Test an inverter circuit using the ML741 discrete component opamp
This is a demonstration of an inverter/inverting buffer circuit using the ML741 discrete component opamp. The general operation of the circuit is for the output to follow the inverse of the inverting input. This is a special case of the general inverting amplifier configuration with gain set to -1, i.e.
Vout = -Vin Rf = Rin
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 midpoint of the Rin:Rf voltage divider. Hence the op amp achieves inverting/non-inverting input equilibrium when Vout = -Vin.
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”.
ML741 on the right, and a standard UA741CN on the left waiting to be put to the test..
ML741 v “real” 741 Test
Here are some results comparing the behaviour of a standard UA741CN chip with the ML741 (protoboard version).
- power is 5V single rail, i.e. V- = GND
- non-inverting input is fed a sine wave 1Vpp 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: non-inverting input
- CH2: output
I’ve purposely driven the input signal such that it clips the lower output rail of both the ML741 and UA741CN, because things get interesting around the rails.
- ML741 is performing nicely
- UA741CN hitting the lower rail at ~1.9v
- ML741 starting to distort at the lower end
- UA741CN still performing similar to 1kHz
Both ML741 and UA741CN exhibiting similar distortion patterns: waveforms have lost definition, are phase-shifted and attenuated
Measurements in action…