Input Impedance for my Microamp

[HeaD]

Hi, I've just fiinished to build a microamp booster. I was wondering..... how to measure the input impedance of this circuit?

http://www.generalguitargadgets.com/diagrams/microamp_sc.gif

Thanks to all!

[David]

Look at the schematic.  It's 22M.  No measurements required.

[Peter Snowberg]

There is also the 10M after the coupling cap which is in parallel so the real value is more like 6.9M.

¹/_Rtotal = ¹/_R1 + ¹/_R2
.

[David]

I stand corrected.  That'll teach me to shoot off my typing fingers...   

[BrianJ]

You can measure the input Z of a device by putting an impedance(usually large, try a 1meg 1%) in line with the input and measuring the drop across it.  You are effectively increasing the output impedance of the device before yours.  Use a sine wave from a real signal generator or other low output impedance device, otherwise its output z will be added to your 1M (that's ok, but you have to compensate in you calculations).  If the input Z is 1meg at most audio frequencies you will lose half of your signal across the 1 meg you added.  If the input Z is 5M you would lose 1/6th of your signal. 


When using opamps the input Z is usually dominated by the parallel combination of the resistors involved.  When the first device is a transistor thing can be much more compicated.   ie: the parallel combination of the bias resistors in parallel with [(the emmitter resistor in parallel with the load) xHFE]...

When that happens just measure it.

Good luck