# Input impedance of this circuit

Please see images of question and solution. Please see my working out using the small-signal model

I'm really confused about how they went about solving the input impedance of this circuit? Did they use DC biasing or the small-signal model to work out the input impedance?

• The input impedance is found from the small-signal model. Why don't you draw out the small signal model of the circuit and add it to your question so we can discuss it? There's a schematic editor in the post editor. Commented May 1, 2016 at 16:46
• en.wikipedia.org/wiki/Common_base (scroll down to low frequency characteristics. Commented May 1, 2016 at 17:00
• Still have a couple of questions. 1)when finding input/output resistances do you ground all nodes in the small signal model? 2) Is there other method in working out input/output impedances without having to resort to the small-signal model? Commented May 1, 2016 at 17:32
• 1) no, only those that connect to a small-signal ground. 2) No there isn't. The in and output impedance are by definition small signal properties so you need a small signal equivalent circuit to determine their values. Commented May 1, 2016 at 18:42