Hi,

I am trying to simulate a scintilator signal. I wont get into details, but I need to achieve the following waveform: it looks like triangle signal, only the falling edge has to be ideally four times longer than the rising one, also the waveform should not have any sharp transitions, but a curve instead of them...
I managed to simulate a working sawtooth generator, then after quite some experiments I added the opamp with a 47uF capacitor in the feedback. By doing this, I achieve the best possible waveform I am looking after... But then I encounter problems... The signal still has to be amplified and the DC offset has to be removed. I was planning to do the mentioned steps one opamp at the time, but somewhy I can not amplify my 50 mV signal (signal gets severely attenuated after U3 opamp). I tried playing around with the component values, picked a quite fast opamp for the amplification, but I am not getting a single mV of signal from the U4, the output always stays high...

I am quite new to this small signal design, any ideas or observations would help me a lot. Thank you in advance.

Sorry for the sketchy EasyEDA schematic. I am not yet used to the net labels in this software...

 

What frequency is this signal?

 

In this simultion the frequency is around 30 kHz, on a real circuit, I will make it up to 800 kHz. Simulation software starts to kick in some parasytics and other non-linear effects, so I can not make the simulation at real design frequency, because the output of the simulation becomes chaotic...

 

My main question is: why is the U4 opamp not amplifying signal at all? Slew rate of this opamp is 3V/us, which is more than enough at this frequency and amplitude...

 

...
The signal still has to be amplified and the DC offset has to be removed. I was planning to do the mentioned steps one opamp at the time, but somewhy I can not amplify my 50 mV signal (signal gets severely attenuated after U3 opamp). I tried playing around with the component values, picked a quite fast opamp for the amplification, but I am not getting a single mV of signal from the U4, the output always stays high...

Hi Vilius,

I don't understand, how the left side of Your circuit works. But, You set U4 up to a gain of 11. It also gains the DC at it's input. By R10 and R11 You give 0.555V at the input of U4, it tries to amplify it by 11, so, the output DC level must be 6.1V. Must be, but it isn't since the supply voltage is 5V and it limits the output.
Try to change R10, from 80k to 330k - 470k, and You'll see the AC part of the signal too, certainly with a DC offset. The type of OpAmp is not very critical for this 20kHz signal. It must have a GBW parameter (Gain * Bandwith) of minimum 250 kHz - not a challenge today.
In case You need to remove the DC offset, You will need a symmetrical +/- power. In this case You don't need to give a DC offset for U4 like now.

 

Hi Vilius,

I don't understand, how the left side of Your circuit works. But, You set U4 up to a gain of 11. It also gains the DC at it's input. By R10 and R11 You give 0.555V at the input of U4, it tries to amplify it by 11, so, the output DC level must be 6.1V. Must be, but it isn't since the supply voltage is 5V and it limits the output.
Try to change R10, from 80k to 330k - 470k, and You'll see the AC part of the signal too, certainly with a DC offset. The type of OpAmp is not very critical for this 20kHz signal. It must have a GBW parameter (Gain * Bandwith) of minimum 250 kHz - not a challenge today.
In case You need to remove the DC offset, You will need a symmetrical +/- power. In this case You don't need to give a DC offset for U4 like now.

Thats a good point. I forgot that the opamp amplifies everything at its input, DC including Thank you. However, after adjusting the divider, I still get the same problem... This time DC offset is around 50 mV, and gain is set to 8. 8*50 mV still should not result in 5V, but somehow it does Any ideas now?

 

I agree: there shoudn't be 5V on Your channel B. You have to see there the AC signal, amplified by 8, superposed to a DC of about 0.4V.
Shutdown is floating, it's OK, also could be tied to V+, common mode range is within spec, I don't know what happens there.

Did You run the simulation again, when You have changed the values? It doesn't happen automatically.

I would try to pull the DC a little bit higher - R10=330k or 470k - but with Your values, it must work well, too. In case Your simulator has a component called "Ideal OpAmp" (which only exists in the simulators, never in the real world) You can try to replace U4.

 

Per the datasheet, a 741 *requires* a minimum of 10 V between its two operating rails. This can be +10 V and GND, +/-5 V, etc.

Also, the 741 is most definitely *not* a rail-to-rail device. It has limitations on both its input and output operating ranges.

Many simulators ignore both of these limitations, but yours might be showing you their effects.

My main question is: why is the U4 opamp not amplifying signal at all? Slew rate of this opamp is 3V/us, which is more than enough at this frequency and amplitude...

Maybe, but besides the operating voltage range, input common mode voltage range, and output voltage range issues, the 741 has a relatively low gain-bandwidth product and slew rate. My guess is that with five strikes against it, there is no signal for U4 to amplify.

ak

 

My main question is: why is the U4 opamp not amplifying signal at all? Slew rate of this opamp is 3V/us, which is more than enough at this frequency and amplitude...

U4 has its +input biased at 0.555VDC and the opamp U4 has an AC and DC gain of 11 times. Then its output is saturated at the +5V supply but produces some negative-going severe distortion.

 

Since the signal of interest is nowhere near DC, you can solve the offset issue by adding a capacitor in series with R9. This will reduce the circuit's DC gain down to 1. The R10-R11 voltage will be the DC operating point of the output.

The added capacitor and R9 form a high-pass filter. A 1 kHz corner frequency should be fine for a 30 kHz signal.

ak

 

Alright, I just replaced all opamps with good old LM358s and simulation worked fine. I got rid of DC component after all stages, my gain is now just what I was looking for... Replacing a precision opamp with a general purpose jellybean one feels a bit counterintuitive, but it seems that for the EasyEDA simulator, that was exactly the case

 

Its me again... I changed my sim software to LTSpice, as it is presumably more accurate and reliable... I also did some updates to the circuit including the pnp transistor which switches the signal to ground and releases it when needed. I encountered a voltage spike problem during the transistor transition. I used the ,,low noise fast switching" transistor even for the simulation but it did not work. I know that there exist snubber circuits of all kinds, but in this case I can not use them, because I would be affecting my signal (this is not a PSU project, I do care about my signal form.) You dont need to investigate the circuit deeply, but maybe you can suggest ways to reduce switching spikes without using a snubber circuit?

 

hi VZ,
Please post your LTS asc file and any non standard models.
E