Open this schematic of a summing amplifier in another browser tab.

What does this amplifier do? Develop an expression for $\bm{v_o}$ in terms of $\bm{v_{ina}}$, $\bm{v_{inb}}$ and the resistor values.

show answer

From the video, the gain expression for a summing amplifier is,

$v_o = - \left (\dfrac{\text{Rf}}{\text{Ra}} v_{ina} + \dfrac{\text{Rf}}{\text{Rb}} v_{inb} \right )$

Fill in the resistor values,

$v_o = - \left (\dfrac{\text{20\,\text K}}{10\,\text k} v_{ina} + \dfrac{10\,\text k}{15\,\text k} v_{inb} \right )$

$v_o = - \left (2 v_{ina} + 0.75 v_{inb} \right )$

Observe the inputs: Add voltage probes to the two input voltages. Click on TRAN to run a transient simulation.

One input voltage is a square wave. The other is a single step voltage.

Sketch what you think the output will look like.

Knowing the shape of the inputs and the amplifier’s function, make a guess.

Add a voltage probe to the output node. Run another TRAN.

Does the simulated output match your sketch?

Starting from a blank schematic, design a summing amplifier that performs this function: $\bm{v_o = -\left ( 4\,v_{ina} + 3\,v_{inb}\right )}$.

Circuit sandbox

Hint: Pick new values for the resistors with the right ratios.

Verify your new amplifier does what you expect.

Change the input waveforms to something else.

For example, make one of them a sine wave. Does the output do what you expect?

Make them both sine waves with a different frequency.

Make up a signal pattern in your head and see if you can build it.

Summing amplifier with probes

Simulation model of a summing amplifier with probes.