NPN Transistor Switch or Linear Circuit Designer

Another cool tool I will share to you is NPN transistor switch or linear circuit designer (download the tool here for free for now). From the name itself this tool can calculate either linear or switching circuits. All you have to do is to fill the required fields then let the template do the task for you. In long hand circuit design, the analysis of a transistor operating in linear region is different to the analysis at saturation region. At linear, the device beta is a factor on the analysis while at saturation it is the circuit beta that dominates. There are two ways to determine the operation of a transistor circuit whether linear or switching. The first one is to assume that the circuit is saturation then compute for the circuit maximum beta then compare the result to the minimum beta of the device. If the circuit beta is lower than the minimum device beta, the assumption is correct. Indeed the transistor saturates. If the circuit beta is greater than the device beta, the circuit is operating at linear.

Another method is to assume the circuit linear. Compute for the circuit minimum collector-emitter voltage (VCE) then compare it to the maximum collector-emitter voltage saturation of the transistor. If the former is higher than the latter, yes the assumption is true. The transistor operates in linear region. If the result is otherwise, the circuit is operating at saturation (switch mode).

These two methods of judgement are integrated inside the template so don’t worry anymore. The template can determine if the circuit is operating at linear or saturation mode.

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NPN Transistor Switch or Linear Circuit Designer – Application

The template is developed using excel. Excel is a very common office application so therefore you can design your circuit even without using circuit simulators. If you have circuit simulators you can use this tool in complement since this is also capable of computing the worst case current values of the circuit. For those who are use to design by long computations, this is the tool for you to speed up your task. The tool is proven with actual circuit results, simulations and long hand computations. It will give exact result as you entered values to it. This is just a tool, so the output will depends on your inputs. Supposing you compare it to a simulation wherein you are using a BC817-25 device, the currents may be different to result of the tool if you are not entering the same beta to the tool as the actual beta in the simulation.

Circuit Schematic

Below is the applicable schematic of the template/tool. As you can see, the schematic is generic. If there is no Rc the tool can be used. if there is no Re, still the tool is can be used. if there is no Rb2 still the tool can be used. It is very flexible. In case there is no value for Rc, just make the value very small but do not make it zero to avoid “computation error”. Likewise, if there is no value for Re, just make it very small but not zero. In case Rb2 is not present in your actual circuit, just make it very very big to simulate open circuit.

NPN Transistor Circuit

User Interface

Below is the snapshot of the input fields.

NPN Transistor Switch or Linear Circuit Designer

Below is the snapshot of the output fields.

Output Fields


NPN Transistor Switch or Linear Circuit Designer – Capability

The tool can compute the following:

1. Nominal, Minimum and Maximum Base currents
2. Nominal, Minimum and Maximum Collector currents
3. Nominal, Minimum and Maximum Emitter currents
4. Nominal emitter voltage 
5. Nominal collector voltage
(you can also get the minimum and maximum values by doing little computations)
6. Power dissipation of all the resistors
7. Power dissipation of the transistor
8. Identify the operation of the circuit whether saturation or linear and give the VCE
9. The tool also considers the ambient temperature

For more information, read the instruction provided in the template itself. You can download the tool in the download button above or simply click here.

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