# How to estimate input and output impedance of an amplifier?

I'm trying to figure out how to design power amplifiers which input and output are properly matched to 50 Ohm. My thinking was that the most bullet proofed approach would be to model an amplifier in LTspice and estimate R as U/I on both input and output. After all usually LTspice models are more or less accurate and impedance is nothing more than U/I ratio (assuming U and I are in phase, of course).

I took "Hand-on Radio Experiments" book by Ward Silver, N0AX and modeled a few amplifiers described in this book (Experiment #45 on page 93 and Experiment #46 on page 95). Since both schematic were published in QST and then in the book, they went through review by the editors and readers many times, and supposably are well designed.

Here is the first one: Not surprisingly the output is 50 Ohms: However it seems that the input is reactive: OK, maybe the input impedance is not that important in this case, since the amplifier was described as a buffer for an oscillator.

Let's check the second schematic: This one supposed to amplify the output of the first circuit. The output impedance is OK: The input current and voltage are almost in phase, but U/I is far from 50 Ohms: Regardless of what model tells I've built the first circuit with minor changes and in practice it works OK, giving about 10 dB amplification on HF.

• Great experiments! But you're not measuring the output impedance. If you measure $V_{50\Omega}/I_{50\Omega}$ on the resistor, it will always be $50\Omega$ ! The simplest way to find the (real) impedance of an unknown source is to measure $V_{OC}/I_{SC}$. In this case it will push the transistor too far from its operating point, so rather measure $V_{49\Omega}$ and $V_{51\Omega}$ and you can calculate $R_{out}$. Aug 1 '20 at 3:20
• Second, you are measuring $Z_{in}$ correctly. In the first case you can estimate Zin, its magnitude is $2500\Omega$ and its phase is $-30^\circ$, so a bit capacitive. In the second, red qraph it appears your input signal is much too big, it's driving the transistor far out of its linear region. Try again with a 1 mV input signal. I think you also need a decoupling capacitor in series with V2, otherwise it will spoil the bias point of the transistor. Aug 1 '20 at 3:27