Doubling the capacitance will half the reactance. A 100uF capacitor will have a reactance of about 80 ohms 20Hz. Let's assume you are designing an audio stage. The bigger the value the lower the cut off frequency. This basically sets the low frequency response of the stage. The size of the decoupling capacitor is next. ( the collector voltage will be set around 3V and have a +/- swing of 2V) Once again not a preferred value so we can choose from 2k0 or 2k2. As the emitter resistor represents 10% supply drop we can simply calculate the collector resistor at about 4 X emitter resistor. (to give maximum output swing at collector). Allowing for a minimum saturation voltage (say 0.2V) we need about 40% of the supply dropped across R2. This value represents 10% supply drop leaving 90% to be divided between the transistor (c-e) and the collector resistor. Now we need a little bit of (simple) maths (using V=IR).įor a 0.5V drop 1mA we need a 500R resistor - BUT this is not a 'standard' E24 or 5% value so choose a nearest preferred value (n.p.v.) - either 470R or 510R Do you need to minimise current used or do you need to drive a high current into the next stage?ĭecision 2. 100uA, 1mA, 10mA? This will depend on your application. Now choose the current you want through the transistor. Let's take Ve as 10% of the supply (= 0.5V for a 5V supply). The voltage across it should be between 10 and 20% of the supply (rule of thumb) to give an 80 - 90% voltage swing at the collector.ĭecision 1. ![]() Let's start with the value of emitter resistor (R4). The circuit is pretty much standard, well understood and lends itself to a simple analysis. ![]() (Note: I still need to use Ohm's law V= IxR and an understanding of percentages/fractions with this approach!) ![]() The thinking behind this approach is very much based in the theory developed by scientists and engineers over decades and if you are to advance in any design work I would recommend that you try to develop a solid mathematical approach. However, there are other 'rule of thumb' type" approaches that might suit a 'practical' designer to get started. Its not really practical to design any circuit without calculations and Alfred's answer gives a comprehensive approach (so +1 from me).
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