# Selection of Op Amps

Discussion in 'General Electronics Discussion' started by z0d, Aug 13, 2015.

1. ### z0d

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Aug 13, 2015
Hello, I'd like to get some perspective in this area. 2 main enquiries.

1st being, in a Relaxation Oscillator Circuit, what are the parameters of the Op Amp that define the output voltage? [eg. slew rate, GBW, R-to-R] So say for example I have a ±5V supply [not V- or V+] for the OpAmp and I'd like to reduce the output voltage to 3V, what are the things I should look out for when selecting the suitable Op Amp.

My second part is related, that is if I were to follow up the circuit with a voltage follower/impedance converter to eliminate the effect of the extra 100 Ω at the output of the Oscillator Circ and maintain the output, which OpAmp would be suitable for the voltage follower - or better yet how does one begin to choose?

Thank you.

2. ### BobK

7,682
1,688
Jan 5, 2010
If you want 3V at the output with a ±5V supply, you will need an opamp that can output a minimum of V+ - 2V.

Other characteristics will be based on the frequencies you need. If you are talking about audio frequencies (< 20KHz), pretty much any opamp will do.

Bob

3. ### AnalogKid

2,573
740
Jun 10, 2015
Reasonable questions, but woefully short on details. Please post a schematic of your ideas so far. There are 1000 different opamps that can be used as a voltage follower. What frequency? What output voltage range? What output current?

Also, "a ±5V supply [not V- or V+]" doesn't make sense. In electronics terms, a +/- 5V supply is two power supplies, one making +5 V with respect to the system ground potential and another one making -5 V with respect to the same ground. If you are trying to say that you have only one power supply and you are running your opamps single-ended, that would be just a single +5 V supply.

ak

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4. ### z0d

15
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Aug 13, 2015
Hello Bob. Thanks Bob. For taking the time. Sorrry I couldn't respond earlier. Okay I think it's better if i just upload a sample circuit.
So, I have achieved a pretty stable ±2.5V square wave from a ±3.3V supply with these 2 Op Amps. My question is, if I have to parametrize it such that I want a ±1V square wave from the SAME supply, which parameters of the new op amps should I be looking out for? For both the Relaxation Oscillator Circuit as well as the v-follower? Or perhaps there is something I could add to the circuit, but I don't think so.

Seems to me that the GBW as well as SR AND output current is pretty important.

EDIT : The frequency is constant as well at 4 kHz - a significant point I failed to mention earlier, apologies.

5. ### z0d

15
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Aug 13, 2015
I hear you Analog, 'twas my bad. So the DEETS.

Frequency. 4 kHz. Output voltage range -2.5V...2.5V (achieved) and -1V...1V (attempting). Output current I am not too sure about BUT it should be able to deliver up to 100 mA AFAIK.

Sorry for the misunderstanding. By ±5V I meant to say the op amp is supplied with that voltage, as in Vcc+ & Vcc-. Basically, it's a double supply op amp.

Anyway, I realized how silly it is NOT to include the circuit, I am new here. Yeah so I've done just that in the reply above. Please refer there and thanks your time m8!

6. ### AnalogKid

2,573
740
Jun 10, 2015
Relying on the peak output voltage capability of the opamp to set the signal amplitude is not a good approach. That parameter varies significantly from one device or lot to another. You have no choice with the relaxation oscillator, but you do with the output buffer. Between the 1208 and the 1809, add a 2-resistor divider to scale the 2.5 down to 1.0.

ak

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7. ### z0d

15
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Aug 13, 2015
Hello AK, thank you for making that part clear to me. Personally I have no experience when it comes to choosing the right op amp but yeah how I managed to get to this point was via trial-and-error with a bit of simple calculation here and there.

yeah that makes sense, just thought that maybe there were a couple of specifications with regards to op amps I could be more precise about to help get the desired output. thanks again for your input!