Impedance, Schmedance....

[HankMcSpank]

When knocking up a circuit with simple signal chain that includes a few series connected opamps...how do you establish the correct opamp interconnecting input or output load resistors - and are they actually necessary?! (my circuit seems to work without them, ie where the ouput of one opamp goes directly into the input of another - ie low output impedance into high input impedance isn't it?)

Also how do you establish the best value pot/trimmer to put in parallel with an opamp's output - is this info gleaned easily from the datasheet? (to give an example - http://sound.westhost.com/p64-f3.gif - VR24 .....how would the designer know to use a 100k Variable resistor as opposed to say a 5k or 47k pot?)

 

[Conrad Hoffman]

It's true, this is a fairly non-fussy thing. Look at the output current in the data sheet. Chances are that the typical op-amp shouldn't drive less than about 2 kohms. CMOS op-amps often have less drive capability and the minimum number might be 10 kohms or so. Others are designed to go as low as 600 or below. Ideally, no current flows through the input terminals, but you still have to drive the feedback network, so include that. Resistance between stages usually isn't necessary, but stability can sometimes be enhanced with a small resistance to isolate any capacitance that might be present, or even a resistive pad like some RF circuits use. There's also the issue of best performance. Just because an op-amp can drive a load doesn't mean it performs best there. Experience is the best teacher, but I keep most of my impedances between 5k and 50k unless there's some reason not to. For example, in my active crossover I wanted some output level controls. I used 2k pots so the subsequent cable capacitance wouldn't roll off the high frequencies too much. It was ok because the op-amps in question had high drive capability and performed well with that constant load, plus the load of the cables and following amp. Remember to look at what the op-amp has to drive at the high frequency end. There are other cases like preamps, where the signal level will never be more than a couple hundred millivolts. You can often use far lower feedback values and optimize noise, a couple hundred ohms can easily be driven.

 

[sawreyrw]

U2A won't work unless there is some resistance in series with the signal at "In". U2B is a voltage follower (unit gain stage), so it doesn't need any input resistor.

 

[HankMcSpank]

Thanks guys.

sawreyrw ....that's not my circuit - just a random one pulled off the web (just so I could be specific about the Variable resistors placement)

You say U2A needs some series resistance? ....that's the point of my post is how do you glean what that particular resistance should be! Also, what about that output Variable resistor on the output of U2A- why not say a 5k (vs the 100k used?)..... is this value dependent (derived) from the following device's specififccation?

 

[Conrad Hoffman]

Sawreyrw is right, unless the circuit is supposed to be an AC coupled current to voltage converter, which seems doubtful. The input resistance works with the feedback resistance to set voltage gain, so you choose it by that. That resistor at the output just keeps the cap bled off. Value isn't critical. It's high to prevent loading of the signal, no good reason to make it low as that would limit the low frequency response (as will the normal load).