Input Impedance of Op Amp Buffers

I learned a few things about input impedances of op amp input buffers.  Different op amp arrangements are measured differently so I’ll walk you through the different types.

 

Image result for non inverting buffer op amp

Image result for non inverting op amp

Non-inverting buffers are the quintessential setup for a buffer.  Bring the signal in on the non-inverting (+) input and tie the output to the inverting (-) input.  This will give a unity gain buffer that provides high input impedance  is typically 1 MΩ to 10 TΩ.  An ideal op amp has infinite resistance.  The output of this buffer is very low, typically much less than 1Ω.  In the guitar FX world we would add a resistor to Vb (Vb is half of the input voltage) in order to allow the guitar signal to pass through since we typically use a single sided 9V power supply.  This resistor will negate the high input impedance of the op amp though since you are essentially putting it in parallel.  This resistor will determine your input impedance so always keep it above ~470kΩ.  1MΩ is generally the go-to resistor here.

 

Image result for inverting op amp

Inverting op amp circuits change the game completely so be aware of which op amp you are using.  Rin alone will determine your input impedance here.  Generally, this circuit is preceded by a transistor buffer or a non-inverting op amp buffer so that lower resistor values can be used.  Higher resistor values in series with your signal translate to higher noise levels.