Op amps and input resistance.

[SeanHatch]

Hey, I'm supposed to answer the following question:

A certain microphone is capable of delivering + 0.1 V when someone claps their hands at a distance of 20 feet. A particular electronic switching circuit has an input resistance of 670 ohms and requires + 20 mA to activate the switching action. Design a circuit that will connect the microphone to the electronic switching circuit in such a way that the switch is activated by someone clapping their hands.

I don't understand what the input resistance is about. Do they mean the resistance in the switching circuit, that would be connected to the output of an op amp?

I don't think they mean the resistance of the op amp, becuase we're doing ideal op amp analysis, in which case i htink we're supposed to assume the input resistance is infinite.

We're supposed to be solving these problems using basic op amp building blocks.

Can anybody help me about, just explain what htey mean by input resistance?

thanks;
sean

 

[Nigel Goodwin]

I'll give you a clue!.

You've got a resistance specified (670 ohms), and a current (20mA), what formula do you know that uses those two, and gives a third?.

This (very simple) question makes it easy, because you can treat the load as a simple resistor.

 

[Joel Rainville]

Do they mean the resistance in the switching circuit, that would be connected to the output of an op amp?

My beginner's knowledge of electronics tells me yes. So you only need to figure out what voltage is needed to get 20ma at the switch, and how to produce that voltage using your input of 0.1V (or higher?)

 

[SeanHatch]

So input resistance is synonomous with load? that seems weird.

 

[DigiTan]

I though the input resistance was just the resistance between your signal and the +/- input--because of the virtual ground.

 

[SeanHatch]

that's what i originally imagined, but then I'd have to generate 20ma across a load or resistance they don't give me.

 

[Nigel Goodwin]

So input resistance is synonomous with load? that seems weird.

The OUTPUT resistance of one circuit feeds to the INPUT resistance of the next.

The question is really very badly phrased though, you wouldn't normally use input and output resistances for something like this.

You also have the added complication of having to 'guess' exactly what's wanted? - it's fairly uncommon to match impedances, normally you feed low output into high input, for maximum signal transfer. Matching impedances gives maximum POWER transfer, but only transfers 50% of the original. The first would be the correct method for this, but which do they want?.