Hi All,

I am trying to understand the differences in op amps. For the longest time I just thought there were general purpose op amps and comparators.

Recently I built a circuit using a 4558D dual op amp IC and all was well. I then tried to build the same circuit again using two OP27G op amps and didn't get the same results.

I even tried swapping the 4558D with OP200 and the circuit failed to work!

So, obviously there are gaps in my knowledge about op amps.

I am trying to understand what specific type of op amp the 4558D is. The data sheet says "especially well-suited for applications in differential-in, differential-out" which leads me to believe this is a "differential "type op amp. The OP200 data sheet says "ideal choice for applications requiring multiple
precision op amps and where low power consumption is critical." which leads me to believe this is more of a general purpose type.

Either way I seem to be lacking in my ability to discern what is what here. For now I am going with 4558D is differential type and OP200 is GP so that is why it did not work the same. But that is just where my "likely false" understanding is at the moment.

Anty thoughts welcome.

Thanks,

Joe

 

I even tried swapping the 4558D with OP200 and the circuit failed to work!

They do no appear to be that different.
Post the circuit schematic, and how did it fail to work?

 

They do no appear to be that different.
Post the circuit schematic, and how did it fail to work?

there was no output at all

 

View attachment 331611

there was no output at all

try setting these resistors to 220
If that noninv node must be half Vdd then Requal will affect it.

 

try setting these resistors to 220
If that noninv node must be half Vdd then Requal will affect it.
View attachment 331612

how do you mean requal will affect it

 

What is the power supply voltage?

 

12V, I tried 5V up to 15V aswell and still no output

 

For now I am going with 4558D is differential type and OP200 is GP so that is why it did not work the same. But that is just where my "likely false" understanding is at the moment.

All opamps, except for LM3900, have differential inputs. The LM3900 Norton amplifier uses a current mirror instead of a true non-inverting input.

4558 is a general purpose opamp. OP200 is a special purpose (low noise) amplifier.

 

The two things that might cause dramatic failure are the input common mode range and output range. Unfortunately, neither datasheet characterizes these at other than ±15V supply.

Edit: Question why the pullup resistors on opamps?

 

12V, I tried 5V up to 15V aswell and still no output

If no output is 0V, then I suspect a wiring error or a bad op amp.

 

how do you mean requal will affect it

Since that node also connects to the output through a third 100k Resistor , may that voltage divider will not works as you expect. Just an idea

 

Should amplifiers be listed first or last in understanding transconductance?
https://w.wiki/BCAB

If you want to understand Norton op amps and OTA op amps you have to read what they are.
https://w.wiki/BC9T
https://w.wiki/BC9o

4 kinds of op amps

Voltage amplifiers take voltage in and produce a voltage at the output.
Current amplifiers receive a current input and produce a current output.
Transconductance amplifiers convert a voltage input to a current output.
Transresistance amplifiers convert a current input and produces a voltage output.

Voltage Comparator compares voltage inputs, and drives the output to the supply rail of whichever input is higher. This configuration is considered open-loop operation because there is no feedback. Voltage comparators have the benefit of operating much faster than the closed-loop topologies

 

try setting these resistors to 220
If that noninv node must be half Vdd then Requal will affect it.
View attachment 331612

View attachment 331611

there was no output at all

That is a comparator circuit not an op-amp circuit.
Some op-amps will work as (slow) comparators, some won't (mostly the ones with back to back diodes across the inputs)
(And it's a perfectly good comparator circuit regardless as to whether the resistors are 100k or 200k)

 

That is a comparator circuit not an op-amp circuit.

That explains the pullup resistors.

 

That explains the pullup resistors.

. . and the positive feedback.

 

The circuit will not work. The inverting inputs have no DC reference level.
The first op amp is confused. It has both positive and negative feedback.

Remove the positive feedback resistor and now both inverting inputs will be at 1/2 Vcc.
You don't need the 2.2 kΩ pullup resistors at the outputs if these are op amps and not comparators with open collector outputs.

Edit: If these are comparators then that requires a totally different analysis. The first stage is an oscillator.

 

So that folks can be more helpful it would be very useful to show the actual voltages at the device inputs and outputs. And either an opamp or a comparator can go into saturation with excessive positive feedback.
Also, for the sake of discussion, identify each resistor by number or letter. Calling a component " Requal " tells me absolutely nothing.