Operational Amplifier Fundamentals Unit 5 – The Inverting Summer
Exercise 1 – Amplifier Operation
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to operate an inverting summing
amplifier. You will verify circuit operation with a multimeter.
EXERCISE DISCUSSION
- A summing amplifier combines multiple input voltages and applies individual gain factors to
each input. - To minimize summing errors, R4 is added to the circuit. R4 equals the parallel combination
of R1, R2, R3, and R5. - The summing configuration generates an output voltage equal to the algebraic sum of each
circuit input voltage multiplied by its respective circuit gain factor. - Due to the inverting configuration, the polarity of the summed input voltages is inverted at
the circuit output. - U1, working through the feedback path, keeps the differential input voltage (VD) at 0V.
- If VD is greater than 0V (positive or negative), U1 is driven into saturation.
- One of the circuit inputs can be used to null, or trim, the circuit offset voltage to 0V.
- Circuit input, feedback, and load current distribution depend on the magnitude and polarity
of the input and output voltages. - Apply Ohm's law to determine the magnitude of each current.
- For positive input voltages, the summed voltage polarity is positive, and electrons flow out of
the op amp output terminal. - For negative input voltages, the summed voltage polarity is negative, and electrons flow into
the op amp output terminal. - If the circuit input voltages are positive and negative, electrons flow in either direction.
- The direction of the op amp output terminal current is based on the resultant polarity of the
summed input voltages.