Calculate Input Resistance: Formula & Tips

[IronaSona]

Homework Statement

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Relevant Equations

R1(1+BA0)

I want to calculate the input resistance ,but i don't really know how to do it .am not sure is the formula R1(1+BA0)?

 

[DaveE]

I think you'll need to use some reasoning to solve this. Guessing at a formula isn't a great approach.

Are you looking for the input impedance at a particular frequency (or range of frequencies)? Or do you need the more general impedance for all frequencies? You can usually simplify the network for know frequencies because some reactive components will have very small (or very large) magnitudes compared to others. For example, if I asked you what is 10KΩ + 1mΩ, you can ignore the 1mΩ for all practical purposes and just say 10KΩ.

The transistors make this a bit confusing. Input impedance is nearly always a question about the AC small signal performance of the circuit. So, try substituting a simple AC small signal model in place of the transistors (hint: it's the simplest transistor model, with only 2 components). Next try to identify the parts that have no effect on the input current based on the features of that model.

Use this approach to create a simplified schematic with only the parts that matter for this particular question.