capacitors with open-circuit (because of the fact that capacitors do not allow DC elements) and eliminate the unnecessary parts to have the following.Įvery circuit needs a supply, so I don’t see any reason stopping us from choosing V CC=20V for this one. The Base-emitter voltage and output characteristics shown below are enough information to go.Ĭlass-A design requires the transistor to be in its active region even when no input signal is provided. Options were limited, so i picked 2SC4102, a small current transistor from ROHM. Let’s start with defining the desired input-output requirements and load conditions of the amplifier.įor a simple tutorial, I decided to pick a transistor that has good datasheet and good pre-installed model in LTspice.

It works with single supply and has DC-biasing stage as well as AC-coupling elements. While more detailed best-practice designs exist, the circuit above is the best for teaching purposes. Let’s see the circuit schematic for our amplifier.

Download transistor as an amplifier