Hallo,

I am trying to implement a simple pulse discriminator for pulses with 10 Hz frequency using TL084N using the scheme from the attachment.

I have put that circuit on my breadboard using an input generator and an oscilloscope to watch out the response.

However, the output level of the op amp isalways 5 V.

Any hints where I am going wrong?

Thank you!

 

The amplitude of the pulses is 250mV but what two voltages are the pulses between?
They could be from 0V to 250mV and then your circuit would work fine or they could be between 1V and 1.25V and then your circuit will always output 5V.

 

I'm attaching the oscillograms: Ch1 is the input and Ch2 the output of the circuit, I've reduced the Vref to 160 mV

 

Are you sure that you have not mixed up the inverting and non inverting inputs. With your circuit I would expect the output to be at almost zero volts. I would not expect to see anything on the output unless the input goes above the reference voltage. Are you sure R2 will adjust low enough to get down to a reference voltage of 230 mV ? I would have thought a 3.3 K potentiometer would have been a better choice. I can't find any information on the data sheet The minimum supply voltage, None of the graphs show it with less than + and -5 volts supply. Try connecting the - supply pin connected to - 5 volts rather than 0 volts.

Les.

 

So I have to interpret the display of an oscilloscope that I have no knowledge of?
What does 'ofs -202mV' mean?

What happens if you disconnect the input pulses?

 

"ofs" is the offset - I just pulled it up and down so that the oscillograms will not overlap. Here is the oscillogram at 0 offset:



Following your question of disconnecting the Generator, I see that the open circuit voltage (that is without generator) at the non-inverting input is about 360 mV. Where it comes from? Via the resistance R4?

 

The TLO84 will not operate well with a single 5V supply. It needs a minimum of ±5V (see data sheet snip below):


You need a single supply or rail-rail type op amp.
An inexpensive example is an LM324/LM358 although its maximum output is about 3.5V with a 5V supply.
You could also use a comparator such as the LM339/LM393.

 

Are you sure that you have not mixed up the inverting and non inverting inputs. With your circuit I would expect the output to be at almost zero volts. I would not expect to see anything on the output unless the input goes above the reference voltage. Are you sure R2 will adjust low enough to get down to a reference voltage of 230 mV ? I would have thought a 3.3 K potentiometer would have been a better choice. I can't find any information on the data sheet The minimum supply voltage, None of the graphs show it with less than + and -5 volts supply. Try connecting the - supply pin connected to - 5 volts rather than 0 volts.

Les.

I can change th epotentiometer later on. I was thinking about the supply voltage too, but looking at the data sheet I saw that it can also operate with a single supply.

 

And it looks like the input pulse is only 220mV.

 

And it looks like the input pulse is only 220mV.

Yes but I reduced the Vref to 160 mV

 

Is the output still +5V with the input pulses disconnected?

 

Yes. There is something wrong ... I guess it is better to rebuild it and start all over again. I'm out of my wits for such a simple circuit ... I'll come back with news asap.

 

looking at the data sheet I saw that it can also operate with a single supply.

That's if the input is biased at about 1/2 the supply voltage and the supply voltage is ≥10V.
Look at the common-mode voltage range (below).
It does not go to the minus voltage.

The input voltage range is 3V above the negative supply voltage (±15V) or 3V above ground if the negative supply is ground.
You can keep fussing with that op amp but it's not likely to do what you want.

 

Crutschow is correct you will never get that circuit to work correctly without a duaI supply, verified myself using a TL082. MCP602 is a better choice.
SG

 

Indeed the power supply is a must for this op amp. I tried with a dual power supply but still I have troubles running the above circuit.
Therefore I replaced the op amp with a comparator 8049 LM311N and adapted a bit the circuit as follows:


Using a single 5 V power supply I have the required result:


I guess this will give me a better behavior at higher frequencies than the op amp.
Thank you all for your help!

 

This can be done with a single supply by raising the power supply voltage to about 12V or higher and putting the bias back to 50% of the supply.

 

The TL08x, TL07x and a few other old opamps have a problem where the output suddenly goes as high as possible if an input voltage gets within a few volts from the negative supply voltage. It is shown here: