Test an inverting amplifier circuit using the ML741 discrete component opamp
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
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.
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.
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 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
- CH1: non-inverting input
- CH2: output
- 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
- 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
Measurements in action…