TroniC-pedia

ELEN2016A / interactive circuit lab

Inverting Summing Amplifier

Set each source to DC, AC or AC + DC. Watch the node voltages, current paths and output trace change over one cycle.

An interactive teaching model by Dr Dangor

AC/DCTwo inputs
One output

01 / live circuit

Current paths

Linear operation
Two-input inverting summing amplifier V1 enters R1 on the upper branch and V2 enters R2 on the lower branch. Their currents meet at the inverting input node. RF connects that node to the output. The non-inverting input is grounded. Coloured moving dots show signed current; the amplifier input pins draw zero current. V1 V2 R1 R2 RF +2.00 V +3.00 V 20.0 kΩ 10.0 kΩ 10.0 kΩ V− V+ Vo +Vs −Vs −4.00 V → →
V1 through R1 V2 through R2 RF feedback 0 A into OA1 input pins

The two input currents combine at the summing node and flow through RF. No current enters either ideal amplifier input.

Dots show current direction, not individual electron motion.

Current at the summing node

KCL
I1 · V1 → R1+0.10 mA
I2 · V2 → R2+0.30 mA
IF · node → RF+0.40 mA

+0.10 mA + (+0.30 mA) = +0.40 mA

⊘ Ideal OA1 inputs: I− = I+ = 0 A

02 / live monitor

Signals and output

STEADY DC
Measured output · Vo to GND−4.00 V

The amplifier output is within the supply rails.

Node voltage relative to GND

−15 V / +15 V
−0 V+
V1
+2.00 V
V2
+3.00 V
V−
0.00 V
V+
0.00 V
Vo
−4.00 V

GND = 0 V · supply rails −15.00 V / +15.00 V

Oscilloscope

DC window · 1.00 s

t = 0.000 s
+15 0 −15 0 s1.00 s
Voltage scale ±15.8 VTime window 0.00–1.00 s
Drag on the plot or move the slider to pause at any instant.
Scope notes

The DC inputs appear as flat traces. The moving line marks the values shown above.

V1 and V2 remain constant; the output stays between the supply rails.

03 / workbench

Change the circuit

Adapted from the CircuitLab schematic. This teaching diagram pairs V1 with R1 and V2 with R2.

Independent input sources

V1 through R1
V
−20 to +20 V
V2 through R2
V
−20 to +20 V

Select DC, AC or AC + DC for each source. Choose a sine, square or triangle waveform and set its peak and frequency. Inputs outside the displayed limits are rejected.

Input resistors

kΩ
1 to 100 kΩ
kΩ
1 to 100 kΩ

Feedback and supply

kΩ
1 to 100 kΩ
V
3 to 24 V, symmetric rails

The idea

The input pin is not a path for current.

In linear operation, negative feedback holds V− at the same voltage as grounded V+: 0 V. The algebraic sum of the V1 and V2 branch currents flows through RF, not into either OA1 input pin. A negative branch current flows in the opposite direction. When Vo reaches a supply rail, V− can move away from 0 V, while ideal input current remains zero.

Linear operation onlyVo = −RF (V1 / R1 + V2 / R2)

Each term represents the current from one input branch.