# Choosing Resistor Values

I am going through analog electronics.

I have seen many designs but I have a few questions below :

My Questions :

I understand that resistors are used to limit current in some places. And in other places, resistors are used to pull up/pull down the voltage and make sure that node is in a defined voltage state instead of the node floating.

I have gone through some articles and websites that recommend to take the resistor value between 10k to 100k if its for pull up or pull down purpose. I agree that you can pick depending on your application.

But I see in someplaces in other designs where the analog circuits having transistors, have resistor values like 22.6k / 11.5k / 82k. Why is there so much disparity in picking the resistor values?

Is it to limit current in those places or is it to hold up the voltage to a defined state? Or like biasing a transistor or to bleed the current of the node slowly?

• Your question appears to boil down to why do resistor manufacturer's make so many different value resistors. Apr 17 '20 at 10:46

Selecting a resistor depends on the circuit/block in question. As you already mentioned, different applications demand different requirements.

For example, here are a few things to consider:

Pull up / down

Small resistors (aka "strong" pull up/down) are used when some nodes must be held at a given state "regardless" of the other impedances connected to the same net. However, in most cases, it leads to higher idle current consumption. Furthermore, due to their low resistive path, they pull the nodes faster to a given state.

i2C

The resistors used to pull up the bus line have a direct impact on the data transfer speed which can be used, and also how far apart the master and slave devices can located.

Bleeder resistor

Some circuits require a passive discharging of capacitive elements. A good example is a AC/DC power supply which should have its input capacitors (connected to the mains) discharged upon removing it from the mains, in order to prevent exposure of energized pins.

Voltage supply regulation

Regulated voltage supplies rely on feedbacks to have their output voltage controlled. Most of the time this is accomplished with a resistive voltage divider. Due to the limited amount of resistor values, selecting a combination of resistors which lead to the required output voltage can be challenging. Here you can usually find such resistors $$\22.6k, 11.5k\$$ etc.

Compensation Network Compensating loops, such as those of oamps, require placing poles and zeros at specific locations. As mentioned before, the limited amount of resistor and capacitor values may require to pick unique combinations in order to guarantee stability.

Note: These are just a few examples. The number of applications are countless.

Generally by limiting current flow resistors are altering voltage levels. Now, you can halve a voltage by using two resistors as a divider network, with input voltage at the top, ground at the bottom and tapping off between the resistors. However, choosing the value is not quite so simple. You might think that to reduce power requirements a couple of one megaOhm resistors would work, and they might. But because they are also limiting current there might not be enough current available to drive the rest of the circuit from the tap-off point. Also, because currents would be so small it also becomes susceptible to noise. So, how about (say) one Ohm resistors? No worry about tiny currents and noise, but now you have to worry about large currents, resistor heating and power drain on your supply.

So, quite often a rule of thumb is to choose a 10K resistor, or occasionally a 1K or 100K as a compromise for pullups/downs. In other cases the resistor values are chosen to provide very specific voltages, but then you also have to take into account what currents are needed to drive the circuitry. It's not simple.

I worked in the same building as a guy who, as part of amplifying a signal on a test board, needed a gain of 5X. He choose to use an operational amplifier (opamp) because after all how hard are opamps to use??

Problem was his choice of resistors. He ran afoul of the finite output current of the opamp.

Most opamps give you 10 or 30 or 30 milliAmps output current, because people use moderate values of resistors such as 1Kohm (which has only 4 nanoVolts/rootHertz noise, which is 4,000 nanoVolts or 4 microVolts or 0.000004 volts RMS in 1MHz bandwidth) or 2Kohm or 10Kohm or 5.01Kohm (because they wants 5.000Kohm and 5.01K was in stock).

This engineer, excellent and focused and dedicated and effective in his usual tasks, did not use Ohms Law to compute the current ---- after all, these are opamps and there are no application issues with opamps, right?

So, to get the gain of 5X, using inverting opamp topology (circuit), he chose to use

ONE OHM and FIVE OHM resistors.

This would have been fine, for input signals less than 10 milliVolts or perhaps 20 milliVolts.

But at one volt input, the opamp needed to handle 1 amp, or 1,000 milliAmps.

He, by not reading the datasheet of the OpAmp, or likely having no prior experience with opamps as NOT infinite power devices, was surprised.

Summary: lower value resistors, such as the very popular 51 ohms (5-1 ohms) have much less random electron-movement-thermal-agitation voltage noise; thus in reading back the music from vinyl discs, one of the design parameters for the PreAmplifier is LOW NOISE.

I've seen vinyl playback amplifiers using 4.7 ohms resistors as part of the circuit directly connected to the Moving Coil pickup cartridge.

One caveat is the non-zero resistance of PCB copper foil. Standard thickness foil, which is 1 ounce of copper per square foot of PCB area before etching, is 0.0014 inch thick (or 35 microns), and that foil has 0.00500 ohms (yes, 500 microOhms) resistance per square of foil. Thus a PCB trace of size 0.2 inches long and 0.005 inches wide, has 40 squares of foil and 20 milliOhms resistance.