Article: Andy Collinson
Measuring Input Impedance
This method can be used with Tinaplus and earlier versions, the TinaPro version has its own comprehensive impedance meter. The circuit below shows a single stage circuit under test :
The current source is fixed at 1amp. As the source is perfect, i.e. infinite impedance, you need to use a high value parallel resistance. I used 100G. It is important to measure before the capacitor, as you will then see the additional impedance at low frequencies due to the 1uF coupling capacitor. The results are generated below :
Mid-band impedance is roughly uniform, low frequency impedance increases due to the series reactance of the coupling capacitor and high frequency impedance is reduced by the transistors internal B-E capacitance. Note that from Ohm's Law as V= I * Z that if the current source is set to 1 amp, as in this case, then the input voltage will equal the input impedance.
Measuring Output Impedance
A similar technique to measuring input impedance is employed for output impedance analysis. A sample circuit for analysis is shown below:
A current source is used, wired in parallel with the load resistor, or point in the circuit where analysis is required, with a fixed magnitude of 1 amp. A shunt resistance is not required as the load resistor Rl is in parallel with the current source. From Ohm's Law V=I*Z as the current source is set to 1 amp then in this case the magnitude of voltage will equal the impedance. An ac analysis is then performed, the y-axis, which will indicate voltage, now also correctly reads impedance and is labelled accordingly. A linear scale for the y-axis is also used, results for the above circuit are displayed below:
This technique is not needed with Tina Pro as an impedance meter is already included. Also this method is computed in seconds compared to conventional meter methods.