# Choosing resistor for an op-amp

Ok, so I am new to electronics and unclear about something. Let's say I have a closed loop inverting op-amp. So there's 2 resistors, 1 feedback resistor, and the resistor at Vin. So I want a gain of perhaps 10 and I know voltage gain is the feedback resistor divided by the resistor at Vin. What should my resistor values be? Whats the difference between 1ohm & 10ohm resistors or 1kohm and 10kohm resistors? Appreciated if anyone can help

Whats the difference between 1ohm & 10ohm resistors or 1kohm and 10kohm resistors?

Consider the voltage at the inverting input $$\V_{IN-}\$$ when looking at the circuit: -

We know that an op-amp has massive open-loop gain hence, for practical reasons (in most op-amp circuits) we can assume that the voltage at $$\V_{IN-}\$$ equals the voltage at $$\V_{IN+}\$$ and, because $$\V_{IN+}\$$ is 0 volts, we can regard the feedback resistor (R2) as loading the output of the op-amp.

Let me repeat that: -

$$\color{red}{\boxed{\text{the basic op-amp load is the feedback resistor R2}}}$$

It's then a simple case of reading the data sheet to see how much current an op-amp output can produce - you'll generally find that it's about 10 mA. It might only be 5 mA or it might be 25 mA but, it's in that rough area for 99% of all op-amps.

So, if your output voltage might be as high as 10 volts then work out what the minimum value of R2 should be. So, 10 volts divided by 10 mA equals 1000 ohms. So, we should aim to ensure that R2 is significantly greater than 1000 ohms.

If you are planning on an external load of 2 kohm then the minimum value for the feedback resistor is 2 kohm. From this you calculate what R1 needs to be to meet your amplification requirements.

• So was looking at the datasheet of an op-amp. Saw a parameter called "Supply current, no load" and its max was 3.7mA. Is that the correct parameter to check? What if my Vout is as high as 12.5V and I have an external load of 2kOhm? – Fiidisks Jul 11 at 16:41
• No, that's not it. Link me the data sheet and I'll show you. – Andy aka Jul 11 at 16:57
• google.com/… – Fiidisks Jul 11 at 17:01
• The LF351 has this data in figure 3 and figure 4 for two load resistors namely 2 kohm and 10 kohm. On page 4 it is also summarised in words ($V_{opp}$). In gives the information indirectly as some op-amps do but, more modern op-amps are less ambiguous about it. Immediately above it there is the short circuit current and this cannot be relied upon to be greater than 10 mA (hence me using 10 mA in my answer). More modern chips usually give more details. – Andy aka Jul 11 at 17:06
• Also, this recent answer gives details about the LF351 that is similar to what you are asking. – Andy aka Jul 11 at 17:10

Difference is in current. If you use 1 ohm and 10 ohm, then 1 volt of input voltage draws 1 amp of current which the op-amp must handle. Now, op-amps typically can't handle 1 A current, so you have to choose higher resistor values.

If you go for the 1kohm and 10kohm resistors, then 1V of input voltage only leads to 1mA of current. That's perfectly fine for generic op-amp circuits.

However, there are different kinds of op-amps, such as current feed-back op-amps, so they might have different requirements for the feedback resistors. Too large feedback resistors can lead to more noise and problems with parasitic capacitance of the circuit, so they are not good either.

• So the difference is in the current produced? So what should my resistor values be? If my Vin is 0.5Vpp and I want to amplify it to 25Vpp(Gain 50). Is there any minimum current value I should have and when you say an op-amp cant handle 1A, can the amount of current hat can be handled by an op-amp be found in its datasheets? – Fiidisks Jul 11 at 16:16
• Current is the biggest difference. 1 ohm / 10 ohm is extremely off the scale, so is 1 megaohm / 10 megaohm, 1 kohm / 10 kohm is approximately the right ballpark for 10x gain. But now you changed the specs, 50x gain is a bit more difficult, because you need 1kohm / 50 kohm and the 50 kohm starts to be quite large, even if total impedance for the node is still about 1kohm. Yes the typical output current should be in the datasheets. Depending on what frequencies your signal has, and what op-amp you use, you might have to split it up to two stages with about 7.07 gain. – Justme Jul 11 at 17:02
• The frequency of my input signal is 50kHz – Fiidisks Jul 11 at 17:03
• Is 50 kHz the bandwith? Or is it a 50 kHz sine wave? 50 kHz square wave? – Justme Jul 11 at 17:06
• Its a sine wave – Fiidisks Jul 11 at 17:07