# Simulation of op-amps in Proteus

I want to simulate the given circuit and get see the outputs from two opamps.

In reality, I use L272D dual opamp, but for simulation, I decided to use just ideal opamps. The IN+ for the first amplifier is the output of DAC (out of the circuit) which is a sinusoidal signal with an amplitude of 2.5V and an offset of +2.5V. IN+ for U2 is two parallel resistors, one of which is connected to REF194ESZ (4.5V from the datasheet). R3 is actually potentiometer, and I set its value to 1k Ohm. Finally, the load resistors of 4 Ohm are connected to the opamps' outputs. Something that I expect to get is on the image.

However, what I get from the Transient Analysis graph is

and from oscilloscope

yellow - Vout1 blue - Vout2 pink - Vout_DAC

which I suspect is something different...

Am I doing something wrong, or the graph output seems to be correct? Please, give some advice on the simulation of circuits with opamps... I am very new to the simulation in Proteus. Thanks

• Are you trying to get rid of the 2.5V offset? Just asking. Oct 28, 2022 at 8:14
• No, I just expect to get a similar Vout_DAC signal (with an offset) on Vout1, and an inverted signal on Vout2 Oct 28, 2022 at 8:16
• what's the voltage supply of the two OpAmps ? Oct 28, 2022 at 8:58
• it's 3.3V, but in a simulation with ideal opamps, does it matter? or I should choose L272D dual opamp after all... Oct 28, 2022 at 9:05
• the output of U1 is Vout1 = 1.2*Vout_dac (so both your offset and ac amplitude get amplified). The output of U2 is Vout2 = Vref-Vout1. What are you trying to do ? Oct 28, 2022 at 9:38

## 1 Answer

You need to double check all your connections. There are a few connection dots that shouldn't be there (such as the GND connection of R1) which might indicate the line is terminated before making a connection as Proteus will add a connection dot to a terminated line.

I have built the circuit on my version which is using all the same values as yours, and included an early terminated connection on the output of Vout2, so you can see what I mean with the connection dot:

When simulating this, here is my output:

It looks like you have early termination at the inputs of both op-amps which would explain the unexpected outputs, as well as a few other potential ones. If you go back and check, maybe re-do them all so it looks similar to mine, with no additional connection dots, then it should work.