Solving Common Base Amplifier Problems: Homework Statement & Solutions

[hisotaso]

Homework Statement

The problem statement, given solutions, and my small signal drawing are attached

Homework Equations

Rout, Rin, Av

The Attempt at a Solution

To begin with, our text seems to alternate between using approximate equations and exact derived equations without stating why at any given point. Also, circuit elements are frequently ignored as you will see below.

Looking at my small signal drawing, I don't understand why r_∏ is not being considered in any of the calculations. By my calculations, r_∏ and R₄ are the same size: r_∏ = β₀/gm = 100 kΩ and R₄ is given as 100 kΩ. I see that R_I is only 50 Ω. Should r_∏ not be considered in the parallel combination in the equation for R_out?

Also, R_c is not considered at all. I know it is not given, does that mean I am supposed to assume there isn't a bias resistor on the collector? Or would I normally need to try and solve for R_c somehow?

The problem is, I have found several errors in the text and in the given solutions so far, and the text gives up to three different equations for each characteristic at times (the derivation and up to two different approximations). This is making me doubt everything I do, and making my homework take forever.

Regards

 

[rude man]

In 'real life' you would say:

A_v = R_L/(R_l + 25mV/25uA) = 200e3/1050 = 190, R_in = 50 + 25mV/25uA = 1050 ohms, R_out = infinity (this is not including R_L in the computation), A_i = +1.

But that obviously would not satisfy the problem issuer!

So comfort yourself in the thought that on a job you would probably never use all the variables & constants given in your answer sheet ...

 

[Drao92]

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