When we connect our instrument to the input, the Q point will swing around this standby point following the input signal: The signal swings around the Q point The Q point represents the state of the transistor in “standby”, when no input signal is applied. 2 – Choosing a Q pointĪs we said earlier the Q point should be in the active region, but where exactly? If you look for the quick answer, in the middle of the load line. With our two points we can draw our load line, that contains all the possible working points for the transistor.
R) and all the voltage drops in the transistor (Vce = Vcc).This means that there’s no voltage drop in the resistor (V = I As the transistor acts as a closed switch, the current is limited only by the resistor. If Vce = 0 the transistor is in the saturation region so we have maximum current.We can choose anything we want so we’ll take the two that are easier to solve: To get a straight line only two points are needed, so we need two conditions. In the next picture the load line is painted in red, the x axis represents Vce and the y axis represents Ic. Once we have the load line for our circuit, we’ll pick the best available Q point. It indicates all the possible Q points in which the transistor can work in a given circuit. The load line is a very useful tool when designing bjt amplifiers. We’ll use the following circuit for this section: Example circuit and voltage loops 1 – The load line In our case we want the transistor to amplify so we need pick a Q point in the active region. – Choose the best possible Q point for our application. – Drawing the load line, that is the line that contains every possible working point for the transistor in our circuit. The process to design an amplifier can be summed up in the next two steps: These extra parts set the DC operating point correctly so the input signal will make the transistor work as intended.
A bipolar transistor doesn’t work by just connecting the output of your guitar to it: you have to add a few parts and power signals to place it in a “sweet spot” or Q point (quiescent point) where it can work.
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