LM358 Opamp Electret Amplifier Circuit
Debugging Circuit w/ SoundCard Oscilloscope #basicTronic 01
LM358 is an operational amplifier.
An operational amplifier is amplifying device with very high gain and very high input impedance & low output impedance.
The most common device that we have for amplification is a transistor, but the limitation that is physically impossible for an operational amplifier to generate voltage greater power supply voltage (VCC). For commercial operational amplifiers, the limit is approximately 1.5 V less than the VCC.
But first, we need an Oscilloscope. But they tend to be expensive. So what to do?
Here is a SoundCard Oscilloscope.
Back to the device, an LM358 IC has dual OPAMP inside, pin 2,3 are inputs & pin 1 output of 1st OPAMP and pin 5,6 & pin 7 are of other.
Resistor R4 and R5 (10k both) make voltage divider circuit for 2V5 supply and set the desired potential at non-inverting (PIN 3) input of OPAMP.
R2 is a 100 K Resistor which provides feedback path and determines the gain. R2 and R1 both together decide the closed loop gain of the amplifier as the gain of inverting amplifier is given by G=R2/R1 (100/1). Capacitor C1 is a coupling capacitor. The audio signal needs to be coupled with DC voltage for amplification which done by capacitor C1. And finally, C2 is a bypass capacitor that helps in providing a stable voltage at voltage rails.
Debugging Using an Oscilloscope
By measuring the signal in the output of the amplifier, PIN 1, we can get a nice view of the result.
First, you can put your finger lightly on the electret and still perceive signal easily, demonstrating how powerful this opamp is.
OPAMP Operation
For an OPAMP there is the rule that the potential at both inputs will be the same which is done through feedback.
If at non-inverting input there is some potential of 2v5 then same will appear at inverting input and the current through feedback resistor will according to increase or decrease and that will decide the potential at an output.
This whole gives the idea for audio amplification.
It’s not the White Horse but…this is an incredible device!
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References & Credits:
Soundcard Oscilloscope- by Christian Zeitnitz
Making a mini Audio Amplifier using Operational Amplifier(LM358)- by riteshnitmn
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— Edited at Dez 2018 — Capacitors & Fritz: thanks to @ Radim Vansa — LM358 pinout included — Grammarly corrections —
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