Operational Amplifier (op-amp) Introduction – Part 1 of 5

All this theory and knowledge about op amp is what I learned from a book called ‘Practical Electronics for Inventors’ by Paul Scherz and Simon Monk. This is a fantastic book for electronic hobbyist like myself, quite in depth and clear in explanation.

I am not associated with the authors and I do not get any commission if you buy this book. This is really a good book for electronics enthusiast. This blog is my learning journey.

Parts of a op amp are:

• +ve power supply(+Vs) and -ve power supply(-Vs).
• non inverting input (V+) and inverting input (V-).
• Output Vout.
• Some specialized leads.

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Basic operation of op amp:

• When V+ (non inverting input) greater than V- ( inverting input) , the out Vout will saturates towards +Vs (+ve supply).
• When V- (inverting input) greater than V+ (non inverting input), the Vout will saturates towards -Vs (-ve supply).

Refer to below diagram for visual explaination:

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Different op amp setup(configurations):

• Inverting amplifier: Vin is connected to V- , and this caused the output (Vout) to be inverted compared to its input (180° shift).
• Non inverting amplifier: Vin is connected to V+ , and this caused the output (Vout) to be in phase with its input ( +ve input +ve output, -ve input -ve output ).
• Take note that Vout reaches Vs (supply) saturation in both -ve and +ve direction.

Negative feedback configuration:

• make op amp more useful
• the gain of the output can be controlled, and prevent Vout from saturation
• components are places between the Vout and V- (usually resistors and capacitors) and this make the op amp to be more than just amplification.
• with a -ve feedback circuit, op amp can be voltage regulator, current to voltage convertor, voltage to current convertor, oscillator, math circuit, waveform generator, active filter, active rectifier etc…
• -ve feedback is more common than +ve feedback

Inverting amplifier formula: Vout = – (RF/Rin)*Vin. Negative sign because the output signal in inverted.

Some theory needed to use op amp:

• The formula: Vout = Ao(V+ – V-). This is for ideal op amp but the formula structure is the same for real life op amp, just that there will be some additional resistance components.

3 rules to note:

• Rule 1: Ideal op amp, Ao = ∞ ———— Real life op amp Ao = 10⁴ to 10⁶
• Rule 2 : Ideal op amp, R_in = ∞ ———— Real life op amp R_in = 10⁶ Ω to 10¹² Ω. Ideal op amp, R_out = 0 Ω ———— Real life op amp R_out = 10 Ω to 1000 Ω.
• Rule 3: Ideal Op amp draws no current, true for real Op amp also (very small current: picoamps to nanoamps)