Input and Output Resistance Help

[andy257]

Hi Everyone,

I am a little confussed as to how to measure inout and output resistance.

I think i have managed to measure the input ressitance but the output is confussing.

In the circuit i am trying to measure, to get the input resistance i have used a variable resistor in series with the input to find out when the voltage becomes half. (using voltage divider rule) when the voltage becomes half i measure the variable resistor and this is my input resistance.

However my problem is with output resistance

can i use the same theory as input resistance to measure the output? I have placed a variable resistor in series with the output but the voltage does not fall by half, it almost increases by half? is this correct?

if anyone could give me a quick fix on this i would be very greatful

cheers

andy

 

[Nigel Goodwin]

However my problem is with output resistance

can i use the same theory as input resistance to measure the output? I have placed a variable resistor in series with the output but the voltage does not fall by half, it almost increases by half? is this correct?

if anyone could give me a quick fix on this i would be very greatful

The variable resistor shouldn't be in series, it should go from the output down to 0V.

You musn't do this on anything with a low output impedance, you could well damage it - it a case like that you would use a higher value resistor and measure the very small signal drop it causes, then use ohms law to work it out.