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Project Notes

#159 ML741/InvertingAmplifier

Test an inverting amplifier circuit using the ML741 discrete component opamp

Notes

This is a demonstration of an inverting amplifier circuit using the ML741 discrete component opamp.

The inverting amplifier is a generalised case of the basic Inverter circuit, but with an arbitrary gain determined by the input and feedback resistor selection:

Vout = - Rf/Rin * Vin

Op-Amp_Inverting_Amplifier

How it works

Fundamentally, an op-amp strives to keep its inverting an non-inverting inputs equal by modulating the output.

In the inverter/inverting amplifier configuration, the inverting input is at the inflexion point of the Rin:Rf voltage divider. Hence the op amp achieves inverting/non-inverting input equilibrium when Vout = - Rf/Rin * Vin.

Construction

In this circuit, I am using a single rail supply (V- = GND) instead of the “conventional” dual rail supply (V+/V-).

For this reason, the non-inverting input is pegged to V+/2 with a voltage divider. In a dual rail configuration, V+/2 is usually “ground”.

The breadboard build has a fixed gain of -2 but of course the feedback resistor can be replaced with a pot for manual variable gain.

Breadboard

The Schematic

The Build

ML741 v “real” 741 Test

Here are some results comparing the behaviour of a standard UA741CN chip with the ML741 (protoboard version).

Setup:

  • power is 5V single rail, i.e. V- = GND
  • non-inverting input is fed a sine wave 200mVpp with 2.5V DC offset
  • the function generator sine wave input replaces the manual 10kΩ input pot in the schematic above (at node FG)
  • CH1 and CH2 signals are DC coupled and vertical shifted by -2.5V in the screenshots that follow

Scope connections

  • CH1: non-inverting input
  • CH2: output

At 10kHz

  • very slight phase shift for both opamps
  • UA741CN is delivering almost exactly -2x gain
  • ML741 is also generating -2x gain, but the output is DC shifted up by ~20mV

ML741:

ML741_10kHz

UA741CN:

UA741CN_10kHz

At 100kHz

  • phase shift has increased for both opamps
  • output has started to attenuate. By 150kHz or so, already hitting unit gain
  • ML741 output remains pulled higher by ~20mV

ML741:

ML741_100kHz

UA741CN:

UA741CN_100kHz

Measurements in action…

Inverter_benchtest

Credits and References

About LEAP#159 OpAmp
Project Source on GitHub Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.