Virtual Ground & Impedances Connected to Grounds

Thread Starter

elec_system_design

Joined Jun 23, 2017
49
Hi, I've run into a homework problem that I don't really know the answer to.

I understand the concept of virtual ground, but this problem kinda stumped me.

Here's the problem:
Explain why impedances connected between ground and virtual ground have no impact at low frequency, but are very important at high frequencies.

Thanks!
 

OBW0549

Joined Mar 2, 2015
3,405
Explain why impedances connected between ground and virtual ground have no impact at low frequency, but are very important at high frequencies.
The impedance of a virtual ground, such as the one that exists at the (-) input of an op amp in the classical inverting amplifier, tends to be largely inductive; that is, it is low at low frequency and increases proportional to frequency, since it is (approximately) equal to the feedback impedance divided by the ratio of the op amp's open-loop gain to the circuit's closed-loop gain. As the op amp's open-loop gain falls off with rising frequency, the impedance seen at the op amp's (-) input increases correspondingly.

Any capacitance connected between this virtual ground and actual ground (that is, between the (-) and (+) inputs of the op amp) will resonate with the virtual ground's equivalent inductance, and this resonance can have perverse effects on the circuit's operation including ringing and even instability leading to oscillation. Even the op amp's own input capacitance can cause trouble, which is why, in some inverting amplifier designs, you see a small neutralizing capacitor (usually a few tens of pF) shunting the feedback resistor to keep the circuit from ringing or oscillating.
 
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